boost_1_45_0/libs/mpl/doc/src/refmanual/HAS_XXX_TEMPLATE_NAMED_DEF.rst

.. 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|