The compiler (but not programmers) look at

Pair p1(2,3);
Pair p2(5,5);
Pair p3, p4;

p3 = p1.add(p2);
p4 = p1 + p2;
   

like this

Pair p1(2,3);
Pair p2(5,5);
Pair p3, p4;

p3 = p1.add(p2);
p4 = p1.operator+(p2);