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## Boost Macro Reference

### Broad C++ Standard Level Checks

These are for the times when you just want to check "does this compiler conform to C++11" or some such.

##### C++ Standard Level

The macro `BOOST_CXX_VERSION` is set to the C++ standard version - it has the same value as `__cplusplus` if `__cplusplus` is set to a sensible value, otherwise the current C++ standard level in effect (for example for MSVC this is set to the value of `_MSVC_LANG`).

Available standard values are:

standard | value
--- | ---
2003 | 199711L
2011 | 201103L
2014 | 201402L
2017 | 201703L
2020 | 202002L

You should use this macro as a basic sanity check that the compiler is minimally at the required standard level - there may still be bugs, unimplemented features, and/or missing headers. The individual feature checks should be used for those.

##### Strict Standard Level Checks

The following macros:

    BOOST_NO_CXX03
    BOOST_NO_CXX11
    BOOST_NO_CXX14
    BOOST_NO_CXX17

Are set if any of our defect macros are set for that standard level, .

Note that these are therefore very strict checks and as a result, other than very recent gcc/clang releases, many compilers will end up defining all of these macros: even when `BOOST_CXX_VERSION` indicates a much more recent standard is in effect.

##### Strict Standard Check Headers

The following headers:

    #include <boost/config/assert_cxx03.hpp>
    #include <boost/config/assert_cxx11.hpp>
    #include <boost/config/assert_cxx14.hpp>
    #include <boost/config/assert_cxx17.hpp>

Can be used to assert that none of our defect macros are set for that standard (or any older standards).

The headers will provide a slightly more useful error message than just checking on `BOOST_NO_CXX03` for example, as they will indicate which defect macro produced the error.

As noted abovem these are very strict checks, and therefore other than recent gcc/clang releases, many compilers will genrate a `#error` on including any of these headers.

##### SD6 Feature Test Macros

Note that none of the above checks take into account any standard library SD6 feature test macros, as doing so would require `#include`ing almost the whole standard library.

### Macros that describe C++03 defects

The following macros all describe features that are required by the C++03 standard, if one of the following macros is defined, then it represents a defect in the compiler's conformance with the 2003 standard.

Macro | Section | Description
--- | --- | ---
`BOOST_BCB_PARTIAL_SPECIALIZATION_BUG` | Compiler | The compiler exhibits certain partial specialisation bug - probably Borland C++ Builder specific.
`BOOST_FUNCTION_SCOPE_USING_DECLARATION_BREAKS_ADL` | Compiler | Argument dependent lookup fails if there is a using declaration for the symbol being looked up in the current scope. For example, using `boost::get_pointer`; prevents ADL from finding overloads of `get_pointer` in namespaces nested inside boost (but not elsewhere). Probably Borland specific.
`BOOST_NO_ADL_BARRIER` | Compiler | The compiler locates and searches namespaces that it should *not* in fact search when performing argument dependent lookup.
`BOOST_NO_ARGUMENT_DEPENDENT_LOOKUP` | Compiler | Compiler does not implement argument-dependent lookup (also named Koenig lookup); see std::3.4.2 [basic.koenig.lookup]
`BOOST_NO_AUTO_PTR` | Standard library | If the compiler / library supplies non-standard or broken `std::auto_ptr`.
`BOOST_NO_COMPLETE_VALUE_INITIALIZATION` | Compiler | Compiler has not completely implemented value-initialization. See also The Utility/Value Init docs
`BOOST_NO_CTYPE_FUNCTIONS` | Platform | The Platform does not provide functions for the character-classifying operations `<ctype.h>` and `<cctype>`, only macros.
`BOOST_NO_CV_SPECIALIZATIONS` | Compiler | If template specialisations for cv-qualified types conflict with a specialisation for a cv-unqualififed type.
`BOOST_NO_CV_VOID_SPECIALIZATIONS` | Compiler | If template specialisations for cv-void types conflict with a specialisation for void.
`BOOST_NO_CWCHAR` | Platform | The Platform does not provide `<wchar.h>` and `<cwchar>`.
`BOOST_NO_CWCTYPE` | Platform | The Platform does not provide `<wctype.h>` and `<cwctype>`.
`BOOST_NO_FENV_H` | Platform, Standard library | The C standard library doesn't provide `<fenv.h>`. `<boost/detail/fenv.hpp>` should be included instead of `<fenv.h>` for maximum portability on platforms which do provide `<fenv.h>`.
`BOOST_NO_DEPENDENT_NESTED_DERIVATIONS` | Compiler | The compiler fails to compile a nested class that has a dependent base class:
    template<typename T>
    struct foo : {
       template<typename U>
       struct bar : public U {};
};
`BOOST_NO_DEPENDENT_TYPES_IN_TEMPLATE_VALUE_PARAMETERS` | Compiler | Template value parameters cannot have a dependent type, for example:
    template<class T, typename T::type value>
    class X { ... };
`BOOST_NO_EXCEPTION_STD_NAMESPACE` | Standard Library | The standard library does not put some or all of the contents of `<exception>` in namespace std.
`BOOST_NO_EXCEPTIONS` | Compiler | The compiler does not support exception handling (this setting is typically required by many C++ compilers for embedded platforms). Note that there is no requirement for boost libraries to honor this configuration setting - indeed doing so may be impossible in some cases. Those libraries that do honor this will typically abort if a critical error occurs - you have been warned!
`BOOST_NO_FUNCTION_TEMPLATE_ORDERING` | Compiler | The compiler does not perform function template ordering or its function template ordering is incorrect.
    // #1
    template<class T> void f(T);

    // #2
    template<class T,class U> void f(T(*)(U));

    void bar(int);

    f(&bar); // should choose #2.
`BOOST_NO_INCLASS_MEMBER_INITIALIZATION` | Compiler | Compiler violates std::9.4.2/4.
`BOOST_NO_INTRINSIC_WCHAR_T` | Compiler | The C++ implementation does not provide `wchar_t`, or it is really a synonym for another integral type. Use this symbol to decide whether it is appropriate to explicitly specialize a template on `wchar_t` if there is already a specialization for other integer types.
`BOOST_NO_IOSFWD` | std lib | The standard library lacks `<iosfwd>`.
`BOOST_NO_IOSTREAM` | std lib | The standard library lacks `<iostream>`, `<istream>` or `<ostream>`.
`BOOST_NO_IS_ABSTRACT` | Compiler | The C++ compiler does not support SFINAE with abstract types, this is covered by Core Language DR337, but is not part of the current standard. Fortunately most compilers that support SFINAE also support this DR. See also BOOST_NO_SFINAE and BOOST_NO_SFINAE_EXPR
`BOOST_NO_LIMITS` | Standard library | The C++ implementation does not provide the `<limits>` header. Never check for this symbol in library code; always include `<boost/limits.hpp>`, which guarantees to provide `std::numeric_limits`.
`BOOST_NO_CXX11_NUMERIC_LIMITS` | Standard library | C++11 additions to `std::numeric_limits` are not available for use. `static function numeric_limits<T>::lowest()` the lowest finite value representable by the numeric type. `static int const max_digits10` the number of decimal digits that are required to make sure that two distinct values of the type have distinct decimal representations. `template<> class numeric_limits<char16_t>;`, see also `BOOST_NO_CXX11_CHAR16_T`, `template<> class numeric_limits<char32_t>;` see also `BOOST_NO_CXX11_CHAR32_T`. Replaces BOOST_NO_NUMERIC_LIMITS_LOWEST.
`BOOST_NO_LIMITS_COMPILE_TIME_CONSTANTS` | Standard library | Constants such as `numeric_limits<T>::is_signed` are not available for use at compile-time.
`BOOST_NO_LONG_LONG_NUMERIC_LIMITS` | Standard library | There is no specialization for `numeric_limits<long long>` and `numeric_limits<unsigned long long>`. `<boost/limits.hpp>` will then add these specializations as a standard library "fix" only if the compiler supports the `long long` datatype.
`BOOST_NO_MEMBER_FUNCTION_SPECIALIZATIONS` | Compiler | The compiler does not support the specialization of individual member functions of template classes.
`BOOST_NO_MEMBER_TEMPLATE_KEYWORD` | Compiler | If the compiler supports member templates, but not the template keyword when accessing member template classes.
`BOOST_NO_MEMBER_TEMPLATE_FRIENDS` | Compiler | Member template friend syntax (`template<class P> friend class frd;`) described in the C++ Standard, 14.5.3, not supported.
`BOOST_NO_MEMBER_TEMPLATES` | Compiler | Member template functions not fully supported.
`BOOST_NO_MS_INT64_NUMERIC_LIMITS` | Standard library | There is no specialization for `numeric_limits<__int64>` and `numeric_limits<unsigned __int64>`. `<boost/limits.hpp>` will then add these specializations as a standard library "fix", only if the compiler supports the `__int64` datatype.
`BOOST_NO_NESTED_FRIENDSHIP` | Compiler | Compiler doesn't allow a nested class to access private members of its containing class. Probably Borland/CodeGear specific.
`BOOST_NO_OPERATORS_IN_NAMESPACE` | Compiler | Compiler requires inherited operator friend functions to be defined at namespace scope, then using'ed to boost. Probably GCC specific. See `<boost/operators.hpp>` for example.
`BOOST_NO_PARTIAL_SPECIALIZATION_IMPLICIT_DEFAULT_ARGS` | Compiler | The compiler does not correctly handle partial specializations which depend upon default arguments in the primary template.
`BOOST_NO_POINTER_TO_MEMBER_CONST` | Compiler | The compiler does not correctly handle pointers to const member functions, preventing use of these in overloaded function templates. See `<boost/functional.hpp>` for example.
`BOOST_NO_POINTER_TO_MEMBER_TEMPLATE_PARAMETERS` | Compiler | Pointers to members don't work when used as template parameters.
`BOOST_NO_PRIVATE_IN_AGGREGATE` | Compiler | The compiler misreads 8.5.1, treating classes as non-aggregate if they contain private or protected member functions.
`BOOST_NO_RESTRICT_REFERENCES` | Compiler | Compiler-specific `restrict` keyword can not be applied to references.
`BOOST_NO_RTTI` | Compiler | The compiler may (or may not) have the typeid operator, but RTTI on the dynamic type of an object is not supported.
`BOOST_NO_SFINAE` | Compiler | The compiler does not support the "Substitution Failure Is Not An Error" meta-programming idiom. This is the lightweight pre-C++11 version of SFINAE.
`BOOST_NO_SFINAE_EXPR` | Compiler | The compiler does not support usage of SFINAE with arbitrary expressions. This is the post-C++11 SFINAE, but excludes a few specific corner cases, see also BOOST_NO_CXX11_SFINAE_EXPR.
`BOOST_NO_STD_ALLOCATOR` | Standard library | The C++ standard library does not provide a standards conforming `std::allocator`.
`BOOST_NO_STD_DISTANCE` | Standard library | The platform does not have a conforming version of `std::distance`.
`BOOST_NO_STD_ITERATOR` | Standard library | The C++ implementation fails to provide the `std::iterator` class. Note that post C++17, this macro is re-purposed to indicate that std::iterator has been removed or deprecated.
`BOOST_NO_STD_ITERATOR_TRAITS` | Standard library | The compiler does not provide a standard compliant implementation of `std::iterator_traits`. Note that the compiler may still have a non-standard implementation.
`BOOST_NO_STD_LOCALE` | Standard library | The standard library lacks `std::locale`.
`BOOST_NO_STD_MESSAGES` | Standard library | The standard library lacks a conforming `std::messages` facet.
`BOOST_NO_STD_MIN_MAX` | Standard library | The C++ standard library does not provide the `min()` and `max()` template functions that should be in `<algorithm>`.
`BOOST_NO_STD_OUTPUT_ITERATOR_ASSIGN` | Standard library | Defined if the standard library's output iterators are not assignable.
`BOOST_NO_STD_TYPEINFO` | Standard library | The <typeinfo> header declares `type_info` in the global namespace instead of namespace std.
`BOOST_NO_STD_USE_FACET` | Standard library | The standard library lacks a conforming `std::use_facet`.
`BOOST_NO_STD_WSTREAMBUF` | Standard library | The standard library's implementation of `std::basic_streambuf<wchar_t>` is either missing, incomplete, or buggy.
`BOOST_NO_STD_WSTRING` | Standard library | The standard library lacks `std::wstring`.
`BOOST_NO_STDC_NAMESPACE` | Compiler, Platform | The contents of C++ standard headers for C library functions (the `<c...>` headers) have not been placed in namespace std. This test is difficult - some libraries "fake" the std C functions by adding using declarations to import them into namespace std, unfortunately they don't necessarily catch all of them...
`BOOST_NO_STRINGSTREAM` | Standard library | The C++ implementation does not provide the `<sstream>` header.
`BOOST_NO_SWPRINTF` | Platform | The platform does not have a conforming version of `swprintf`.
`BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION` | Compiler | Class template partial specialization (14.5.4 [temp.class.spec]) not supported.
`BOOST_NO_TEMPLATED_IOSTREAMS` | Standard library | The standard library does not provide templated iostream classes.
`BOOST_NO_TEMPLATED_ITERATOR_CONSTRUCTORS` | Standard library | The standard library does not provide templated iterator constructors for its containers.
`BOOST_NO_TEMPLATE_TEMPLATES` | Compiler | The compiler does not support template template parameters.
`BOOST_NO_TYPEID` | Compiler | The compiler does not support the typeid operator at all.
`BOOST_NO_TYPENAME_WITH_CTOR` | Compiler | The typename keyword cannot be used when creating a temporary of a Dependent type.
`BOOST_NO_UNREACHABLE_RETURN_DETECTION` | Compiler | If a return is unreachable, then no return statement should be required, however some compilers insist on it, while other issue a bunch of warnings if it is in fact present.
`BOOST_NO_USING_DECLARATION_OVERLOADS_FROM_TYPENAME_BASE` | Compiler | The compiler will not accept a using declaration that brings a function from a typename used as a base class into a derived class if functions of the same name are present in the derived class.
`BOOST_NO_USING_TEMPLATE` | Compiler | The compiler will not accept a using declaration that imports a template class or function from another namespace. Originally a Borland specific problem with imports to/from the global namespace, extended to MSVC6 which has a specific issue with importing template classes (but not functions).
`BOOST_NO_VOID_RETURNS` | Compiler | The compiler does not allow a void function to return the result of calling another void function.
    void f() {}
    void g() { return f(); }

