Question

In what situation would generate an error if you don’t return something by reference for a function?

Answer 1

In what situation would generate an error if you don’t return something by reference for a function?

Restricting the answer to what is being exactly asked,
Whenever you want the return value of the function to act as an l-value, and you do not return by reference then it will generate an error.

The more common example of this is overloading the operator [](the array subscription operator), You have to return by reference in order to use the [] on the l.h.s or more correctly to use it as an l-value.

An example:

class Myclass 
{ 
     int i;
     public:
     /*Not returned by reference- gives error*/
     int operator[](int idx){ return i;}

     /*Returned by reference- gives no error*/
     //int& operator[](int idx){ return i;}
};

int main()
{
   Myclass  obj;
   obj[1]= 10;
   return 0;
}

Output:

prog.cpp: In function ‘int main()’:
prog.cpp:16: error: lvalue required as left operand of assignment