How to convert anything to string implicitly?

Question

My goal is to design a String class that decorates std::string in order to provide some functionality my program needs. One functionality I want to add is the ability to con

Answer 1

This or similar should fix it:

namespace HasFormattedOutput {

    namespace Detail
    {
        struct Failure{};
    }

    template
    Detail::Failure operator << (OutputStream&, const T&);

    template
    struct Result : std::integral_constant<
        bool,
        ! std::is_same<
            decltype((*(OutputStream*)0) << std::declval()),
            Detail::Failure
        >::value
    > {};
} // namespace HasFormattedOutput

template 
struct has_formatted_output
:   HasFormattedOutput::Result
{};

class X  {
    public:
    X() {}

    template 
    X(const T& t) {
        static_assert(
             has_formatted_output::value, 
             "Not supported type.");
        std::ostringstream s;
        s << t;
        str = s.str();
    }

    private:
    std::string str;
};
std::ostream& operator << (std::ostream& stream, const X&) { return stream; }

struct Y  {
    Y() {}
};

int main() {
    Y y;
    X x(y);
    return 0;
}