### Macros that describe optional features

The following macros describe features that are not required by the C++ standard. The macro is only defined if the feature is present.

Macro | Section | Description
--- | --- | ---
`BOOST_HAS_BETHREADS` | Platform | The platform supports BeOS style threads.
`BOOST_HAS_CLOCK_GETTIME` | Platform | The platform has the POSIX API `clock_gettime`.
`BOOST_HAS_DIRENT_H` | Platform | The platform has the POSIX header `<dirent.h>`.
`BOOST_HAS_EXPM1` | Platform | The platform has the functions `expm1`, `expm1f` and `expm1l` in `<math.h>`
`BOOST_HAS_FLOAT128` | Compiler | The compiler has `__float128` as a native type which is distinct from all the regular C++ floating point types.
`BOOST_HAS_FTIME` | Platform | The platform has the Win32 API type FTIME.
`BOOST_HAS_GETSYSTEMTIMEASFILETIME` | Platform | The platform has the Win32 API GetSystemTimeAsFileTime.
`BOOST_HAS_GETTIMEOFDAY` | Platform | The platform has the POSIX API `gettimeofday`.
`BOOST_HAS_HASH` | Standard library | The C++ implementation provides the (SGI) hash_set and hash_map classes. When defined, `BOOST_HASH_SET_HEADER` and `BOOST_HASH_MAP_HEADER` will contain the names of the header needed to access hash_set and hash_map; `BOOST_STD_EXTENSION_NAMESPACE` will provide the namespace in which the two class templates reside.
`BOOST_HAS_INT128` | Compiler | The compiler has `__int128` and `unsigned __int128` as native types which are distinct from all the regular C++ integer types.
`BOOST_HAS_LOG1P` | Platform | The platform has the functions `log1p`, `log1pf` and `log1pl` in `<math.h>`.
`BOOST_HAS_MACRO_USE_FACET` | Standard library | The standard library lacks a conforming `std::use_facet`, but has a macro `_USE(loc, Type)` that does the job. This is primarily for the Dinkumware std lib.
`BOOST_HAS_MS_INT64` | Compiler | The compiler supports the `__int64` data type.
`BOOST_HAS_NANOSLEEP` | Platform | The platform has the POSIX API nanosleep.
`BOOST_HAS_NL_TYPES_H` | Platform | The platform has an `<nl_types.h>`.
`BOOST_HAS_NRVO` | Compiler | Indicated that the compiler supports the named return value optimization (NRVO). Used to select the most efficient implementation for some function. See `<boost/operators.hpp>` for example.
`BOOST_HAS_PARTIAL_STD_ALLOCATOR` | Standard Library | The standard library has a partially conforming `std::allocator` class, but without any of the member templates.
`BOOST_HAS_PRAGMA_ONCE` | Compiler | The compiler recognizes the `#pragma once` directive which tells that the containing header should be included only once while preprocessing the current translation unit. The pragma may improve compile times of large projects with some compilers.
`BOOST_HAS_PRAGMA_DETECT_MISMATCH` | Compiler | The compiler recognizes the `#pragma detect_mismatch("name", "value")` directive which tells that the link stage should be terminated with error if values for provided `"name"` missmatch. This pragma may be a help in preventing ODR violations and ensuring that different modules are compiled with same flags.
`BOOST_HAS_PTHREAD_DELAY_NP` | Platform | The platform has the POSIX API `pthread_delay_np`.
`BOOST_HAS_PTHREAD_MUTEXATTR_SETTYPE` | Platform | The platform has the POSIX API `pthread_mutexattr_settype`.
`BOOST_HAS_PTHREAD_YIELD` | Platform | The platform has the POSIX API `pthread_yield`.
`BOOST_HAS_PTHREADS` | Platform | The platform support POSIX style threads.
`BOOST_HAS_SCHED_YIELD` | Platform | The platform has the POSIX API `sched_yield`.
`BOOST_HAS_SGI_TYPE_TRAITS` | Compiler, Standard library | The compiler has native support for SGI style type traits.
`BOOST_HAS_STDINT_H` | Platform | The platform has a `<stdint.h>`
`BOOST_HAS_SLIST` | Standard library | The C++ implementation provides the (SGI) slist class. When defined, `BOOST_SLIST_HEADER` will contain the name of the header needed to access `slist` and `BOOST_STD_EXTENSION_NAMESPACE` will provide the namespace in which `slist` resides.
`BOOST_HAS_STLP_USE_FACET` | Standard library | The standard library lacks a conforming `std::use_facet`, but has a workaround class-version that does the job. This is primarily for the STLport std lib.
`BOOST_HAS_TR1_ARRAY` | Standard library | The library has a TR1 conforming version of `<array>`. This macro is only guaranteed to be defined after including one of the headers from Boost.TR1. Further this macro is now deprecated in favour of BOOST_NO_CXX11_HDR_ARRAY.
`BOOST_HAS_TR1_COMPLEX_OVERLOADS` | Standard library | The library has a version of `<complex>` that supports passing scalars to the complex number algorithms.
`BOOST_HAS_TR1_COMPLEX_INVERSE_TRIG` | Standard library | The library has a version of `<complex>` that includes the new inverse trig functions from TR1.
`BOOST_HAS_TR1_REFERENCE_WRAPPER` | Standard library | The library has TR1 conforming reference wrappers in `<functional>`. This macro is only guaranteed to be defined after including one of the headers from Boost.TR1. Further this macro is now deprecated in favour of BOOST_NO_CXX11_HDR_FUNCTIONAL.
`BOOST_HAS_TR1_RESULT_OF` | Standard library | The library has a TR1 conforming result_of template in `<functional>`. This macro is only guaranteed to be defined after including one of the headers from Boost.TR1. Further this macro is now deprecated in favour of BOOST_NO_CXX11_HDR_FUNCTIONAL.
`BOOST_HAS_TR1_MEM_FN` | Standard library | The library has a TR1 conforming mem_fn function template in `<functional>`. This macro is only guaranteed to be defined after including one of the headers from Boost.TR1. Further this macro is now deprecated in favour of BOOST_NO_CXX11_HDR_FUNCTIONAL.
`BOOST_HAS_TR1_BIND` | Standard library | The library has a TR1 conforming bind function template in `<functional>`. This macro is only guaranteed to be defined after including one of the headers from Boost.TR1. Further this macro is now deprecated in favour of BOOST_NO_CXX11_HDR_FUNCTIONAL.
`BOOST_HAS_TR1_FUNCTION` | Standard library | The library has a TR1 conforming function class template in `<functional>`. This macro is only guaranteed to be defined after including one of the headers from Boost.TR1. Further this macro is now deprecated in favour of BOOST_NO_CXX11_HDR_FUNCTIONAL.
`BOOST_HAS_TR1_HASH` | Standard library | The library has a TR1 conforming hash function template in `<functional>`. This macro is only guaranteed to be defined after including one of the headers from Boost.TR1. Further this macro is now deprecated in favour of BOOST_NO_CXX11_HDR_FUNCTIONAL.
`BOOST_HAS_TR1_SHARED_PTR` | Standard library | The library has a TR1 conforming `shared_ptr` class template in `<memory>`. This macro is only guaranteed to be defined after including one of the headers from Boost.TR1. Further this macro is now deprecated in favour of BOOST_NO_CXX11_SMART_PTR.
`BOOST_HAS_TR1_RANDOM` | Standard library | The library has a TR1 conforming version of `<random>`. This macro is only guaranteed to be defined after including one of the headers from Boost.TR1. Further this macro is now deprecated in favour of BOOST_NO_CXX11_HDR_RANDOM.
`BOOST_HAS_TR1_REGEX` | Standard library | The library has a TR1 conforming version of `<regex>`. This macro is only guaranteed to be defined after including one of the headers from Boost.TR1. Further this macro is now deprecated in favour of BOOST_NO_CXX11_HDR_REGEX.
`BOOST_HAS_TR1_TUPLE` | Standard library | The library has a TR1 conforming version of `<tuple>`. This macro is only guaranteed to be defined after including one of the headers from Boost.TR1. Further this macro is now deprecated in favour of BOOST_NO_CXX11_HDR_TUPLE.
`BOOST_HAS_TR1_TYPE_TRAITS` | Standard library | The library has a TR1 conforming version of `<type_traits>`. This macro is only guaranteed to be defined after including one of the headers from Boost.TR1. Further this macro is now deprecated in favour of BOOST_NO_CXX11_HDR_TYPE_TRAITS.
`BOOST_HAS_TR1_UTILITY` | Standard library | The library has the TR1 additions to `<utility>` (tuple interface to `std::pair`). This macro is only guaranteed to be defined after including one of the headers from Boost.TR1. Further this macro is now deprecated in favour of BOOST_NO_CXX11_HDR_TUPLE.
`BOOST_HAS_TR1_UNORDERED_MAP` | Standard library | The library has a TR1 conforming version of `<unordered_map>`. This macro is only guaranteed to be defined after including one of the headers from Boost.TR1. Further this macro is now deprecated in favour of BOOST_NO_CXX11_HDR_UNORDERED_MAP.
`BOOST_HAS_TR1_UNORDERED_SET` | Standard library | The library has a TR1 conforming version of `<unordered_set>`. This macro is only guaranteed to be defined after including one of the headers from Boost.TR1. Further this macro is now deprecated in favour of BOOST_NO_CXX11_HDR_UNORDERED_SET.
`BOOST_HAS_TR1` | Standard library | Implies all the other `BOOST_HAS_TR1_*` macros should be set.
`BOOST_HAS_THREADS` | Platform, Compiler | Defined if the compiler, in its current translation mode, supports multiple threads of execution.
`BOOST_HAS_TWO_ARG_USE_FACET` | Standard library | The standard library lacks a conforming std::use_facet, but has a two argument version that does the job. This is primarily for the Rogue Wave std lib.
`BOOST_HAS_UNISTD_H` | Platform | The Platform provides `<unistd.h>`.
`BOOST_HAS_WINTHREADS` | Platform | The platform supports MS Windows style threads.
`BOOST_MSVC_STD_ITERATOR` | Standard library | Microsoft's broken version of `std::iterator` is being used. This implies that `std::iterator` takes no more than two template parameters.
`BOOST_MSVC6_MEMBER_TEMPLATES` | Compiler | Microsoft Visual C++ 6.0 has enough member template idiosyncrasies (being polite) that `BOOST_NO_MEMBER_TEMPLATES` is defined for this compiler. `BOOST_MSVC6_MEMBER_TEMPLATES` is defined to allow compiler specific workarounds. This macro gets defined automatically if `BOOST_NO_MEMBER_TEMPLATES` is not defined - in other words this is treated as a strict subset of the features required by the standard.
`BOOST_HAS_STDINT_H` | Platform | There are no 1998 C++ Standard headers `<stdint.h>` or `<cstdint>`, although the 1999 C Standard does include `<stdint.h>`. If `<stdint.h>` is present, `<boost/stdint.h>` can make good use of it, so a flag is supplied (signalling presence; thus the default is not present, conforming to the current C++ standard).

