Member functions returning pointers to member functions

问题

I'd like to have a class with member functions that return pointers to member functions.

That is, something like:

class Foo {
// typedef ????(Foo::* func)????
public:
   Func s1();
   Func s2();
   Func s3();
}

Func Foo::s1() {
  // do stuff
  return &Foo::s2;
}

Func Foo::s2() {
  // do stuff
  return &Foo::s3;
}

Func Foo::s3() {
  // do stuff
  return 0;
}

Basically, what I try to do is to implement a state machine, where each state nows the next state and returns it by means of a function pointer.

Note: I'm not interested in other ways of implementing a state machine. I really like to know if it can be implemented in a way that looks like the above.

Context: I got inspired by this talk: http://www.youtube.com/watch?v=HxaD_trXwRE and wondered if a similar pattern can be used in C++.

回答1:

Blatently ripping off the solution at GotW and adapting it to member functions:

class Foo {
  public:
    struct FuncPtrRet;
    typedef FuncPtrRet(Foo::*FuncPtr)();
    struct FuncPtrRet {
        FuncPtrRet(FuncPtr pp) : p(pp) { }
        operator FuncPtr() { return p; }
        FuncPtr p;
    };
    FuncPtrRet s1() { return &Foo::s2; }
    FuncPtrRet s2() { return &Foo::s3; }
    FuncPtrRet s3() { return &Foo::s3; }
};

int main() {
    Foo test;
    Foo::FuncPtr state = test.s1();
    state = (test.*state)();
    state = (test.*state)();
    state = (test.*state)();
    state = (test.*state)();
    state = (test.*state)();
    return 0;
}

This seems to work on Clang 3.3. I don't think returning 0 is a good choice for an idle state (s3 here), but I might be wrong there. Having an idle state that returns itself seems more intuitive to me.

来源：https://stackoverflow.com/questions/19600044/member-functions-returning-pointers-to-member-functions