[boost] [mpl] holder class for types with external linkage (similar to integral contants)

Subject: [boost] [mpl] holder class for types with external linkage (similar to integral contants)
From: klaus triendl (klaus_at_[hidden])
Date: 2009-02-04 08:57:24

Recently, I've been in need of a holder class for strings with external
linkage but I didn't find an mpl datatype that could wrap an externally
defined entity.

Instead of creating a quick solution working for my needs I thought the
mpl library could benefit from a generic holder class similar to the
integral constant holders.

<code>
template<typename T, T N>
struct external_c
{
    // make it a nullary metafunction
    typedef external_c type;
    typedef T value_type;
    // correct? requirements?
    typedef integral_c_tag tag;

    // can't make a static compile time constant but only a static
    // function
    static value_type value()	{	return N;	}

    // implicitly convertible to its runtime type value
    operator value_type() const	{	return this->value();	}
};

// e.g.
extern const char string1[];
const char string1[] = "string1";
extern const std::string string2;
const std::string string2("string2");

int main()
{
    external_c<const char* const, string1> x1;
    external_c<const std::string&, string2> x2;
    std::cout << x1 << "\n";
    std::cout << x2.value() << "\n";
    return 0;
}
</code>

The name 'external_c' is just a quick idea and would have to be changed
anyway with c++09's possibility to use types with internal linkage in
template programming.

Would it be reasonable to integrate this into the mpl? Any comments?

Klaus Triendl