### Macros that describe C++11 features not supported

The following macros describe features in the 2011 ISO C++ standard, formerly known as C++0x, that are not yet supported by a particular compiler or library.

Macro | Description
--- | ---
`BOOST_NO_CXX11_ADDRESSOF` | The standard library header <memory> has no working std::addressof.
`BOOST_NO_CXX11_ALIGNAS` | The compiler does not support the `alignas` keyword.
`BOOST_NO_CXX11_ALIGNOF` | The compiler does not support the `alignof` keyword.
`BOOST_NO_CXX11_ALLOCATOR` | The standard library does not provide a C++11 version of `std::allocator` in <memory>.
`BOOST_NO_CXX11_ATOMIC_SMART_PTR` | The standard library <memory> does not support atomic smart pointer operations.
`BOOST_NO_CXX11_AUTO_DECLARATIONS` | The compiler does not support type deduction for variables declared with the `auto` keyword (`auto var = ...;`).
`BOOST_NO_CXX11_AUTO_MULTIDECLARATIONS` | The compiler does not support type deduction for multiple variables declared with the `auto` keyword (`auto var = ..., *ptr = ...;`).
`BOOST_NO_CXX11_CHAR16_T` | The compiler does not support type `char16_t`.
`BOOST_NO_CXX11_CHAR32_T` | The compiler does not support type `char32_t`.
`BOOST_NO_CXX11_CONSTEXPR` | The compiler does not support `constexpr`.
`BOOST_NO_CXX11_DECLTYPE` | The compiler does not support `decltype`.
`BOOST_NO_CXX11_DECLTYPE_N3276` | The compiler does not support the extension to `decltype` described in N3276, accepted in Madrid, March 2011.
`BOOST_NO_CXX11_DELETED_FUNCTIONS` | The compiler does not support deleted (`= delete`) functions.
`BOOST_NO_CXX11_DEFAULTED_FUNCTIONS` | The compiler does not support defaulted (`= default`) functions.
`BOOST_NO_CXX11_DEFAULTED_MOVES` | The compiler does not support defaulted move constructor or assignment. Other defaulted functions may still be supported.
`BOOST_NO_CXX11_EXPLICIT_CONVERSION_OPERATORS` | The compiler does not support explicit conversion operators (`explicit operator T()`).
`BOOST_NO_CXX11_EXTERN_TEMPLATE` | The compiler does not support explicit instantiation forward declarations for templates (`extern template ...`).
`BOOST_NO_CXX11_FINAL` | The compiler does not support the C++ class-virt-specifier final.
`BOOST_NO_CXX11_FIXED_LENGTH_VARIADIC_TEMPLATE_EXPANSION_PACKS` | The compiler does not support expanding a variadic template parameter pack into a template containing one or more fixed arguments
`BOOST_NO_CXX11_FUNCTION_TEMPLATE_DEFAULT_ARGS` | The compiler does not support default template arguments for function templates.
`BOOST_NO_CXX11_HDR_ATOMIC` | The standard library does not provide header <atomic>.
`BOOST_NO_CXX11_HDR_ARRAY` | The standard library does not provide header <array>.
`BOOST_NO_CXX11_HDR_CHRONO` | The standard library does not provide header <chrono>.
`BOOST_NO_CXX11_HDR_CODECVT` | The standard library does not provide header <codecvt>. Note that this header is deprecated post C++17, and therefore the macro may be set as a result of the feature being deliberately removed.
`BOOST_NO_CXX11_HDR_CONDITION_VARIABLE` | The standard library does not provide header <condition_variable>.
`BOOST_NO_CXX11_HDR_EXCEPTION` | The standard library does not provide a C++11 compatible version of <exception>.
`BOOST_NO_CXX11_HDR_FORWARD_LIST` | The standard library does not provide header <forward_list>.
`BOOST_NO_CXX11_HDR_FUNCTIONAL` | The standard library does not provide a C++11 compatible version of <functional>.
`BOOST_NO_CXX11_HDR_FUTURE` | The standard library does not provide header <future>.
`BOOST_NO_CXX11_HDR_INITIALIZER_LIST` | The standard library does not provide header <initializer_list>.
`BOOST_NO_CXX11_HDR_MUTEX` | The standard library does not provide header <mutex>.
`BOOST_NO_CXX11_HDR_RANDOM` | The standard library does not provide header <random>.
`BOOST_NO_CXX11_HDR_RATIO` | The standard library does not provide header <ratio>.
`BOOST_NO_CXX11_HDR_REGEX` | The standard library does not provide header <regex>.
`BOOST_NO_CXX11_HDR_SYSTEM_ERROR` | The standard library does not provide header <system_error>.
`BOOST_NO_CXX11_HDR_THREAD` | The standard library does not provide header <thread>.
`BOOST_NO_CXX11_HDR_TUPLE` | The standard library does not provide header <tuple>.
`BOOST_NO_CXX11_HDR_TYPEINDEX` | The standard library does not provide header <typeindex>.
`BOOST_NO_CXX11_HDR_TYPE_TRAITS` | The standard library does not provide header <type_traits>.
`BOOST_NO_CXX11_HDR_UNORDERED_MAP` | The standard library does not provide header <unordered_map>.
`BOOST_NO_CXX11_HDR_UNORDERED_SET` | The standard library does not provide header <unordered_set>.
`BOOST_NO_CXX11_INLINE_NAMESPACES` | The compiler does not support inline namespaces.
`BOOST_NO_CXX11_LAMBDAS` | The compiler does not support Lambdas.
`BOOST_NO_CXX11_LOCAL_CLASS_TEMPLATE_PARAMETERS` | The compiler does not allow to pass local classes as template parameters (this macro intentionally does not control passing of unnamed types as template parameters, see also N2657).
`BOOST_NO_CXX11_NON_PUBLIC_DEFAULTED_FUNCTIONS` | The compiler does not support defaulted (`= default`) functions in access control sections other than `public`. Public defaulted functions may still be supported, as indicated by `BOOST_NO_CXX11_DEFAULTED_FUNCTIONS`. Some compilers implementing an early draft of the C++11 standard (in particular, incorporating DR906) are susceptible to this problem.
`BOOST_NO_CXX11_NOEXCEPT` | The compiler does not support `noexcept`.
`BOOST_NO_CXX11_NULLPTR` | The compiler does not support `nullptr`.
`BOOST_NO_CXX11_NUMERIC_LIMITS` | The standard library `<limits>` header does not support the C++11 version of `numeric_limits`.
`BOOST_NO_CXX11_OVERRIDE` | The compiler does not support `override`.
`BOOST_NO_CXX11_POINTER_TRAITS` | The standard library does not provide a C++11 version of `std::pointer_traits` in <memory>.
`BOOST_NO_CXX11_RANGE_BASED_FOR` | The compiler does not support range-based for statements.
`BOOST_NO_CXX11_RAW_LITERALS` | The compiler does not support raw string literals.
`BOOST_NO_CXX11_REF_QUALIFIERS` | The compiler does not support ref-qualifiers on member functions as described in N2439.
`BOOST_NO_CXX11_RVALUE_REFERENCES` | The compiler does not support r-value references.
`BOOST_NO_CXX11_SCOPED_ENUMS` | The compiler does not support scoped enumerations (`enum class`).
`BOOST_NO_CXX11_SFINAE_EXPR` | The compiler does not support usage of C++11 SFINAE with arbitrary expressions. Use this macro only if you are using all of the features of SFINAE including substitution-failure-on-private-member-access. Otherwise use BOOST_NO_SFINAE_EXPR or BOOST_NO_SFINAE which get defined for fewer compilers.
`BOOST_NO_CXX11_SMART_PTR` | The standard library header <memory> has no shared_ptr and unique_ptr.
`BOOST_NO_CXX11_STATIC_ASSERT` | The compiler does not support `static_assert`.
`BOOST_NO_CXX11_STD_ALIGN` | The standard library header <memory> has no working std::align.
`BOOST_NO_CXX11_STD_UNORDERED` | The standard library does not support <unordered_map> and <unordered_set>.
`BOOST_NO_CXX11_TEMPLATE_ALIASES` | The compiler does not support template aliases.
`BOOST_NO_CXX11_THREAD_LOCAL` | The compiler does not support the `thread_local` storage specifier.
`BOOST_NO_CXX11_TRAILING_RESULT_TYPES` | The compiler does not support the new function result type specification syntax (e.g. `auto foo(T) -> T;`).
`BOOST_NO_CXX11_UNICODE_LITERALS` | The compiler does not support Unicode (`u8`, `u`, `U`) literals.
`BOOST_NO_CXX11_UNIFIED_INITIALIZATION_SYNTAX` | The compiler does not support the C++11 Unified Initialization Syntax.
`BOOST_NO_CXX11_UNRESTRICTED_UNION` | The compiler does not support an unrestricted union. This is a union that may contain static data as well as user-defined member data with non-trivial special member functions.
`BOOST_NO_CXX11_USER_DEFINED_LITERALS` | The compiler does not support user defined literals.
`BOOST_NO_CXX11_VARIADIC_TEMPLATES` | The compiler does not support variadic templates.
`BOOST_NO_CXX11_VARIADIC_MACROS` | The compiler does not support variadic macros.
`BOOST_NO_LONG_LONG` | The compiler does not support `long long`.

