| .. Macros/Introspection//BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF |
| |
| BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF |
| ================================= |
| |
| Synopsis |
| -------- |
| |
| .. parsed-literal:: |
| |
| #define BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF(trait, name, default\_) \\ |
| |unspecified-token-seq| \\ |
| /\*\*/ |
| |
| |
| Description |
| ----------- |
| |
| Expands into a definition of a boolean unary |Metafunction| ``trait`` |
| such that for any type ``x`` ``trait<x>::value == true`` if and only |
| if ``x`` is a class type and has a nested type memeber ``x::name``. |
| |
| On the deficient compilers not capabale of performing the detection, |
| ``trait<x>::value`` always returns a fallback value ``default_``. |
| A boolean configuraion macro, |BOOST_MPL_CFG_NO_HAS_XXX|, is provided |
| to signal or override the "deficient" status of a particular compiler. |
| |Note:| The fallback value can also be provided at the point of the |
| metafunction invocation; see the `Expression semantics` section for |
| details |-- end note| |
| |
| |
| Header |
| ------ |
| |
| .. parsed-literal:: |
| |
| #include <boost/mpl/has_xxx.hpp> |
| |
| |
| Parameters |
| ---------- |
| |
| +---------------+-------------------------------+---------------------------------------------------+ |
| | Parameter | Requirement | Description | |
| +===============+===============================+===================================================+ |
| | ``trait`` | A legal identifier token | A name of the metafunction to be generated. | |
| +---------------+-------------------------------+---------------------------------------------------+ |
| | ``name`` | A legal identifier token | A name of the member being detected. | |
| +---------------+-------------------------------+---------------------------------------------------+ |
| | ``default_`` | An boolean constant | A fallback value for the deficient compilers. | |
| +---------------+-------------------------------+---------------------------------------------------+ |
| |
| |
| Expression semantics |
| -------------------- |
| |
| For any legal C++ identifiers ``trait`` and ``name``, boolean constant expression ``c1``, |
| boolean |Integral Constant| ``c2``, and arbitrary type ``x``: |
| |
| .. parsed-literal:: |
| |
| BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF(trait, name, c1) |
| |
| :Precondition: |
| Appears at namespace scope. |
| |
| :Return type: |
| None. |
| |
| :Semantics: |
| Expands into an equivalent of the following class template definition |
| |
| .. parsed-literal:: |
| |
| template< typename X, typename fallback = boost::mpl::bool_<c1> > |
| struct trait |
| { |
| // |unspecified| |
| // ... |
| }; |
| |
| where ``trait`` is a boolean |Metafunction| with the following semantics: |
| |
| .. parsed-literal:: |
| |
| typedef trait<x>::type r; |
| |
| :Return type: |
| |Integral Constant|. |
| |
| :Semantics: |
| If |BOOST_MPL_CFG_NO_HAS_XXX| is defined, ``r::value == c1``; |
| otherwise, ``r::value == true`` if and only if ``x`` is a class type |
| that has a nested type memeber ``x::name``. |
| |
| |
| .. parsed-literal:: |
| |
| typedef trait< x,c2 >::type r; |
| |
| :Return type: |
| |Integral Constant|. |
| |
| :Semantics: |
| If |BOOST_MPL_CFG_NO_HAS_XXX| is defined, ``r::value == c2::value``; |
| otherwise, equivalent to |
| |
| .. parsed-literal:: |
| |
| typedef trait<x>::type r; |
| |
| |
| Example |
| ------- |
| |
| .. parsed-literal:: |
| |
| BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF(has_xxx, xxx, false) |
| |
| struct test1 {}; |
| struct test2 { void xxx(); }; |
| struct test3 { int xxx; }; |
| struct test4 { static int xxx(); }; |
| struct test5 { template< typename T > struct xxx {}; }; |
| struct test6 { typedef int xxx; }; |
| struct test7 { struct xxx; }; |
| struct test8 { typedef void (\*xxx)(); }; |
| struct test9 { typedef void (xxx)(); }; |
| |
| BOOST_MPL_ASSERT_NOT(( has_xxx<test1> )); |
| BOOST_MPL_ASSERT_NOT(( has_xxx<test2> )); |
| BOOST_MPL_ASSERT_NOT(( has_xxx<test3> )); |
| BOOST_MPL_ASSERT_NOT(( has_xxx<test4> )); |
| BOOST_MPL_ASSERT_NOT(( has_xxx<test5> )); |
| |
| #if !defined(BOOST_MPL_CFG_NO_HAS_XXX) |
| BOOST_MPL_ASSERT(( has_xxx<test6> )); |
| BOOST_MPL_ASSERT(( has_xxx<test7> )); |
| BOOST_MPL_ASSERT(( has_xxx<test8> )); |
| BOOST_MPL_ASSERT(( has_xxx<test9> )); |
| #endif |
| |
| BOOST_MPL_ASSERT(( has_xxx<test6,true\_> )); |
| BOOST_MPL_ASSERT(( has_xxx<test7,true\_> )); |
| BOOST_MPL_ASSERT(( has_xxx<test8,true\_> )); |
| BOOST_MPL_ASSERT(( has_xxx<test9,true\_> )); |
| |
| |
| See also |
| -------- |
| |
| |Macros|, |BOOST_MPL_HAS_XXX_TRAIT_DEF|, |BOOST_MPL_CFG_NO_HAS_XXX| |
| |
| |
| .. copyright:: Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) |