22.2 `operator()` and function pointers

C++ allows a user to define a function operator () for any class. This operator can have return values, take arguments and be overloaded. For instance bool operator()(const OEAtomBase *atom) is a functor which takes a const pointer to an OEAtomBase as an argument and returns boolean. The operator() function is called when the class instance name is followed by the arguments to the function.

If we have a class MyPredicate which defines bool operator()(const OEAtomBase *), then the function will be called in the example below.

  MyPredicate pred; //create functor
  bool returnVal;
  OEAtomBase *atom;

  ...define atom...

  returnVal = pred(atom);

Note that while pred in the example above is actually a class. However, the syntax for calling a function and calling operator() of a functor is indistinguishable.

While this idiom may be new to non-C++-aficionados, it is actually quite similar to using function pointers in C or other languages.

Note: For technical reasons, operator() functions cannot be virtual.

				OEChem - C++ Theory Manual Version 1.3.1

				OEChem - C++ Theory Manual Version 1.3.1

22.2 operator() and function pointers

22.2 `operator()` and function pointers