### Macros that allow use of C++11 features with C++03 compilers

The following macros allow use of C++11 features even with compilers that do not yet provide compliant C++11 support.

Macro | Description
--- | ---
`BOOST_ALIGNMENT(X)`, `BOOST_NO_ALIGNMENT` | Some compilers don't support the `alignas` keyword but provide other means to specify alignment (usually, through compiler-specific attributes). The macro `BOOST_ALIGNMENT(X)` will expand to the `alignas(X)` keyword if the compiler supports it or to some compiler-specific attribute to achieve the specified alignment. If no such compiler-specific attribute is known then `BOOST_ALIGNMENT(X)` will expand to nothing and `BOOST_NO_ALIGNMENT` will be defined. Unlike native `alignas`, `X` must always be a compile-time integer constant. The macro can be used to specify alignment of types and data:
    struct BOOST_ALIGNMENT(16) my_data
    {
        char c[16];
    };
    BOOST_ALIGNMENT(8) int arr[32];
`BOOST_CONSTEXPR` | Some compilers don't support the use of `constexpr`. This macro expands to nothing on those compilers, and `constexpr` elsewhere. For example, when defining a constexpr function or constructor replace:
    constexpr tuple();
with:
    BOOST_CONSTEXPR tuple();
`BOOST_CONSTEXPR_OR_CONST` | Some compilers don't support the use of `constexpr`. This macro expands to `const` on those compilers, and `constexpr` elsewhere. For example, when defining const expr variables replace:
    static constexpr UIntType xor_mask = a;
with:
    static BOOST_CONSTEXPR_OR_CONST UIntType xor_mask = a;
`BOOST_OVERRIDE` | If `BOOST_NO_CXX11_OVERRIDE` is not defined (i.e. C++11 compliant compilers), expands to `override` keyword, otherwise expands to nothing.
`BOOST_STATIC_CONSTEXPR` | This is a shortcut for `static BOOST_CONSTEXPR_OR_CONST`. For example, when defining const expr variables replace:
    static constexpr UIntType xor_mask = a;
with:
    BOOST_STATIC_CONSTEXPR UIntType xor_mask = a;
`BOOST_DEFAULTED_FUNCTION(fun, body)` | This macro is intended to be used within a class definition in order to declare a default implementation of function `fun`. For the compilers that do not support C++11 defaulted functions the macro will expand into an inline function definition with the `body` implementation. For example:
    struct my_struct
    {
        BOOST_DEFAULTED_FUNCTION(my_struct(), {})
    };
is equivalent to:
    struct my_struct
    {
        my_struct() = default;
    };
or:
    struct my_struct
    {
        my_struct() {}
    };
`BOOST_DELETED_FUNCTION(fun)` | This macro is intended to be used within a class definition in order to declare a deleted function `fun`. For the compilers that do not support C++11 deleted functions the macro will expand into a private function declaration with no definition. Since the macro may change the access mode, it is recommended to use this macro at the end of the class definition. For example:
    struct noncopyable
    {
        BOOST_DELETED_FUNCTION(noncopyable(noncopyable const&))
        BOOST_DELETED_FUNCTION(noncopyable& operator= (noncopyable const&))
    };
is equivalent to:
    struct noncopyable
    {
        noncopyable(noncopyable const&) = delete;
        noncopyable& operator= (noncopyable const&) = delete;
    };
or:
    struct noncopyable
    {
    private:
        noncopyable(noncopyable const&);
        noncopyable& operator= (noncopyable const&);
    };
    BOOST_NOEXCEPT
    BOOST_NOEXCEPT_OR_NOTHROW
    BOOST_NOEXCEPT_IF(Predicate)
    BOOST_NOEXCEPT_EXPR(Expression)
> | If `BOOST_NO_CXX11_NOEXCEPT` is defined (i.e. C++03 compliant compilers) these macros are defined as:
>     #define BOOST_NOEXCEPT
>     #define BOOST_NOEXCEPT_OR_NOTHROW throw()
>     #define BOOST_NOEXCEPT_IF(Predicate)
>     #define BOOST_NOEXCEPT_EXPR(Expression) false
> If `BOOST_NO_CXX11_NOEXCEPT` is not defined (i.e. C++11 compliant compilers) they are defined as:
>     #define BOOST_NOEXCEPT noexcept
>     #define BOOST_NOEXCEPT_OR_NOTHROW noexcept
>     #define BOOST_NOEXCEPT_IF(Predicate) noexcept((Predicate))
>     #define BOOST_NOEXCEPT_EXPR(Expression) noexcept((Expression))
`BOOST_FINAL` | If `BOOST_NO_CXX11_FINAL` is not defined (i.e. C++11 compliant compilers), expands to `final` keyword, otherwise expands to nothing.
`BOOST_MSVC_ENABLE_2012_NOV_CTP` | For Microsoft Visual C++ 2012, enable the C++11 features supplied by the November 2012 Community Technology Preview. These features are not automatically enabled because the CTP is non-supported alpha code that is not recommended for production use. This macro must be defined before including any Boost headers, and must be defined for all translation units in the program, including Boost library builds. This macro will no longer have any effect once an official Microsoft release supports the CTP features.
`BOOST_NULLPTR` | If `BOOST_NO_CXX11_NULLPTR` is not defined (i.e. C++11 compliant compilers), expands to `nullptr`, otherwise expands to `0`.

### Macros that describe C++14 features not supported

The following macros describe features in the 2014 ISO C++ standard, formerly known as C++0y, that are not yet supported by a particular compiler or library.

