Operator overloading

Operator overloading is syntactic sugar, and is used because it allows programming using notation nearer to the target domain[1] and allows user-defined types a similar level of syntactic support as types built into a language. It is common, for example, in scientific computing, where it allows computing representations of mathematical objects to be manipulated with the same syntax as on paper.

Operator overloading does not change the expressive power of a language (with functions), as it can be emulated using function calls. For example, consider variables a, b and c of some user-defined type, such as matrices:

a + b * c

In a language that supports operator overloading, and with the usual assumption that the '*' operator has higher precedence than the '+' operator, this is a concise way of writing:

Add(a, Multiply(b, c))

However, the former syntax reflects common mathematical usage.

In this case, the addition operator is overloaded to allow addition on a user-defined type Time in C++:

Time operator+(const Time& lhs, const Time& rhs) {
  Time temp = lhs;
  temp.seconds += rhs.seconds;
  temp.minutes += temp.seconds / 60;
  temp.seconds %= 60;
  temp.minutes += rhs.minutes;
  temp.hours += temp.minutes / 60;
  temp.minutes %= 60;
  temp.hours += rhs.hours;
  return temp;
}

Addition is a binary operation, which means it has two operands. In C++, the arguments being passed are the operands, and the temp object is the returned value.

The operation could also be defined as a class method, replacing lhs by the hidden this argument; However, this forces the left operand to be of type Time:

// This "const" means that |this| is not modified.
//  \
//   ------------------------------------\
//                                       |
//                                       V
Time Time::operator+(const Time& rhs) const {
  Time temp = *this;  // |this| should not be modified, so make a copy.
  temp.seconds += rhs.seconds;
  temp.minutes += temp.seconds / 60;
  temp.seconds %= 60;
  temp.minutes += rhs.minutes;
  temp.hours += temp.minutes / 60;
  temp.minutes %= 60;
  temp.hours += rhs.hours;
  return temp;
}

Note that a unary operator defined as a class method would receive no apparent argument (it only works from this):

bool Time::operator!() const {
  return hours == 0 && minutes == 0 && seconds == 0;
}

The less-than(<) operator is often overloaded to sort a structure or class:

class Pair {
 public:
  bool operator<(const Pair& p) const {
    if (x_ == p.x_) {
      return y_ < p.y_;
    }
    return x_ < p.x_;
  }

 private:
  int x_;
  int y_;
};

Like with the previous examples, in the last example operator overloading is done within the class. In C++, after overloading the less-than operator (<), standard sorting functions can be used to sort some classes.

Operator overloading has often been criticized[2] because it allows programmers to reassign the semantics of operators depending on the types of their operands. For example, the use of the << operator in C++:

a << b

shifts the bits in the variable a left by b bits if a and b are of an integer type, but if a is an output stream then the above code will attempt to write a b to the stream. Because operator overloading allows the original programmer to change the usual semantics of an operator and to catch any subsequent programmers by surprise, it is considered good practice to use operator overloading with care (the creators of Java decided not to use this feature,[3] although not necessarily for this reason).

Another, more subtle, issue with operators is that certain rules from mathematics can be wrongly expected or unintentionally assumed. For example, the commutativity of + (i.e. that a + b == b + a) does not always apply; an example of this occurs when the operands are strings, since + is commonly overloaded to perform a concatenation of strings (i.e. "bird" + "song" yields "birdsong", while "song" + "bird" yields "songbird"). A typical counter to this argument comes directly from mathematics: While + is commutative on integers (and more generally any complex number), it is not commutative for other "types" of variables. In practice, + is not even always associative, for example with floating-point values due to rounding errors. Another example: In mathematics, multiplication is commutative for real and complex numbers but not commutative in matrix multiplication.

A classification of some common programming languages is made according to whether their operators are overloadable by the programmer and whether the operators are limited to a predefined set.

• Function overloading
• Polymorphism (computer science)
• Subroutine
• Operator (programming)
• Operators in C and C++
• Mutator method
• Indexer (programming)
• Property (programming)