has_ostream_operator for C++98

has_ostream_operator.cpp

#include <iostream>
#include <typeinfo>

namespace detail {

    namespace has_ostream_operator_impl {
        
            //!Typedef a char array of size one.
            typedef char no;

            //! Typedef a char array of size two.
            typedef char yes[2];

            //! A fallback struct which can create itself
            //  from any object of any type.
            struct fallback {
                template<typename T>
                fallback(const T &);
            };

            //! Declare a dummy << operator which operates on
            //  any fallback object.
            no operator << (std::ostream const &, fallback const &);

            //! If the class does have an << operator overloaded,
            //  then the return type of invoking it has to be a
            //  std::ostream &. In that case, this function will
            //  be invoked.
            yes & test(std::ostream &);

            //! If the class did not have an << operator overloaded
            //  to begin with, it will use the global << operator
            //  defined in this namespace. It will convert itself into
            //  fallback object and then invoke this function. In this
            //  case, the return type in no.
            no & test(no);

        template<typename T>
        struct has_ostream_operator {
            static std::ostream &s;
            static T &t;
            // compiler complains that test(s << t) is ambiguous
            // for Foo
            static bool const value = sizeof( test(s << t) ) == sizeof( yes );
        };
    }

    template<typename T>
    struct has_ostream_operator :
        has_ostream_operator_impl::has_ostream_operator<T> {
    };

    // template<class, class = std::void_t<>>
    //     struct has_ostream_operator : std::false_type {};

    // template<class T>
    // struct has_ostream_operator<
    //     T,
    //     std::void_t<
    //         decltype(std::declval<std::ostream&>() << std::declval<const T&>())>>
    //     : std::true_type {};

}


template<class X>
void prettyPrint(std::ostream& o, const X& x)
{
        o <<  typeid(x).name() << ": " << detail::has_ostream_operator<X>::value;
}

struct Foo {
    friend std::ostream& operator>>(std::ostream& o, Foo& f) {
            return o;
        }
        
    friend std::ostream& operator<<(std::ostream& o, Foo& f) {
            return o;
        }
};

struct Bar {};

int main()
{
    Bar x;
    Foo y;

    prettyPrint(std::cout, x); // 3Bar: 0
    std::cout << std::endl;


    prettyPrint(std::cout, y); // 3Foo: 1
    std::cout << std::endl;

    return 0;
}

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