C++ Cookbook

8.14. Overloading Arithmetic and Assignment Operators for Intuitive Class Behavior

Problem

You have a class for which some of C++’s unary or binary operators make sense, and you want users of your class to be able to use them when working with objects of your class. For example, if you have a class named Balance that contains, essentially, a floating-point value (i.e., an account balance), it would be convenient if you could use Balance objects with some standard C++ operators, like this:

Balance checking(50.0), savings(100.0);

checking += 12.0;
Balance total = checking + savings;

Solution

Overload the operators you want to use as member functions and standalone functions to allow arguments of various types for which the given operator makes sense, as in Example 8-15.

Example 8-15. Overloading unary and binary operators

#include <iostream> using namespace std; class Balance { // These have to see private data friend const Balance operator+(const Balance& lhs, const Balance& rhs); friend const Balance operator+(double lhs, const Balance& rhs); friend const Balance operator+(const Balance& lhs, double rhs); public: Balance() : val_(0.0) {} Balance(double val) : val_(val) {} ~Balance() {} // Unary operators Balance& operator+=(const Balance& other) { val_ += other.val_; return(*this); } Balance& operator+=(double other) { val_ += other; return(*this); } double getVal() const {return(val_);} private: double val_; }; // Binary operators const Balance operator+(const Balance& lhs, ...