C++ Primer, Fifth Edition

10.3. Customizing Operations

Many of the algorithms compare elements in the input sequence. By default, such algorithms use either the element type’s < or == operator. The library also defines versions of these algorithms that let us supply our own operation to use in place of the default operator.

For example, the sort algorithm uses the element type’s < operator. However, we might want to sort a sequence into a different order from that defined by <, or our sequence might have elements of a type (such as Sales_data) that does not have a < operator. In both cases, we need to override the default behavior of sort.