Macro | Description
--- | ---
> `BOOST_NO_CXX14_AGGREGATE_NSDMI` | The compiler does not support member initializer for aggregates as in the following example:
>     struct Foo
>     {
>       int x, y = 42;
>     };

>     Foo foo = { 0 };
`BOOST_NO_CXX14_BINARY_LITERALS` | The compiler does not binary literals (e.g. `0b1010`).
`BOOST_NO_CXX14_CONSTEXPR` | The compiler does not support relaxed `constexpr`.
`BOOST_NO_CXX14_DECLTYPE_AUTO` | The compiler does not support `decltype(auto)`.
`BOOST_NO_CXX14_DIGIT_SEPARATORS` | The compiler does not support digit separators (e.g. `1'000'000`).
`BOOST_NO_CXX14_STD_EXCHANGE` | The compiler does not support `std::exchange()`.
`BOOST_NO_CXX14_GENERIC_LAMBDAS` | The compiler does not support generic lambda (e.g. `[](auto v){ }`).
`BOOST_NO_CXX14_HDR_SHARED_MUTEX` | The standard library does not provide header <shared_mutex>.
`BOOST_NO_CXX14_INITIALIZED_LAMBDA_CAPTURES` | The compiler does not support initialized lambda capture (e.g. `[foo = 42]{ }`).
`BOOST_NO_CXX14_RETURN_TYPE_DEDUCTION` | The compiler does not support return type deduction for normal functions (e.g. `auto f() { return val; }`).
`BOOST_NO_CXX14_VARIABLE_TEMPLATES` | The compiler does not support variable template (e.g. `template <class T> T kibi = T(1024);`).

### Macros that allow use of C++14 features with C++11 or earlier compilers

The following macros allow use of C++14 features even with compilers that do not yet provide compliant C++14 support.

Macro | Description
--- | ---
`BOOST_CXX14_CONSTEXPR` | This macro works similar to `BOOST_CONSTEXPR`, but expands to `constexpr` only if the C++14 "relaxed" `constexpr` is available.

### Macros that describe C++17 features not supported

The following macros describe features in the 2017 ISO C++ standard, formerly known as C++1z, that are not yet supported by a particular compiler or library.

Macro | Description
--- | ---
`BOOST_NO_CXX17_HDR_ANY` | The compiler does not support the header `<any>`.
`BOOST_NO_CXX17_HDR_CHARCONV` | The compiler does not support the header `<charconv>`.
`BOOST_NO_CXX17_HDR_EXECUTION` | The compiler does not support the header `<execution>`.
`BOOST_NO_CXX17_HDR_FILESYSTEM` | The compiler does not support the header `<filesystem>`.
`BOOST_NO_CXX17_HDR_MEMORY_RESOURCE` | The compiler does not support the header `<memory_resource>`.
`BOOST_NO_CXX17_HDR_OPTIONAL` | The compiler does not support the header `<optional>`.
`BOOST_NO_CXX17_HDR_STRING_VIEW` | The compiler does not support the header `<string_view>`.
`BOOST_NO_CXX17_HDR_VARIANT` | The compiler does not support the header `<variant>`.
`BOOST_NO_CXX17_STD_APPLY` | The compiler does not support `std::apply()`.
`BOOST_NO_CXX17_STD_INVOKE` | The compiler does not support `std::invoke()`.
`BOOST_NO_CXX17_ITERATOR_TRAITS` | The compiler does not support SFINAE-friendly `std::iterator_traits`.
`BOOST_NO_CXX17_IF_CONSTEXPR` | The compiler does not support `if constexpr`.
`BOOST_NO_CXX17_INLINE_VARIABLES` | The compiler does not support `inline` variables.
`BOOST_NO_CXX17_DEDUCTION_GUIDES` | The compiler does not support class template argument deduction (CTAD) guides.

### Macros that allow use of C++17 features with C++14 or earlier compilers

The following macros allow use of C++17 features even with compilers that do not yet provide compliant C++17 support.

Macro | Description
--- | ---
`BOOST_INLINE_VARIABLE` | This macro expands to `inline` on compilers that support C++17 inline variables and to nothing otherwise. Users may need to check for `BOOST_NO_CXX17_INLINE_VARIABLES` for further adjustments to the code.
`BOOST_IF_CONSTEXPR` | Expands to `if constexpr` when supported, or `if` otherwise.
`BOOST_INLINE_CONSTEXPR` | This is a shortcut for `BOOST_INLINE_VARIABLE BOOST_CONSTEXPR_OR_CONST`.

### Macros that describe C++20 features not supported

The following macros describe features in the 2020 ISO C++ standard, formerly known as C++2a, that are not yet supported by a particular compiler or library.

Macro | Description
--- | ---
`BOOST_NO_CXX20_HDR_BARRIER` | The compiler does not support the header `<barrier>`.
`BOOST_NO_CXX20_HDR_FORMAT` | The compiler does not support the header `<format>`.
`BOOST_NO_CXX20_HDR_SOURCE_LOCATION` | The compiler does not support the header `<source_location>`.
`BOOST_NO_CXX20_HDR_BIT` | The compiler does not support the header `<bit>`.
`BOOST_NO_CXX20_HDR_LATCH` | The compiler does not support the header `<latch>`.
`BOOST_NO_CXX20_HDR_SPAN` | The compiler does not support the header `<span>`.
`BOOST_NO_CXX20_HDR_COMPARE` | The compiler does not support the header `<compare>`.
`BOOST_NO_CXX20_HDR_NUMBERS` | The compiler does not support the header `<numbers>`.
`BOOST_NO_CXX20_HDR_STOP_TOKEN` | The compiler does not support the header `<stop_token>`.
`BOOST_NO_CXX20_HDR_CONCEPTS` | The compiler does not support the header `<concepts>`.
`BOOST_NO_CXX20_HDR_RANGES` | The compiler does not support the header `<ranges>`.
`BOOST_NO_CXX20_HDR_SYNCSTREAM` | The compiler does not support the header `<syncstream>`.
`BOOST_NO_CXX20_HDR_COROUTINE` | The compiler does not support the header `<coroutine>`.
`BOOST_NO_CXX20_HDR_SEMAPHORE` | The compiler does not support the header `<semaphore>`.
`BOOST_NO_CXX20_HDR_VERSION` | The compiler does not support the header `<version>`.

### Macros that describe features that have been removed from the standard.

The following macros describe features which were required by one version of the standard, but have been removed by later versions.

Macro | Description
--- | ---
`BOOST_NO_CXX98_RANDOM_SHUFFLE` | The standard library no longer supports `std::random_shuffle()`. It was deprecated in C++11 and is removed from C++14.
`BOOST_NO_AUTO_PTR` | The standard library no longer supports `std::auto_ptr`. It was deprecated in C++11 and is removed from C++14.
`BOOST_NO_CXX98_FUNCTION_BASE` | The standard library no longer supports `std::unary_function` and `std::binary_function`. They were deprecated in C++11 and is removed from C++14.
`BOOST_NO_CXX98_BINDERS` | The standard library no longer supports `std::bind1st`, `std::bind2nd`, `std::ptr_fun` and `std::mem_fun`. They were deprecated in C++11 and is removed from C++14.
`BOOST_NO_CXX11_ATOMIC_SMART_PTR` | The standard library no longer supports atomic overloads for `std::shared_ptr`: use `std::atomic<shared_ptr<T>>` instead.
`BOOST_NO_CXX11_HDR_CODECVT` | The standard library no longer supports `<codecvt>`, there is as yet no replacement.

### Boost Helper Macros

The following macros are either simple helpers, or macros that provide workarounds for compiler/standard library defects.

Macro | Description
--- | ---
`BOOST_WORKAROUND` | This macro is used where a compiler specific workaround is required that is not otherwise described by one of the other Boost.Config macros. To use the macro you must first
    #include <boost/config/workaround.hpp>
usage is then:
    #if BOOST_WORKAROUND(MACRONAME, CONDITION)
       // workaround code goes here...
    #else
       // Standard conforming code goes here...
    #endif
where `MACRONAME` is a macro that usually describes the version number to be tested against, and `CONDITION` is a comparison operator followed by a value. For example `BOOST_WORKAROUND(BOOST_INTEL, <= 1010)` would evaluate to `1` for Intel C++ 10.1 and earlier. The macro can also be used with `BOOST_TESTED_AT` if all current compiler versions exhibit the issue, but the issue is expected to be fixed at some later point. For example `BOOST_WORKAROUND(__BORLANDC__, BOOST_TESTED_AT(0x590))` would normally evaluate to `1` for all values of `__BORLANDC__` unless the macro `BOOST_DETECT_OUTDATED_WORKAROUNDS` is defined, in which case evaluates to `(__BORLANDC__ <= 0x590)`. Note: the ultimate source of documentation for this macro is in boost/config/workaround.hpp.
`BOOST_PREVENT_MACRO_SUBSTITUTION` | Sometimes you have a function name with the same name as a C macro, for example "min" and "max" member functions, in which case one can prevent the function being expanded as a macro using:
    someclass.min BOOST_PREVENT_MACRO_SUBSTITUTION(arg1, arg2);
The following also works in most, but not all, contexts:
    (someclass.max)(arg1, arg2);
`BOOST_DEDUCED_TYPENAME` | Some compilers don't support the use of typename for dependent types in deduced contexts. This macro expands to nothing on those compilers, and typename elsewhere. For example, replace: `template <class T> void f(T, typename T::type);` with: `template <class T> void f(T, BOOST_DEDUCED_TYPENAME T::type);`
`BOOST_HASH_MAP_HEADER` | The header to include to get the SGI `hash_map` class. This macro is only available if `BOOST_HAS_HASH` is defined.
`BOOST_HASH_SET_HEADER` | The header to include to get the SGI `hash_set` class. This macro is only available if `BOOST_HAS_HASH` is defined.
`BOOST_SLIST_HEADER` | The header to include to get the SGI `slist` class. This macro is only available if `BOOST_HAS_SLIST` is defined.
`BOOST_STD_EXTENSION_NAMESPACE` | The namespace used for std library extensions (hashtable classes etc).
`BOOST_STATIC_CONSTANT(Type, assignment)` | On compilers which don't allow in-class initialization of static integral constant members, we must use enums as a workaround if we want the constants to be available at compile-time. This macro gives us a convenient way to declare such constants. For example instead of:
    struct foo{
       static const int value = 2;
    };
use:
    struct foo{
       BOOST_STATIC_CONSTANT(int, value = 2);
    };
`BOOST_UNREACHABLE_RETURN(result)` | Normally evaluates to nothing, but evaluates to return x; if the compiler requires a return, even when it can never be reached.
`BOOST_FALLTHROUGH` | The BOOST_FALLTHROUGH macro can be used to annotate implicit fall-through between switch labels:
    switch (x) {
    case 40:
    case 41:
       if (truth_is_out_there) {
          ++x;
          BOOST_FALLTHROUGH;  // Use instead of/along with annotations in
          // comments.
       } else {
         return x;
       }
       case 42:
          ...
As shown in the example above, the BOOST_FALLTHROUGH macro should be followed by a semicolon. It is designed to mimic control-flow statements like 'break;', so it can be placed in most places where 'break;' can, but only if there are no statements on the execution path between it and the next switch label. When compiled with Clang >3.2 in C++11 mode, the BOOST_FALLTHROUGH macro is expanded to `[[clang::fallthrough]]` attribute, which is analysed when performing switch labels fall-through diagnostic ('-Wimplicit-fallthrough'). See clang documentation on language extensions for details. When used with unsupported compilers, the BOOST_FALLTHROUGH macro has no effect on diagnostics. In either case this macro has no effect on runtime behavior and performance of code.
`BOOST_EXPLICIT_TEMPLATE_TYPE(t)` `BOOST_EXPLICIT_TEMPLATE_NON_TYPE(t,v)` `BOOST_APPEND_EXPLICIT_TEMPLATE_TYPE(t)` `BOOST_APPEND_EXPLICIT_TEMPLATE_NON_TYPE(t,v)` | Some compilers silently "fold" different function template instantiations if some of the template parameters don't appear in the function parameter list. For instance:
    #include <iostream>
    #include <ostream>
    #include <typeinfo>

