Re: (Boost) Static Assert and behavior on msvc/gcc

Hello All,

I compiled the following code:-

/*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
#include <iostream>
#include <boost/static_assert.hpp>

namespace beta{

    struct composition_root{};

    template<typename composition>
    struct composition_base: composition_root
    {
    private:
        typedef composition C;
        typedef typename C::alpha_type alpha_type;
        typedef typename C::value_type value_type;

    public:
        const composition&
            derived() const
        {
            return static_cast<const composition&>(*this);
        }

        composition&
            derived()
        {
            return static_cast<composition&>(*this);
        }

        static alpha_type to_alpha(const value_type&)
        {
            BOOST_STATIC_ASSERT(false);
        }

        static value_type from_alpha(const alpha_type&)
        {
            BOOST_STATIC_ASSERT(false);
        }
    };

    struct basic_composition_tag;
}

struct alpha_
{
    typedef int alpha_type;
    typedef char value_type;
    char from_alpha(const alpha_type& x)
    {

        return 0;
    }

    int to_alpha(const value_type& x)
    {
        return 0;
    }
};

int main(void)
{
    typedef beta::composition_base<alpha_> a;
    sizeof(a);
    return 0;
}

/*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/

on msvc(Version 9.0) and gcc 4.3.3.

Behavior:-
a) msvc -- compiles well
b) gcc -- throws error, basically assert failure

Even when I write,

int main(void)
{
    //typedef beta::composition_base<alpha_> a;
    //sizeof(a);
    return 0;
}

The behavior is same.

$1. Is there any problem in BOOST_STATIC_ASSERT ? I will ask in boost
forum for it. Or else you know then please answer it.

$2. Why gcc is complaining error even when template instantiation has
not been done ? (second case)

$3. I understand that any direct use of compositon_base::{to/from }
_alpha is bound to cause problem. But if any code is not touching it,
in that case why compiler is throwing error.

$4. When does static function definition gets instantiated ? If in a
compilation unit, presence of a static function name and definition
anywhere (inside class or outside class) causes it to get instantiated
also, then does the gcc compiler is checking extra for a possible
error in instantiation of a composition_base class.