''From FunctorObject:''

Here's a stripped down version of a C++ implementation: -- PhilGoodwin

''This declares an interface for functors that is used by clients. Other versions of this interface include parameter types and return values''
 struct Functor
 { virtual void operator()()=0; };

''This declares an adaptor that turns an object/method pair into a Functor that can be passed to a client''
 template< typename Type_ >
 class Functor''''''For: public Functor
 {
 public:
        typedef void(Type_::*Member''''''Function)();

 private:
        Type_           *theInstance;
        Member''''''Function  theMethod;

 public:
        Functor''''''For(Type_ *anInstance, Member''''''Function aMethod): theInstance(anInstance), theMethod(aMethod){}

        void operator()() { (theInstance->*theMethod)(); }
 };

'' This declares an adaptor for normal function pointers It doesn't actually '''do''' anything except add type safety''
 class Function''''''Functor: public Functor
 {
 public:
        typedef void(*Function''''''Type)();

 private:
        Function theFunction;

 public:
        Function''''''Functor(Function''''''Type aFunction): theFunction(aFunction){}

        void operator() () { theFunction(); }
 };

Here are a couple of snippets that show how to add parameter and return types:

 template< typename Return''''''Type, typename Parameter''''''Type >
 struct FunctorR1
 { Return''''''Type operator() (Parameter''''''Type)=0; };

 template< typename Type_, typename Return''''''Type, typename Parameter''''''Type >
 class FunctorR1For: class FunctorR1<Return''''''Type, Parameter''''''Type>
 {
       ...
       typedef Return''''''Type(Type_::*Member''''''Function)(Parameter''''''Type);
       ...
       Return''''''Type operator() (Parameter''''''Type param) { return (theInstance->*theMethod(param); }
 };

----
See BlocksInJava for an example in the JavaLanguage