我该如何修复此 FSM C++ 语法错误 C3867

Question

so I'm new to c++ and trying to create a FSM for this RPG game but i get 1 syntax error: C3867 non-standard syntax; use '&' to create a pointer to member C3867 non-standard syntax; use '&' to create a pointer to member but when i use the '&' i get another error C2276: '&': illegal operation on bound member function expression here's my codes:

Class CNPC
{
private : 
    State <CNPC>* CurrentState;
    State<CNPC>* PrevState;
public:
    void ChangeState(State <CNPC> * NewState)
    void revertToPrevState(); 
    void OnUpdate(Uint32 time, Uint32 deltaTime)
}

CNPC.cpp for the change state function

void CNPC::changeState(State<CNPC>* pnewState)
{

    assert(CurrentState && pnewState);

    PrevState = CurrentState;

    CurrentState->Exit(this);

    CurrentState = pnewState;

    CurrentState->Enter(this);

}

the states header file

class P_Attack : public State<CNPC>
{
public:
    static P_Attack* Instance();
    virtual void Enter(CNPC* npc);
    virtual void Exucute(CNPC* npc);
    virtual void Exit(CNPC* npc);
private:
    P_Attack() {};
};

cpp file

P_Attack* P_Attack::Instance() // the same layout in all of the states 
{
    static P_Attack* S;
    return S;
}

void P_Attack::Exucute(CNPC*npc)
{ 

    npc->Attacked();


    if(npc->GetHealth() <= 0)
    {
        npc->changeState(Die::Instance()->Enter); // this is where im getting the error
    }

im sure the answer is obvious but it continues to elude me, pretty sure im doing something very stupid

Answer 1

The error you are receiving is due to the fact that you are accidentally passing a bound member function reference as an argument to the changeState function.

When you pass Die::Instance()->Enter as an argument, you aren't passing the return of the Enter member function but rather attempting to pass the memory address of the Enter function itself. This explains the first error telling you to append & as the compiler believes you are actually wanting to pass a function reference.

The following issue is that the syntax for passing member functions by reference requires you to refer to their class name, followed by scope resolution :: followed by the member function name, hence getting an error if you try to pass &Die::Instance()->Enter as you are not properly passing a member function since member functions belong to the class, not to an instance. So you receive an error saying the operation is illegal as you are attempting to pass a member function which is bound to an object as if it were unbound. Essentially what the compiler expects to see is &state<CBNC>::Enter because the Enter method is bound to your state class, but it sees you attempting to take the address of the Enter function as if it belonged to something else.

That all said, assuming that Die::Instance()->Enter() requires no arguments, appending () as I just showed should resolve those errors, but not the underlying issue. I say this because it appears that your Instance() member function is likely a factory which returns the state object you really want to pass. If that is in fact the case simply passing Die::Instance() should suffice since the returned object will then invoke Enter once inside of your changeState function, properly invoking Enter on your instance as expected. I say it won't solve the underlying issue as it appears that your Enter() functions return void, thus simply changing the syntax to the above would attempt to pass the return of Die::Instance()->Enter() which would be void as the last function to execute is Enter() causing another error to appear.