forked from boostorg/type_traits
Added VC++ intrinsic support.
Added intrinsic-hook docs. [SVN r28824]
This commit is contained in:
John Maddock
@ -29,6 +29,7 @@
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[def __examples [*Examples:]]
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[def __type [*type:]]
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[def __transform_workaround [link transform.broken_compiler_workarounds_ compiler workarounds]]
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[def __intrinsics [link boost_typetraits.intrinsics intrinsics]]
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[def __is_void [link boost_typetraits.is_void is_void]]
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[def __is_integral [link boost_typetraits.is_integral is_integral]]
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@ -753,6 +754,61 @@ as appropriate:
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[endsect]
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[section:intrinsics Support for Compiler Intrinsics]
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There are some traits that can not be implemented within the current C++ language:
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to make these traits "just work" with user defined types, some kind of additional
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help from the compiler is required. Currently (May 2005) MWCW 9 and Visual C++ 8
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provide the necessary intrinsics, and other compilers will no doubt follow in due
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course.
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The Following traits classes always need compiler support to do the right thing
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for all types
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(but all have safe fallback positions if this support is unavailable):
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* __is_union
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* __is_pod
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* __has_trivial_constructor
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* __has_trivial_copy
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* __has_trivial_assign
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* __has_trivial_destructor
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* __has_nothrow_constructor
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* __has_nothrow_copy
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* __has_nothrow_assign
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* __has_virtual_destructor
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The following traits classes can't be portably implemented in the C++ language,
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although in practice, the implementations do in fact do the right thing on all
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the compilers we know about:
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* __is_empty
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* __is_polymorphic
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The following traits classes are dependent on one or more of the above:
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* __is_class
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* __is_stateless
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The hooks for compiler-intrinsic support are defined in
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[@../../boost/type_traits/intrinsics.hpp boost/type_traits/intrinsics.hpp], adding support for new compilers is simply
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a matter of defining one of more of the following macros:
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[table Macros for Compiler Intrinsics
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[[BOOST_IS_UNION(T)][Should evaluate to true if T is a union type]]
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[[BOOST_IS_POD(T)][Should evaluate to true if T is a POD type]]
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[[BOOST_IS_EMPTY(T)][Should evaluate to true if T is an empty struct or union]]
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[[BOOST_HAS_TRIVIAL_CONSTRUCTOR(T)][Should evaluate to true if the default constructor for T is trivial (i.e. has no effect)]]
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[[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)]]
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[[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)]]
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[[BOOST_HAS_TRIVIAL_DESTRUCTOR(T)][Should evaluate to true if T has a trivial destructor (i.e. ~T() has no effect)]]
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[[BOOST_HAS_NOTHROW_CONSTRUCTOR(T)][Should evaluate to true if `T x;` can not throw]]
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[[BOOST_HAS_NOTHROW_COPY(T)][Should evaluate to true if `T(t)` can not throw]]
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[[BOOST_HAS_NOTHROW_ASSIGN(T)][Should evaluate to true if `T t, u; t = u` can not throw]]
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[[BOOST_HAS_VIRTUAL_DESTRUCTOR(T)][Should evaluate to true T has a virtual destructor]]
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]
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[endsect]
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[section:mpl MPL Interoperability]
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All the value based traits in this library conform to MPL's requirements
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@ -1273,6 +1329,8 @@ templates, then this template can not be used with function types.
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Without some (as yet unspecified) help from the compiler,
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`has_nothrow_assign` will never report that a class or struct has a
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non-throwing assignment-operator; this is always safe, if possibly sub-optimal.
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Currently (May 2005) only Visual C++ 8 has the necessary compiler support to ensure that this
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trait "just works".
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__header ` #include <boost/type_traits/has_nothrow_assign.hpp>` or ` #include <boost/type_traits.hpp>`
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@ -1292,6 +1350,8 @@ templates, then this template can not be used with function types.
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Without some (as yet unspecified) help from the compiler,
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`has_nothrow_constructor` will never report that a class or struct has a
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non-throwing default-constructor; this is always safe, if possibly sub-optimal.
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Currently (May 2005) only Visual C++ 8 has the necessary compiler __intrinsics to ensure that this
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trait "just works".
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__header ` #include <boost/type_traits/has_nothrow_constructor.hpp>` or ` #include <boost/type_traits.hpp>`
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@ -1311,6 +1371,8 @@ templates, then this template can not be used with function types.
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Without some (as yet unspecified) help from the compiler,
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`has_nothrow_copy` will never report that a class or struct has a
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non-throwing copy-constructor; this is always safe, if possibly sub-optimal.
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Currently (May 2005) only Visual C++ 8 has the necessary compiler __intrinsics to ensure that this
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trait "just works".
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__header ` #include <boost/type_traits/has_nothrow_copy.hpp>` or ` #include <boost/type_traits.hpp>`
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@ -1333,7 +1395,7 @@ templates, then this template can not be used with function types.
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Without some (as yet unspecified) help from the compiler,
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has_trivial_assign will never report that a user-defined class or struct has a
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trivial constructor; this is always safe, if possibly sub-optimal. Currently
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(April 2005) only MWCW has the necessary compiler-intrinsics to detect
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(May 2005) only MWCW 9 and Visual C++ 8 have the necessary compiler __intrinsics to detect
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user-defined classes with trivial constructors.
