Overloading Arithmetic Operators

The assignment operator can be declared as

T& operator= (const t&);

in a class, but the arithmetic operators cannot be defined that way. It has to be friend function. I don't understand why? Can you please explain ?

From stackoverflow

• They ideally should be globals and not necessarily friends, so that you can write:

  yourtype v = 1;
  yourtype w = 1 + v;
  

  Since, 1 is not an object of yourtype, if the operator+ were a member it would throw a fit. However, making it global makes it convert the 1 to yourtype and then perform the operation. Making it a friend helps to extract and manipulate the members of yourtype as required -- though not required. As an example: You can implement the member function operator+= and use it in the implementation of the operator+.

• I think that C++ FAQ Lite will give you a definitive answer.

• It is not mandatory that arithmetic operators should be friend

  Well you can define like this:

  MyClass MyClass::operator + (const MyClass& t) const
  {
    MyClass ret(*this);
    ret += t;
    return ret;
  }
  

  The a + b is really a syntax sugar, the compiler will expand it to a.operator+(b). The previous sample will work if all your objects are MyClass instances, but will not work if you have to operate with others types, ie 1 + a, will not work, this can be solved by using friends.

  MyClass operator + (int i, const MyClass& t)
  {
    MyClass ret(i);
    ret += t;
    return ret;
  }
  

  This has to be done when the left hand side of the + operator is not a class, or it is a class but you can't add operator + to its definition.

• The problem is that if you do something like this:

  class A
  {
      A& operator+(int n);
      // ...
  }
  
  int main()
  {
      A my_var;
      int integer = 1;
      A + integer; // compiles
      integer + A // error: no function operator+(int, A) defined
  }
  

  it will not compile. A solution is to define operator+(int, A) and operator+(A, int) as friends of A. As a side note, the Boost Operators library makes this process very easy.