C++ Primer, Fifth Edition

14.8.2. Library-Defined Function Objects

The standard library defines a set of classes that represent the arithmetic, relational, and logical operators. Each class defines a call operator that applies the named operation. For example, the plus class has a function-call operator that applies + to a pair of operands; the modulus class defines a call operator that applies the binary % operator; the equal_to class applies ==; and so on.

These classes are templates to which we supply a single type. That type specifies the parameter type for the call operator. For example, plus<string> applies the string addition operator to string objects; for plus<int> the operands are ints; plus<Sales_data> applies + to Sales_datas; and so on:

plus<int> intAdd;       // ...