Use constexpr for boost::mpl::c_str::value when possible.

[ytj000@…]

Right now boost::mpl::c_str::value is a static const member variable. I think it is a decent idea to add the constexpr keyword here.

I have a patch for this issue. Here is an example why it is useful:

#include<boost/mpl/string.hpp>
int main() {
    using H = boost::mpl::string<'h'>;
    static_assert(boost::mpl::c_str<H>::value[0] == 'h', "");
}

Without my patch, compiler will complain we could not use c_str::value in constant expression. With my patch, compiler will accept this code.

Tested in clang++ 3.4, CentOS 6.3, but I think GCC should works as well.

[ytj000@…]

Patch

[ytj000@…]

example.cpp