Question

I had to build a const_iterator wrapper to exchange generic programming with dynamic binding (never mind this). However std::set<Base*>::const_iterator::operator*() gives me something I didn’t expect, because I have trouble returning it in a Derived* const& where Derived publicly inherits from Base.

template <typename T, typename Container> class StdConstIterator : public ConstIteratable<T> {
  private:
    typename Container::const_iterator it;
  public:
    T const& operator*() const {
      return *it; // g++ says: warning: returning reference to temporary
    }
};

// invokation
StdConstIterator<Derived*,std::set<Base*> > si;

While I see that it might be reasonable for pointers to copy them instead of returning a reference, I fail to find a specialisation in my STL’s implementation. Could you shed some light on the issue, please?

Note: You might know the routine; Unfortunately no C++11 support, so I cannot decltype myself out of this. But this is more of a “what the heck is going on here?” sort of question, anyway.

Answer 1

Most likely is that there’s some conversion going on. Instead of T you should use the actual iterator’s value type, typename Container::const_iterator::value_type. If that type isn’t the same as T, then the conversion creates a temporary, to which you attempt to return a reference.

On the other hand, if you do want the conversion, then return by value, T operator*() const.