derived basic_ostream: "using" keyword and ambiguous overload for operator <<

Question

I have a derived basic_ostream class and an inline modifier (similar to setw). My stream class should also inherit all the operator << behavior from its parent. I get different compiler errors depending on whether I use the "using" keyword or not:

#include <iostream>

struct modifier { };

template <typename C, typename T=std::char_traits<C> >
struct mystream : public std::basic_ostream<C, T>
{
    // this is where the trouble is
    using std::basic_ostream<C, T>::operator <<;

    inline mystream & operator << (const modifier & mod)
    {
        // ...custom behavior...
        return *this;
    }
};

int main()
{
    mystream<char> foo;
    modifier m;
    foo << "string";  // this fails if the using is present
    foo << 123;       // this fails if the using is absent
    foo << m;
}

When I put the using directive in, the compiler is confused about the "string" output, and if I comment it out, it gets confused about the integer 123 output, in both cases giving me "error: ambiguous overload for 'operator<<'". I have the problem with both g++ 4.2.1 and g++4.8. What's the right way forward here?

Answer 1

Rather then inherit from std::basic_ostream, won't it be sufficient to just re-implement << for your modifier struct using a regular stream:

std::ostream & operator << (std::ostream &stream, const modifier & mod)
{
    // ...custom behavior...
    return stream;
}

Your solution seems overcomplicated, but I think the actual error you get comes from your overload of << - it has to accept two arguments (first argument being reference to stream itself).