assert.rst

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.. Macros/Asserts//BOOST_MPL_ASSERT

.. Copyright Aleksey Gurtovoy, David Abrahams 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_ASSERT
================

Synopsis
--------

.. parsed-literal::
    
    #define BOOST_MPL_ASSERT( pred ) \\
        |unspecified-token-seq| \\
    /\*\*/


Description
-----------

Generates a compilation error when the predicate ``pred`` holds false.


Header
------

.. parsed-literal::
    
    #include <boost/mpl/assert.hpp>


Parameters
----------

+---------------+-----------------------------------+-------------------------------+
| Parameter     | Requirement                       | Description                   |
+===============+===================================+===============================+
| ``pred``      | Boolean nullary |Metafunction|    | A predicate to be asserted.   |
+---------------+-----------------------------------+-------------------------------+


Expression semantics
--------------------

For any boolean nullary |Metafunction| ``pred``:


.. parsed-literal::

    BOOST_MPL_ASSERT(( pred ));

:Return type:
    None.

:Semantics:
    Generates a compilation error if ``pred::type::value != true``, otherwise
    has no effect. Note that double parentheses are required even if no commas 
    appear in the condition. 
    
    When possible within the compiler's diagnostic capabilities,
    the error message will include the predicate's full type name, and have a 
    general form of:

    .. parsed-literal::
    
       |...| \*\*\*\*\*\*\*\*\*\*\*\* pred::\*\*\*\*\*\*\*\*\*\*\*\* |...|


Example
-------

::
    
    template< typename T, typename U > struct my
    {
        // ...
        BOOST_MPL_ASSERT(( is_same< T,U > ));
    };
    
    my<void*,char*> test;

    // In instantiation of `my<void, char*>':
    //   instantiated from here
    // conversion from `
    //   mpl_::failed************boost::is_same<void, char*>::************' to 
    //   non-scalar type `mpl_::assert<false>' requested


See also
--------

|Asserts|, |BOOST_MPL_ASSERT_NOT|, |BOOST_MPL_ASSERT_MSG|, |BOOST_MPL_ASSERT_RELATION|