15.5 Comparison operator

The comparison operator can be overloaded to compare two different types or to compare two equal types that are not basic types. The result type of a comparison operator is always a boolean.

The comparison operators that can be overloaded are:

equal to

(=) To determine if two variables are equal.

unequal to

(<>) To determine if two variables are different.

less than

(<) To determine if one variable is less than another.

greater than

(>) To determine if one variable is greater than another.

greater than or equal to

(>=) To determine if one variable is greater than or equal to another.

less than or equal to

(<=) To determine if one variable is greater than or equal to another.

If there is no separate operator for unequal to (<>), then, to evaluate a statement that contains the unequal to operator, the compiler uses the equal to operator (=), and negates the result. The opposite is not true: if no ”equal to” but an ”unequal to” operator exists, the compiler will not use it to evaluate an expression containing the equal (=) operator.

As an example, the following operator allows to compare two complex numbers:

the above definition allows comparisons of the following form:

The comparison operator definition needs 2 parameters, with the types that the operator is meant to compare. Here also, the compiler doesn’t apply commutativity: if the two types are different, then it is necessary to define 2 comparison operators.

In the case of complex numbers, it is, for instance necessary to define 2 comparisons: one with the complex type first, and one with the real type first.

Given the definitions

the following two comparisons are possible:

Note that the order of the real and complex type in the two comparisons is reversed.