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__std_ref 12.8p11.
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@ -1375,7 +1437,7 @@ templates, then this template can not be used with function types.
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Without some (as yet unspecified) help from the compiler,
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has_trivial_constructor will never report that a user-defined class or struct has a
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trivial constructor; this is always safe, if possibly sub-optimal. Currently
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(April 2005) only MWCW has the necessary compiler-intrinsics to detect
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(May 2005) only MWCW 9 and Visual C++ 8 have the necessary compiler __intrinsics to detect
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user-defined classes with trivial constructors.
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__std_ref 12.1p6.
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@ -1415,7 +1477,7 @@ templates, then this template can not be used with function types.
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Without some (as yet unspecified) help from the compiler,
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has_trivial_copy will never report that a user-defined class or struct has a
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trivial constructor; this is always safe, if possibly sub-optimal. Currently
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(April 2005) only MWCW has the necessary compiler-intrinsics to detect
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(May 2005) only MWCW 9 and Visual C++ 8 have the necessary compiler __intrinsics to detect
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user-defined classes with trivial constructors.
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__std_ref 12.8p6.
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@ -1457,7 +1519,7 @@ templates, then this template can not be used with function types.
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Without some (as yet unspecified) help from the compiler,
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has_trivial_destructor will never report that a user-defined class or struct has a
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trivial destructor; this is always safe, if possibly sub-optimal. Currently
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(April 2005) only MWCW has the necessary compiler-intrinsics to detect
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(May 2005) only MWCW 9 and Visual C++ 8 have the necessary compiler __intrinsics to detect
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user-defined classes with trivial constructors.
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__std_ref 12.4p3.
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@ -1488,10 +1550,12 @@ __inherit If T is a (possibly cv-qualified) type with a virtual destructor
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then inherits from __true_type, otherwise inherits from __false_type.
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__compat This trait is provided for completeness, since it's part of the
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Technical Report on C++ Library Extensions, however, there is currently no
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way to portably implement this trait, the default version provided
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Technical Report on C++ Library Extensions. However, there is currently no
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way to portably implement this trait. The default version provided
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always inherits from __false_type, and has to be explicitly specialized for
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types with virtual destructors.
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types with virtual destructors unless the compiler used has compiler __intrinsics
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that enable the trait to do the right thing: currently (May 2005) only Visual C++
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8 has the necessary __intrinsics.
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__std_ref 12.4.
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@ -1664,7 +1728,9 @@ __header ` #include <boost/type_traits/is_class.hpp>` or ` #include <boost/type_
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__compat Without (some as yet unspecified) help from the compiler,
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we cannot distinguish between union and class types, as a result this type
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will erroneously inherit from __true_type for union types. See also __is_union.
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will erroneously inherit from __true_type for union types. See also __is_union.
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Currently (May 2005) only Visual C++ 8 has the necessary compiler __intrinsics to
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correctly identify union types, and therefore make is_class function correctly.
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__examples
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@ -1823,8 +1889,10 @@ __std_ref 10p5.
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__header ` #include <boost/type_traits/is_empty.hpp>` or ` #include <boost/type_traits.hpp>`
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__compat Relies on the compiler implementing zero sized empty base classes in
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order to detect empty classes.
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__compat In order to correctly detect empty classes this trait relies on either:
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* the compiler implementing zero sized empty base classes, or
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* the compiler providing __intrinsics to detect empty classes.
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Can not be used with incomplete types.
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@ -2138,8 +2206,8 @@ of class templates, then this template can not be used with function types.
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Without some (as yet unspecified) help from the compiler, is_pod will
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never report that a class or struct is a POD; this is always safe,
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if possibly sub-optimal. Currently (April 2005) only MWCW provides the
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necessary compiler-intrinsic support.
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if possibly sub-optimal. Currently (May 2005) only MWCW 9 and Visual C++ 8 have the
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necessary compiler-__intrinsics.
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__header ` #include <boost/type_traits/is_pod.hpp>` or ` #include <boost/type_traits.hpp>`
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@ -2316,8 +2384,8 @@ then this template can not be used with function types.
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Without some (as yet unspecified) help from the compiler, is_stateless will never
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report that a class or struct is stateless; this is always safe,
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if possibly sub-optimal. Currently (April 2005) only MWCW has the necessary
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compiler intrinsics to make this template work automatically.
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if possibly sub-optimal. Currently (May 2005) only MWCW 9 and Visual C++ 8 have the necessary
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compiler __intrinsics to make this template work automatically.
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[endsect]
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@ -2364,9 +2432,10 @@ __std_ref 3.9.2 and 9.5.
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__compat Without (some as yet unspecified) help from the
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compiler, we cannot distinguish between union and class types using only standard C++,
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as a result this type will never inherit from __true_type, unless the user explicitly
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specializes the template for their user-defined union types. Now that the type traits
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library is has been accepted as part of the Technical Report on C++ Library Extensions,
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this situation should change as newer compiler versions are released.
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specializes the template for their user-defined union types, or unless the compiler
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supplies some unspecified intrinsic that implements this functionality. Currently
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(May 2005) only Visual C++ 8 has the necessary compiler __intrinsics to make this
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trait "just work" without user intervention.
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__header ` #include <boost/type_traits/is_union.hpp>` or ` #include <boost/type_traits.hpp>`
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