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## Support for Compiler Intrinsics

There are some traits that can not be implemented within the current C++ language: to make these traits "just work" with user defined types, some kind of additional help from the compiler is required. Currently (April 2008) Visual C++ 8 and 9, GNU GCC 4.3 and MWCW 9 provide the necessary intrinsics, and other compilers will no doubt follow in due course.

The Following traits classes always need compiler support to do the right thing for all types (but all have safe fallback positions if this support is unavailable):

- is_union
- is_pod
- has_trivial_constructor
- has_trivial_copy
- has_trivial_assign
- has_trivial_destructor
- has_nothrow_constructor
- has_nothrow_copy
- has_nothrow_assign
- has_virtual_destructor

The following traits classes can't be portably implemented in the C++ language, although in practice, the implementations do in fact do the right thing on all the compilers we know about:

- is_empty
- is_polymorphic

The following traits classes are dependent on one or more of the above:

- is_class
- is_stateless

The hooks for compiler-intrinsic support are defined in boost/type_traits/intrinsics.hpp, adding support for new compilers is simply a matter of defining one of more of the following macros:

Table 1.4. Macros for Compiler Intrinsics

BOOST_IS_UNION(T) | Should evaluate to true if T is a union type
--- | ---
BOOST_IS_POD(T) | Should evaluate to true if T is a POD type
BOOST_IS_EMPTY(T) | Should evaluate to true if T is an empty struct or union
BOOST_HAS_TRIVIAL_CONSTRUCTOR(T) | Should evaluate to true if the default constructor for T is trivial (i.e. has no effect)
BOOST_HAS_TRIVIAL_COPY(T) | Should evaluate to true if T has a trivial copy constructor (and can therefore be replaced by a call to memcpy)
BOOST_HAS_TRIVIAL_ASSIGN(T) | Should evaluate to true if T has a trivial assignment operator (and can therefore be replaced by a call to memcpy)
BOOST_HAS_TRIVIAL_DESTRUCTOR(T) | Should evaluate to true if T has a trivial destructor (i.e. ~T() has no effect)
BOOST_HAS_NOTHROW_CONSTRUCTOR(T) | Should evaluate to true if `T x;` can not throw
BOOST_HAS_NOTHROW_COPY(T) | Should evaluate to true if `T(t)` can not throw
BOOST_HAS_NOTHROW_ASSIGN(T) | Should evaluate to true if `T t, u; t = u` can not throw
BOOST_HAS_VIRTUAL_DESTRUCTOR(T) | Should evaluate to true T has a virtual destructor
BOOST_IS_ABSTRACT(T) | Should evaluate to true if T is an abstract type
BOOST_IS_BASE_OF(T,U) | Should evaluate to true if T is a base class of U
BOOST_IS_CLASS(T) | Should evaluate to true if T is a class type
BOOST_IS_CONVERTIBLE(T,U) | Should evaluate to true if T is convertible to U
BOOST_IS_ENUM(T) | Should evaluate to true is T is an enum
BOOST_IS_POLYMORPHIC(T) | Should evaluate to true if T is a polymorphic type
BOOST_ALIGNMENT_OF(T) | Should evaluate to the alignment requirements of type T.