### BOOST_DECLARE_HAS_MEMBER

The BOOST_DECLARE_HAS_MEMBER macro allows to declare a trait which would then let introspect the existence of a class member (a data member or a member function) with the specified name.

For example, the following declarations introduce `local::has_begin` and `local::has_funop` traits which then allow to test if the supplied class has respectively `begin` and `operator()` members:

    namespace { namespace local
    {
        BOOST_DECLARE_HAS_MEMBER(has_begin, begin);
        BOOST_DECLARE_HAS_MEMBER(has_funop, operator());
    }}

Now these traits can be used as follows:

    namespace { namespace local
    {
        struct test01 { int   begin; };
        struct test02 { char* begin() { return 0; } };
        struct test22 { void operator()() {} };
    }}

    BOOST_TEST(local::has_begin<local::test01>::value == true);
    BOOST_TEST(local::has_begin<local::test02>::value == true);
    BOOST_TEST(local::has_funop<local::test22>::value == true);

As it can be seen from the example the traits only check for the existence of a member (be that a data member or a member function) with the specified name.