Question

Why compiler doesn’t return error if operator+() member function of date class is not returning anything. if i do

date d1,d2;
date any = d1 + d2;

then d1 + d2 will create a temporary, what does this temporary gets initialized with?

date operator+(date d)  
{  
    day += d.day; 
    month += d.month; 
    year += d.year; 
}

Note: its just for test purpose only. Not for commercial use or anything.

Answer 1

Since, it’s operator +() and not operator +=(), you should be creating a temporary and return the same:

date operator + (const date &d) const
{            //  ^^^^ 1         ^^^^^ 2
  date temp = *this;  // copy current object
  ...
  return temp;  // important: -Wall warned you for missing 'return'
}

There are other 2 important changes you can see:

(1) Passing d as const reference; because you don’t need another copy

(2) Making operator + as const correct by adding const at end; because you are not going to modify this object

Update: For your updated question, here is a link which answers it.

Why “not all control paths return a value” is warning and not an error?