operator()defined and accessible. They can be passed as arguments to algorithm templates and used in place of a function pointer. Not only is the resulting expressiveness of the library increased, but the generated code can be more efficient than what you might write by hand. When we refer to "functors," then, generally we include function pointers in the description as well.
Often, functors are only created as temporaries passed to algorithm calls, rather than being created as named variables.
Two examples taken from the standard itself follow. To perform a by-element addition of two vectors
double, and put the result in
transform (a.begin(), a.end(), b.begin(), a.begin(), plus<double>());
transform(a.begin(), a.end(), a.begin(), negate<double>());
The standard functors are derived from structs named
binary_function. These two classes contain nothing but typedefs, to aid in generic (template) programming. If you write your own functors, you might consider doing the same.