clang. binary operator overloading for nested templates

Question

I'm trying to overload binary operator+ for nested template class with clang and got the error invalid operands to binary expression. Note: candidate template ignored: couldn't infer template argument T. Something like:

template<typename T>
class Container
{
  struct Iterator {
    template<typename U>
    friend typename Container<U>::Iterator operator+(size_t, typename Container<U>::Iterator const&);
   };
};

template<typename U>
typename Container<U>::Iterator
operator+(size_t, typename Container<U>::Iterator const&)
{
  return Container<U>::Iterator{};
}

Is there way to overload binary operators for nested template class using clang compiler?

C++17 can be used.

The left side of :: is an undeduced context. It's not possible to deduce U. — cpplearner
– cpplearner, Commented Jan 19, 2018 at 7:57
Also, it is far easier to do these sorts of templates inline within the nested class declaration, otherwise you end up needing to do template<typename T> template<typename U> — SoronelHaetir
– SoronelHaetir, Commented Jan 19, 2018 at 7:59

Barry · Accepted Answer · 2018-01-19 14:46:32Z

2

Don't make the inner function a template. Just make it a non-template non-member friend and define it inline:

template<typename T>
class Container
{
  struct Iterator {
    friend Iterator operator+(size_t, Iterator const&) {
      return {};
    } 
  };
};

This works in C++11. C++03 if you don't {} in the return.

answered Jan 19, 2018 at 14:46

Barry

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