    template <int n>
    void f() { std::cout << n << ' '; }

    template <typename T>
    void g() { std::cout << typeid(T).name() << ' '; }

    int main() {
      f<1>();
      f<2>();

      g<int>();
      g<double>();
    }
incorrectly outputs `2 2 double double` on VC++ 6. These macros, to be used in the function parameter list, fix the problem without effects on the calling syntax. For instance, in the case above write:
    template <int n>
    void f(BOOST_EXPLICIT_TEMPLATE_NON_TYPE(int, n)) { ... }

    template <typename T>
    void g(BOOST_EXPLICIT_TEMPLATE_TYPE(T)) { ... }
Beware that they can declare (for affected compilers) a dummy defaulted parameter, so they a) should be always invoked at the end of the parameter list b) can't be used if your function template is multiply declared. Furthermore, in order to add any needed comma separator, an `APPEND_*` version must be used when the macro invocation appears after a normal parameter declaration or after the invocation of another macro of this same group.
`BOOST_USE_FACET(Type, loc)` | When the standard library does not have a conforming `std::use_facet` there are various workarounds available, but they differ from library to library. This macro provides a consistent way to access a locale's facets. For example, replace: `std::use_facet<Type>(loc);` with: `BOOST_USE_FACET(Type, loc);` Note do not add a `std::` prefix to the front of `BOOST_USE_FACET`.
`BOOST_HAS_FACET(Type, loc)` | When the standard library does not have a comforming `std::has_facet` there are various workarounds available, but they differ from library to library. This macro provides a consistent way to check a locale's facets. For example, replace: `std::has_facet<Type>(loc);` with: `BOOST_HAS_FACET(Type, loc);` Note do not add a `std::` prefix to the front of `BOOST_HAS_FACET`.
`BOOST_NESTED_TEMPLATE` | Member templates are supported by some compilers even though they can't use the `A::template member<U>` syntax, as a workaround replace: `typedef typename A::template rebind<U> binder;` with: `typedef typename A::BOOST_NESTED_TEMPLATE rebind<U> binder;`
`BOOST_STRINGIZE(X)` | Converts the parameter `X` to a string after macro replacement on `X` has been performed.
`BOOST_JOIN(X,Y)` | This piece of macro magic joins the two arguments together, even when one of the arguments is itself a macro (see 16.3.1 in C++ standard). This is normally used to create a mangled name in combination with a predefined macro such a __LINE__.
`BOOST_RESTRICT` | This macro can be used in place of the compiler-specific variant of the C99 `restrict` keyword to notify the compiler that, for the lifetime of the qualified pointer variable, only it and its derivative values will be used to access the object it references. This limits the effect of pointer aliasing and helps optimizers in generating better code. However, if this condition is violated, undefined behavior may occur. Usage example:
    void perform_computation( float* BOOST_RESTRICT in, float* BOOST_RESTRICT out )
    {
      *out = *in * 0.5f;
    }
`BOOST_FORCEINLINE` | This macro can be used in place of the `inline` keyword to instruct the compiler that the function should always be inlined. Overuse of this macro can lead to significant bloat, while good use can increase performance in certain cases, such as computation-intensive code built through generative programming techniques. Usage example:
    template<class T>
    BOOST_FORCEINLINE T& f(T& t)
    {
        return t;
    }
Note that use of this macro can lead to cryptic error messages with some compilers. Consider defining it to `inline` before including the Boost.Config header in order to be able to debug errors more easily.
`BOOST_NOINLINE` | This macro can be used in place of the `inline` keyword to instruct the compiler that the function should never be inlined. One should typically use this macro to mark functions that are unlikely to be called, such as error handling routines. Usage example:
    BOOST_NOINLINE void handle_error(const char* descr)
    {
        // ...
    }
`BOOST_NORETURN` | This macro can be used before the function declaration or definition to instruct the compiler that the function does not return normally (i.e. with a `return` statement or by leaving the function scope, if the function return type is `void`). The macro can be used to mark functions that always throw exceptions or terminate the application. Compilers that support this markup may use this information to specifically organize the code surrounding calls to this function and suppress warnings about missing `return` statements in the functions enclosing such calls. Usage example:
    BOOST_NORETURN void on_error_occurred(const char* descr)
    {
        throw std::runtime_error(descr);
    }
If the compiler does not support this markup, `BOOST_NORETURN` is defined empty and an additional macro `BOOST_NO_NORETURN` is defined.
`BOOST_LIKELY(X)` `BOOST_UNLIKELY(X)` | These macros communicate to the compiler that the conditional expression `X` is likely or unlikely to yield a positive result. The expression should result in a boolean value. The result of the macro is an integer or boolean value equivalent to the result of `X`. The macros are intended to be used in branching statements. The additional hint they provide can be used by the compiler to arrange the compiled code of the branches more effectively. Usage example:
    if (BOOST_UNLIKELY(ptr == NULL))
      handle_error("ptr is NULL");
`BOOST_ATTRIBUTE_UNUSED` | Expands to `__attribute__((unused))` when this is available - can be used to disable compiler warnings relating to unused types or variables.
`BOOST_ATTRIBUTE_NODISCARD` | Expands to `[[nodiscard]]` when this is available - can be used to create a warning when a type or variable is unused.
`BOOST_ATTRIBUTE_NO_UNIQUE_ADDRESS` | Expands to `[[no_unique_address]]` when this is available - can be used to indicate that a non-static data member need not have a unique address (for example empty classes).
`BOOST_MAY_ALIAS`, `BOOST_NO_MAY_ALIAS` | `BOOST_MAY_ALIAS` expands to a type attribute that can be used to mark types that may alias other types. Pointers or references to such marked types can be used to access objects of other types. If the compiler supports this feature `BOOST_NO_MAY_ALIAS` is not defined. Otherwise `BOOST_MAY_ALIAS` expands to nothing and `BOOST_NO_MAY_ALIAS` is defined. Usage example:
    struct BOOST_MAY_ALIAS aliasing_struct;
    typedef unsigned int BOOST_MAY_ALIAS aliasing_uint;
`BOOST_DEPRECATED(M)` | Expands to an attribute for a symbol that generates warnings when that symbol is used in code. The warnings may contain a message `M`, which must be a string literal. This attribute may be applied to types, functions or objects and is typically used to mark parts of the API as deprecated with a recommendation of replacement. Example:
    BOOST_DEPRECATED("Use bar() instead.")
    void foo();

    template< typename T >
    class BOOST_DEPRECATED("Use std::unique_ptr instead.") auto_ptr
    {
    };

    BOOST_DEPRECATED("Use std::numeric_limits<int>::max() instead.")
    const int max_int = 0x7fffffff;
The warnings issued by `BOOST_DEPRECATED` can be suppressed by defining `BOOST_ALLOW_DEPRECATED_SYMBOLS` or `BOOST_ALLOW_DEPRECATED` macros.
`BOOST_PRAGMA_MESSAGE(M)` | Defined in header `<boost/config/pragma_message.hpp>`, this macro expands to the equivalent of `#pragma message(M)`. `M` must be a string literal. Example: `BOOST_PRAGMA_MESSAGE("This header is deprecated.")` The messages issued by `BOOST_PRAGMA_MESSAGE` can be suppressed by defining the macro `BOOST_DISABLE_PRAGMA_MESSAGE`.
`BOOST_HEADER_DEPRECATED(A)` | Defined in header `<boost/config/header_deprecated.hpp>`, this macro issues the message "This header is deprecated. Use `A` instead." via `BOOST_PRAGMA_MESSAGE`. `A` must be a string literal. Example: `BOOST_HEADER_DEPRECATED("<boost/config/workaround.hpp>")` The messages issued by `BOOST_HEADER_DEPRECATED` can be suppressed by defining `BOOST_ALLOW_DEPRECATED_HEADERS` or `BOOST_ALLOW_DEPRECATED` macros.

### Boost Informational Macros

The following macros describe boost features; these are, generally speaking the only boost macros that should be tested in user code.

