Which operand should be passed in the binary overloaded operator function as a second operand?

Answer 1

The right-hand operand; the r-value of the operator.

Unary operators have one operand while tertiary operators have three operands. All binary operators have two operands, the l-value and the r-value. The l-value is the operand to the left of the operator while the r-value is the operand to the right of the operator.

Thus, in the expression x + y, x is the l-value while y is the r-value.

When overloading binary operators in a class, you need only specify the r-value. The l-value is the instance of the class to which the operator applies and therefore does not need to be specified. For instance:

class MyClass

{

public:

MyClass(int data=0):m_data(data){} // default constructor

int operator+ (const int rhs) const {return(m_data+rhs);}

private:

int m_data;

};

While this allows you to return the sum of your class instance and an integer, it does not allow you to return the sum of an integer and an instance of your class. For example:

MyClass obj(5);

int x = 10;

int y = obj + x; // OK! y is 15.

int z = x + obj; // Compiler error! No operator exists that accepts an r-value of type MyClass.

To fix this error and allow for two-way addition, you must declare a binary operator overload outside of the class. You cannot do it inside the class because the l-value is an int, not an instance of MyClass.

The external overload requires two parameters, the l-value and the r-value of the operator:

int operator+(const int lhs,const MyClass& rhs) {return(rhs+lhs);}

Note that the implementation simply reverses the operands. This is functionally equivalent to making the following explicit call:

return(rhs.operator+(lhs));

Note also that since MyClass::operator+ is a public operator of MyClass, this overload does not need to be declared a friend of MyClass (a common misconception). However, the overload must be declared in the same file where the class is declared since it is only of relevance to MyClass but should be made available wherever MyClass is accessible.