Question

Basically I have the following class:

class StateMachine { ... StateMethod stateA(); StateMethod stateB(); ... }; 

The methods stateA() and stateB() should be able return pointers to stateA() and stateB(). How to typedef the StateMethod?

Answer 1

GotW #57 says to use a proxy class with an implicit conversion for this very purpose.

struct StateMethod; typedef StateMethod (StateMachine:: *FuncPtr)();  struct StateMethod {   StateMethod( FuncPtr pp ) : p( pp ) { }   operator FuncPtr() { return p; }   FuncPtr p; };  class StateMachine {   StateMethod stateA();   StateMethod stateB(); };  int main() {   StateMachine *fsm = new StateMachine();   FuncPtr a = fsm->stateA();  // natural usage syntax   return 0; }      StateMethod StateMachine::stateA {   return stateA; // natural return syntax }  StateMethod StateMachine::stateB {   return stateB; } 

This solution has three main strengths:

1. It solves the problem as required. Better still, it’s type-safe and portable.

2. Its machinery is transparent: You get natural syntax for the caller/user, and natural syntax for the function’s own ‘return stateA;’ statement.

3. It probably has zero overhead: On modern compilers, the proxy class, with its storage and functions, should inline and optimize away to nothing.