What does this code do and why does it compile?

Question

I'm creating a Vector2 class in C++ as a template, and I want to define the + operator as a non-member friend function that can simply add two vectors.

This is the friend declaration inside my Vector2 template class:

template <class U>
friend Vector2<T> operator+(const Vector2<T> &lhs, const Vector2<T> &rhs);

This is contained in a .hpp file, but the implementation is in a separate .cpp file:

template <class T>
Vector2<T> operator+(const Vector2<T> &lhs, const Vector2<T> &rhs)
{
    return Vector2<T>(lhs.x_ + rhs.x_, lhs.y_ + rhs.y_);
}

This compiles without any warnings, however, it does not seem to work.

Vector2<int> v1(4, 3);
Vector2<int> v2(3, 4);

Vector2<int> v3 = v1 + v2;

When I try to compile the above snippet, GCC complains:

prog.cpp: In function ‘int main(int, char**)’:
prog.cpp:26:28: error: no match for ‘operator+’ in ‘v1 + v2’

source/vector2.hpp:31:23: note: template<class U> Vector2<int> operator+(const Vector2<int>&, const Vector2<int>&)
source/vector2.hpp:31:23: note:   template argument deduction/substitution failed:
prog.cpp:26:28: note:   couldn't deduce template parameter ‘U’
prog.cpp:26:18: warning: unused variable ‘v3’ [-Wunused-variable]

What am I doing wrong? How can I correctly define the + operator for my template class?

Answer 1

The compiler clearly states what the problem is. It cannot deduce the template parameter 'U'. Your declaration(the .hpp file) is wrong. Should be

template <class T>
friend Vector2<T> operator+(const Vector2<T> &lhs, const Vector2<T> &rhs);