Question

Is nesting overloaded operators possible ? I would like to nest << inside a ()

template<class T>
struct UnknownName
{
   T g;
   T&operator<<(std::ostream&os, const T&v){return os<<v;}
   bool operator()(const T&v)
   {
      if(v==g)
        //do the streaming << then return true
      else return false;
   }
};

Would you please help me out ? I am afraid my example is unreal enough to you, please just ask if you still have any doubts. Sincerely.

Answer 1

The best I can think of would be to have operator<< return a specific type, and then overload operator() to accept that type:

#include <cstdio>

namespace {
    struct Foo {
        struct Bar {
            int i;
        };

        Foo& operator()(const Bar& b)
        {
            std::printf("bar, %d\n", b.i);
            return *this;
        }

        // obviously you don't *have* to overload operator()
        // to accept multiple types; I only did so to show that it's possible
        Foo& operator()(const Foo& f)
        {
            std::printf("foo\n");
            return *this;
        }
    };

    Foo::Bar operator<<(const Foo& f, const Foo& g)
    {
        Foo::Bar b = { 5 };
        return b;
    }
}

int main()
{
    Foo f, g, h;
    f(g << h);
    f(g);
}

This isn’t a common idiom, to say the least.