22.3 Functors have State

One advantage of functors over function pointers is that they can hold state. Since this state is held by the instance of the object it can be thread safe (unlike static-variables inside functions used with function pointers). The state of a functor can be initialized at construction. For instance, OEChem's OEHasAtomicNum functor takes an argument on construction which defines which atomic number is required for the functor to return true. The state of a functor can also change during the operator() function. One group of functors, called accumulators, sum a value each time they are called. Finally, functors can take advantage of all of the other properties of C++ classes. A programmer can define any number of other functions which manipulate or query the state of the functor.

				OEChem - C++ Theory Manual Version 1.3.1

				OEChem - C++ Theory Manual Version 1.3.1