Macro | Header | Description
--- | --- | ---
`BOOST_VERSION` | `<boost/version.hpp>` | Describes the boost version number in XYYYZZ format such that: `(BOOST_VERSION % 100)` is the sub-minor version, `((BOOST_VERSION / 100) % 1000)` is the minor version, and `(BOOST_VERSION / 100000)` is the major version.
`BOOST_NO_INT64_T` | `<boost/cstdint.hpp>` `<boost/stdint.h>` | Defined if there are no 64-bit integral types: `int64_t`, `uint64_t` etc.
`BOOST_NO_INTEGRAL_INT64_T` | `<boost/cstdint.hpp>` `<boost/stdint.h>` | Defined if `int64_t` as defined by `<boost/cstdint.hpp>` is not usable in integral constant expressions.
`BOOST_MSVC` | `<boost/config.hpp>` | Defined if the compiler is really Microsoft Visual C++, as opposed to one of the many other compilers that also define `_MSC_VER`. Has the same value as _MSC_VER.
`BOOST_MSVC_FULL_VER` | `<boost/config.hpp>` | Defined to a normalised 9 digit version of _MSC_FULL_VER (which sometimes only has 8 digits), the macro has the form VVMMPPPPP where VV is the major version number, MM is the minor version number, and PPPPP is the compiler build number.
`BOOST_GCC` | `<boost/config.hpp>` | Defined if the compiler is really GCC, as opposed to one of the many other compilers that also define `__GNUC__`. Has the value: `__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__`.
`BOOST_INTEL` | `<boost/config.hpp>` | Defined if the compiler is an Intel compiler, takes the same value as the compiler version macro.
`BOOST_CLANG` | `<boost/config.hpp>` | Defined to 1 if the compiler is the Clang compiler.
`BOOST_CLANG_VERSION` | `<boost/config.hpp>` | Defined to the version of the Clang compiler, usually `__clang_major__ * 10000 + __clang_minor__ * 100 + __clang_patchlevel__`. On Apple Clang, has a best-effort value reflecting the upstream version, rather than the Apple version.
`BOOST_BORLANDC` | `<boost/config.hpp>` | Defined to the value of __BORLANDC__ if the compiler is the Embarcadero non-clang based compiler.
`BOOST_CODEGEARC` | `<boost/config.hpp>` | Defined to the value of __CODEGEARC__ if the compiler is the Embarcadero non-clang based compiler.
`BOOST_EMBTC` | `<boost/config.hpp>` | Defined to the value of __CODEGEARC__ if the compiler is the Embarcadero clang based compiler.
`BOOST_WINDOWS` | `<boost/config.hpp>` | Defined if the Windows platform API is available.
`BOOST_DINKUMWARE_STDLIB` | `<boost/config.hpp>` | Defined if the dinkumware standard library is in use, takes the same value as the Dinkumware library version macro `_CPPLIB_VER` if defined, otherwise 1.
`BOOST_MSSTL_VERSION` | `<boost/config.hpp>` | Defined if the Microsoft Visual C++ standard library is in use. Has the value of `_MSVC_STL_VERSION` when that is defined, and a synthesized value of the same format otherwise. Example values are 143 for VS2022/msvc-14.3, 142 for VS2019/msvc-14.2, 141 for VS2017/msvc-14.1, 140 for VS2015/msvc-14.0, 120 for VS2013/msvc-12.0, and so on.
`BOOST_COMPILER` | `<boost/config.hpp>` | Defined as a string describing the name and version number of the compiler in use. Mainly for debugging the configuration.
`BOOST_STDLIB` | `<boost/config.hpp>` | Defined as a string describing the name and version number of the standard library in use. Mainly for debugging the configuration.
`BOOST_PLATFORM` | `<boost/config.hpp>` | Defined as a string describing the name of the platform. Mainly for debugging the configuration.
`BOOST_LIBSTDCXX_VERSION` | `<boost/config.hpp>` | Defined if the libstdc++ standard library is in use. Has the value of normalised 5 digit integer of the form VVMMM where VV is the major version number, MM is the minor version number.

### Boost Deprecated Macros

The following have been deprecated; please use the replacements instead. They will be removed in a future version of boost.

Deprecated Macro | Replacement | When deprecated | When removed
--- | --- | --- | ---
`BOOST_NO_0X_HDR_ARRAY` | `BOOST_NO_CXX11_HDR_ARRAY` | Boost 1.50 |
`BOOST_NO_0X_HDR_CHRONO` | `BOOST_NO_CXX11_HDR_CHRONO` | Boost 1.50 |
`BOOST_NO_0X_HDR_CODECVT` | `BOOST_NO_CXX11_HDR_CODECVT` | Boost 1.50 |
`BOOST_NO_0X_HDR_CONDITION_VARIABLE` | `BOOST_NO_CXX11_HDR_CONDITION_VARIABLE` | Boost 1.50 |
`BOOST_NO_0X_HDR_FORWARD_LIST` | `BOOST_NO_CXX11_HDR_FORWARD_LIST` | Boost 1.50 |
`BOOST_NO_0X_HDR_FUTURE` | `BOOST_NO_CXX11_HDR_FUTURE` | Boost 1.50 |
`BOOST_NO_0X_HDR_INITIALIZER_LIST` | `BOOST_NO_CXX11_HDR_INITIALIZER_LIST` | Boost 1.50 |
`BOOST_NO_INITIALIZER_LISTS` | `BOOST_NO_CXX11_HDR_INITIALIZER_LIST` | Boost 1.50 |
`BOOST_NO_0X_HDR_MUTEX` | `BOOST_NO_CXX11_HDR_MUTEX` | Boost 1.50 |
`BOOST_NO_0X_HDR_RANDOM` | `BOOST_NO_CXX11_HDR_RANDOM` | Boost 1.50 |
`BOOST_NO_0X_HDR_RATIO` | `BOOST_NO_CXX11_HDR_RATIO` | Boost 1.50 |
`BOOST_NO_0X_HDR_REGEX` | `BOOST_NO_CXX11_HDR_REGEX` | Boost 1.50 |
`BOOST_NO_0X_HDR_SYSTEM_ERROR` | `BOOST_NO_CXX11_HDR_SYSTEM_ERROR` | Boost 1.50 |
`BOOST_NO_0X_HDR_THREAD` | `BOOST_NO_CXX11_HDR_THREAD` | Boost 1.50 |
`BOOST_NO_0X_HDR_TUPLE` | `BOOST_NO_CXX11_HDR_TUPLE` | Boost 1.50 |
`BOOST_NO_0X_HDR_TYPE_TRAITS` | `BOOST_NO_CXX11_HDR_TYPE_TRAITS` | Boost 1.50 |
`BOOST_NO_0X_HDR_TYPEINDEX` | `BOOST_NO_CXX11_HDR_TYPEINDEX` | Boost 1.50 |
`BOOST_NO_0X_HDR_UNORDERED_SET` | `BOOST_NO_CXX11_HDR_UNORDERED_SET` | Boost 1.50 |
`BOOST_NO_0X_HDR_UNORDERED_MAP` | `BOOST_NO_CXX11_HDR_UNORDERED_MAP` | Boost 1.50 |
`BOOST_NO_STD_UNORDERED` | `BOOST_NO_CXX11_HDR_UNORDERED_SET` | Boost 1.50 |
`BOOST_NO_AUTO_DECLARATIONS` | `BOOST_NO_CXX11_AUTO_DECLARATIONS` | Boost 1.51 |
`BOOST_NO_AUTO_MULTIDECLARATIONS` | `BOOST_NO_CXX11_AUTO_MULTIDECLARATIONS` | Boost 1.51 |
`BOOST_NO_CHAR16_T` | `BOOST_NO_CXX11_CHAR16_T` | Boost 1.51 |
`BOOST_NO_CHAR32_T` | `BOOST_NO_CXX11_CHAR32_T` | Boost 1.51 |
`BOOST_NO_TEMPLATE_ALIASES` | `BOOST_NO_CXX11_TEMPLATE_ALIASES` | Boost 1.51 |
`BOOST_NO_CONSTEXPR` | `BOOST_NO_CXX11_CONSTEXPR` | Boost 1.51 |
`BOOST_NO_DECLTYPE` | `BOOST_NO_CXX11_DECLTYPE` | Boost 1.51 |
`BOOST_NO_DECLTYPE_N3276` | `BOOST_NO_CXX11_DECLTYPE_N3276` | Boost 1.51 |
`BOOST_NO_DEFAULTED_FUNCTIONS` | `BOOST_NO_CXX11_DEFAULTED_FUNCTIONS` | Boost 1.51 |
`BOOST_NO_DELETED_FUNCTIONS` | `BOOST_NO_CXX11_DELETED_FUNCTIONS` | Boost 1.51 |
`BOOST_NO_EXPLICIT_CONVERSION_OPERATORS` | `BOOST_NO_CXX11_EXPLICIT_CONVERSION_OPERATORS` | Boost 1.51 |
`BOOST_NO_EXTERN_TEMPLATE` | `BOOST_NO_CXX11_EXTERN_TEMPLATE` | Boost 1.51 |
`BOOST_NO_FUNCTION_TEMPLATE_DEFAULT_ARGS` | `BOOST_NO_CXX11_FUNCTION_TEMPLATE_DEFAULT_ARGS` | Boost 1.51 |
`BOOST_NO_LAMBDAS` | `BOOST_NO_CXX11_LAMBDAS` | Boost 1.51 |
`BOOST_NO_LOCAL_CLASS_TEMPLATE_PARAMETERS` | `BOOST_NO_CXX11_LOCAL_CLASS_TEMPLATE_PARAMETERS` | Boost 1.51 |
`BOOST_NO_NOEXCEPT` | `BOOST_NO_CXX11_NOEXCEPT` | Boost 1.51 |
`BOOST_NO_NULLPTR` | `BOOST_NO_CXX11_NULLPTR` | Boost 1.51 |
`BOOST_NO_RAW_LITERALS` | `BOOST_NO_CXX11_RAW_LITERALS` | Boost 1.51 |
`BOOST_NO_RVALUE_REFERENCES` | `BOOST_NO_CXX11_RVALUE_REFERENCES` | Boost 1.51 |
`BOOST_NO_SCOPED_ENUMS` | `BOOST_NO_CXX11_SCOPED_ENUMS` | Boost 1.51 |
`BOOST_NO_STATIC_ASSERT` | `BOOST_NO_CXX11_STATIC_ASSERT` | Boost 1.51 |
`BOOST_NO_STD_UNORDERED` | `BOOST_NO_CXX11_STD_UNORDERED` | Boost 1.51 |
`BOOST_NO_UNICODE_LITERALS` | `BOOST_NO_CXX11_UNICODE_LITERALS` | Boost 1.51 |
`BOOST_NO_UNIFIED_INITIALIZATION_SYNTAX` | `BOOST_NO_CXX11_UNIFIED_INITIALIZATION_SYNTAX` | Boost 1.51 |
`BOOST_NO_VARIADIC_TEMPLATES` | `BOOST_NO_CXX11_VARIADIC_TEMPLATES` | Boost 1.51 |
`BOOST_NO_VARIADIC_MACROS` | `BOOST_NO_CXX11_VARIADIC_MACROS` | Boost 1.51 |
`BOOST_NO_NUMERIC_LIMITS_LOWEST` | `BOOST_NO_CXX11_NUMERIC_LIMITS` | Boost 1.51 |
`BOOST_HAS_STATIC_ASSERT` | `BOOST_NO_CXX11_STATIC_ASSERT` (negated) | Boost 1.53 |
`BOOST_HAS_VARIADIC_TMPL` | `BOOST_NO_CXX11_VARIADIC_TEMPLATES` (negated) | Boost 1.53 |
`BOOST_HAS_RVALUE_REFS` | `BOOST_NO_CXX11_RVALUE_REFERENCES` (negated) | Boost 1.53 |
`BOOST_HAS_CHAR16_T` | `BOOST_NO_CXX11_CHAR16_T` (negated) | Boost 1.53 |
`BOOST_HAS_CHAR32_T` | `BOOST_NO_CXX11_CHAR32_T` (negated) | Boost 1.53 |

