instantiation_count (was: [boost] is_incomplete)

From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2002-10-24 07:05:14

Here's a class template that keeps track of its instantiations:

    #include "boost/static_assert.hpp"
    #include "boost/config.hpp"

    typedef char (&no_tag)[1];
    typedef char (&yes_tag)[2];

    template< typename U > no_tag test(U*, ...);
    template< typename T, long > struct instantiation_count;

    template<
          typename T
        , long value_ = 
              sizeof(test(static_cast<instantiation_count<T,1>*>(0)))
            + sizeof(test(static_cast<instantiation_count<T,2>*>(0)))
            + sizeof(test(static_cast<instantiation_count<T,3>*>(0)))
            + sizeof(test(static_cast<instantiation_count<T,4>*>(0)))
            + sizeof(test(static_cast<instantiation_count<T,5>*>(0)))
        >
    struct instantiation_count
    {
        BOOST_STATIC_CONSTANT(long, value = value_ - 5 + 1);
        friend yes_tag test(instantiation_count<T, value>*);
    };

    // here!
    BOOST_STATIC_ASSERT(instantiation_count<int>::value == 1);
    BOOST_STATIC_ASSERT(instantiation_count<int>::value == 2);
    BOOST_STATIC_ASSERT(instantiation_count<int>::value == 3);
    BOOST_STATIC_ASSERT(instantiation_count<int>::value == 4);
    BOOST_STATIC_ASSERT(instantiation_count<int>::value == 5);
    BOOST_STATIC_ASSERT(instantiation_count<int>::value == 6);

    BOOST_STATIC_ASSERT(instantiation_count<long>::value == 1);
    BOOST_STATIC_ASSERT(instantiation_count<short>::value == 1);
    BOOST_STATIC_ASSERT(instantiation_count<long>::value == 2);

Seems well-formed to me, and works with Intel C++ 6.0 and one other
compiler.

Aleksey