| .. Macros/Introspection//BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF |
| |
| .. Copyright Daniel Walker 2007. |
| .. 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) |
| |
| BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF |
| ==================================== |
| |
| Synopsis |
| -------- |
| |
| .. parsed-literal:: |
| |
| #define BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF(trait, name, default\_) \\ |
| |unspecified-token-seq| \\ |
| /\*\*/ |
| |
| |
| Description |
| ----------- |
| |
| Expands into the definition of a boolean |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 template member ``x::template |
| name`` with no more than |BOOST_MPL_LIMIT_METAFUNCTION_ARITY| |
| parameters. |
| |
| On deficient compilers not capable of performing the detection, |
| ``trait<x>::value`` always returns a fallback value ``default_``. A |
| boolean configuration macro, |BOOST_MPL_CFG_NO_HAS_XXX_TEMPLATE|, 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_TEMPLATE_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_TEMPLATE| is defined, ``r::value |
| == c1``; otherwise, ``r::value == true`` if and only if ``x`` |
| is a class type that has a nested template member ``x::template |
| name`` with no more than |BOOST_MPL_LIMIT_METAFUNCTION_ARITY|. |
| |
| |
| .. parsed-literal:: |
| |
| typedef trait< x, c2 >::type r; |
| |
| :Return type: |
| |Integral Constant|. |
| |
| :Semantics: |
| If |BOOST_MPL_CFG_NO_HAS_XXX_TEMPLATE| is defined, ``r::value |
| == c2::value``; otherwise, equivalent to |
| |
| .. parsed-literal:: |
| |
| typedef trait<x>::type r; |
| |
| |
| Example |
| ------- |
| |
| .. parsed-literal:: |
| |
| BOOST_MPL_HAS_XXX_TEMPLATE_NAMED_DEF( |
| has_xxx, xxx, false |
| ) |
| |
| struct test1 {}; |
| struct test2 { void xxx(); }; |
| struct test3 { int xxx; }; |
| struct test4 { static int xxx(); }; |
| struct test5 { typedef int xxx; }; |
| struct test6 { struct xxx; }; |
| struct test7 { typedef void (\*xxx)(); }; |
| struct test8 { typedef void (xxx)(); }; |
| struct test9 { template< class T > struct 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> )); |
| BOOST_MPL_ASSERT_NOT(( has_xxx<test6> )); |
| BOOST_MPL_ASSERT_NOT(( has_xxx<test7> )); |
| BOOST_MPL_ASSERT_NOT(( has_xxx<test8> )); |
| |
| #if !defined(BOOST_MPL_CFG_NO_HAS_XXX_TEMPLATE) |
| BOOST_MPL_ASSERT(( has_xxx<test9> )); |
| #endif |
| |
| BOOST_MPL_ASSERT(( has_xxx<test9, true\_> )); |
| |
| |
| See also |
| -------- |
| |
| |Macros|, |BOOST_MPL_HAS_XXX_TEMPLATE_DEF|, |
| |BOOST_MPL_CFG_NO_HAS_XXX_TEMPLATE|, |BOOST_MPL_LIMIT_METAFUNCTION_ARITY| |
| |