Question

I have the following operator-overloading prototypes:

 ostream& operator<<(ostream & outputstream, my_arr& arr)

 my_arr operator+(const my_arr& left, const my_arr& right)

I call:

 cout << (arr1 + arr2);

This gives me the following compiler error:

error: no match for ‘operator<<’ in ‘std::cout << operator+(const my_array&, const my_array&)((*(const my_array*)(& y)))’

This goes away if I change the signature of << to the following:

  ostream& operator<<(ostream & outputstream, const my_arr& arr)

I might be missing something basic here, but why does this happen?
Thanks for your help.

Answer 1

The problem is that when passing as reference, you cannot pass “temporary” (rvalue) objects such as the result of an addition. When passing a const reference, C++ rules allow passing temporaries because it’s guaranteed they won’t be written to.