A function object, or functor, is a custom type that implements the function call operator: (operator()). This means that a function operator can be called in a way that looks like it is a function. Since we haven't covered classes yet, in this section we will just explore the function objects types that are provided by the Standard Library and how to use them.
The <functional> header file contains various types that can be used as function objects. The following table lists these:
| Purpose | Types |
| Arithmetic | divides, minus, modulus, multiplies, negate, plus |
| Bitwise | bit_and, bit_not, bit_or, bit_xor |
| Comparison | equal_to, greater, greater_equal, less, less_equals, not_equal_to |
| Logical | logical_and, logical_not, logical_or ... |