### Macros for libraries with separate source code

The following macros and helper headers are of use to authors whose libraries include separate source code, and are intended to address several issues:

- Controlling shared library symbol visibility
- Fixing the ABI of the compiled library
- Selecting which compiled library to link against based upon the compilers settings

See Guidelines for Authors of Boost Libraries Containing Separate Source

#### Macros controlling shared library symbol visibility

Some compilers support C++ extensions that control which symbols will be exported from shared libraries such as dynamic shared objects (DSO's) on Unix-like systems or dynamic-link libraries (DLL's) on Windows.

The Microsoft VC++ compiler has long supplied `__declspec(dllexport)` and `__declspec(dllimport)` extensions for this purpose, as do virtually all other compilers targeting the Windows platform.

Modern versions of the GNU GCC compiler provide the `__attribute__((visibility("default")))` extension to indicate that a symbol should be exported. All other symbols may be hidden by using the `-fvisibility-hidden` or `-fvisibility-ms-compat` compiler switches.

Boost supplies several macros to make it easier to manage symbol visibility in a way that is portable between compilers and operating systems.

Macro | Description
--- | ---
`BOOST_SYMBOL_EXPORT` | Defines the syntax of a C++ language extension that indicates a symbol is to be exported from a shared library. If the compiler has no such extension, the macro is defined with no replacement text.
`BOOST_SYMBOL_IMPORT` | Defines the syntax of a C++ language extension that indicates a symbol is to be imported from a shared library. If the compiler has no such extension, the macro is defined with no replacement text.
`BOOST_SYMBOL_VISIBLE` | Defines the syntax of a C++ language extension that indicates a symbol is to be globally visible. If the compiler has no such extension, the macro is defined with no replacement text. Needed for classes that are not otherwise exported, but are used by RTTI. Examples include class for objects that will be thrown as exceptions or used in dynamic_casts, across shared library boundaries. For example, a header-only exception class might look like this:
    class BOOST_SYMBOL_VISIBLE my_exception : public std::runtime_error { ... };
Without BOOST_SYMBOL_VISIBLE, it would be impossible to catch my_exception thrown from a shared library compiled by GCC with the -fvisibility=hidden option.
`BOOST_HAS_DECLSPEC` | The compiler has C++ extensions `__declspec(dllexport)` and `__declspec(dllimport)` to control export/import of symbols from shared libraries. Deprecated. This macro is no longer necessary since BOOST_SYMBOL_EXPORT and BOOST_SYMBOL_IMPORT are now supplied. It is provided to support legacy code.
`BOOST_DISABLE_EXPLICIT_SYMBOL_VISIBILITY` | Disables the effect of the BOOST_SYMBOL_EXPORT, BOOST_SYMBOL_IMPORT and BOOST_SYMBOL_VISIBLE macros, in order to revert to the default compiler behaviour. Note that this option should never be used if Boost libraries are being linking against dynamically, or if you are building a shared library that exposes Boost types in its public API. It is however advisable when statically-linking against Boost to prevent Boost symbols from leaking from the binary: for instance because you are building a plug-in for a software which may itself use Boost which could cause ODR conflicts.

Typical usage:

boost/foo/config.hpp

    ...
    #if defined(BOOST_ALL_DYN_LINK) || defined(BOOST_FOO_DYN_LINK)
    # if defined(BOOST_FOO_SOURCE)
    #   define BOOST_FOO_DECL BOOST_SYMBOL_EXPORT
    # else
    #   define BOOST_FOO_DECL BOOST_SYMBOL_IMPORT
    # endif
    #else
    # define BOOST_FOO_DECL
    #endif
    ...

boost/foo/foo.hpp

    #include <boost/foo/config.hpp>
    ...
    class BOOST_FOO_DECL bar { ... };
    ...
    void BOOST_FOO_DECL f();
    ...

boost/libs/foo/src/foo.cpp

    #define BOOST_FOO_SOURCE
    #include <boost/foo/foo.hpp>
    ...
    void BOOST_FOO_DECL f()
    {
      ...
    }
    ...

#### ABI Fixing

When linking against a pre-compiled library it vital that the ABI used by the compiler when building the library matches exactly the ABI used by the code using the library. In this case ABI means things like the struct packing arrangement used, the name mangling scheme used, or the size of some types (enum types for example). This is separate from things like threading support, or runtime library variations, which have to be dealt with by build variants. To put this in perspective there is one compiler (Borland's) that has so many compiler options that make subtle changes to the ABI, that at least in theory there 3200 combinations, and that's without considering runtime library variations. Fortunately these variations can be managed by `#pragma`'s that tell the compiler what ABI to use for the types declared in your library. In order to avoid sprinkling `#pragma`'s all over the boost headers, there are some prefix and suffix headers that do the job. Typical usage is:

my_library.hpp

    #ifndef MY_INCLUDE_GUARD
    #define MY_INCLUDE_GUARD

    // all includes go here:
    #include <boost/config.hpp>
    #include <whatever>

    #include <boost/config/abi_prefix.hpp> // must be the last #include

    namespace boost {

    // your code goes here

    }

    #include <boost/config/abi_suffix.hpp> // pops abi_prefix.hpp pragmas

    #endif // include guard

my_library.cpp

    ...
    // nothing special need be done in the implementation file
    ...

The user can disable this mechanism by defining `BOOST_DISABLE_ABI_HEADERS`, or they can define `BOOST_ABI_PREFIX` and/or `BOOST_ABI_SUFFIX` to point to their own prefix/suffix headers if they so wish.

#### Automatic library selection

It is essential that users link to a build of a library which was built against the same runtime library that their application will be built against -if this does not happen then the library will not be binary compatible with their own code- and there is a high likelihood that their application will experience runtime crashes. These kinds of problems can be extremely time consuming and difficult to debug, and often lead to frustrated users and authors alike (simply selecting the right library to link against is not as easy as it seems when their are 6-8 of them to chose from, and some users seem to be blissfully unaware that there even are different runtimes available to them).

To solve this issue, some compilers allow source code to contain `#pragma`'s that instruct the linker which library to link against, all the user need do is include the headers they need, place the compiled libraries in their library search path, and the compiler and linker do the rest. Boost.config supports this via the header `<boost/config/auto_link.hpp>`, before including this header one or more of the following macros need to be defined:

`BOOST_LIB_NAME`

  Required: An identifier containing the basename of the library, for example 'boost_regex'.

`BOOST_DYN_LINK`

  Optional: when set link to dll rather than static library.

`BOOST_LIB_DIAGNOSTIC`

  Optional: when set the header will print out the name of the library selected (useful for debugging).

`BOOST_AUTO_LINK_NOMANGLE`

  Optional: whan set specifies that we should link to BOOST_LIB_NAME.lib, rather than a mangled-name version.

`BOOST_AUTO_LINK_TAGGED`

  Optional: Specifies that we link to libraries built with the --layout=tagged option. This is essentially the same as the default name-mangled version, but without the compiler name and version, or the Boost version. Just the build options.

`BOOST_AUTO_LINK_SYSTEM`

  Optional: Specifies that we link to libraries built with the --layout=system option. This is essentially the same as the non-name-mangled version, but with the prefix to differentiate static and dll builds

If the compiler supports this mechanism, then it will be told to link against the appropriately named library, the actual algorithm used to mangle the name of the library is documented inside `<boost/config/auto_link.hpp>` and has to match that used to create the libraries via bjam 's install rules.

my_library.hpp

    ...
    //
    // Don't include auto-linking code if the user has disabled it by
    // defining BOOST_ALL_NO_LIB, or BOOST_MY_LIBRARY_NO_LIB, or if this
    // is one of our own source files (signified by BOOST_MY_LIBRARY_SOURCE):
    //
    #if !defined(BOOST_ALL_NO_LIB) && !defined(BOOST_MY_LIBRARY_NO_LIB) && !defined(BOOST_MY_LIBRARY_SOURCE)
    #  define BOOST_LIB_NAME boost_my_library
    #  ifdef BOOST_MY_LIBRARY_DYN_LINK
    #     define BOOST_DYN_LINK
    #  endif
    #  include <boost/config/auto_link.hpp>
    #endif
    ...

my_library.cpp

    // define BOOST_MY_LIBRARY_SOURCE so that the header knows that the
    // library is being built (possibly exporting rather than importing code)
    //
    #define BOOST_MY_LIBRARY_SOURCE

    #include <boost/my_library/my_library.hpp>
    ...