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I'm quite unfamiliar with C++ and I'm having some issues with executing a static member function pointer I've tried several different variations on the code (included below) and looked at several different tutorials and questions but I'm not having much luck!

Here's a code snippet for the header and source files plus the error's I'm getting:Actions.h

class Actions{
private:
    int actionId;
    int stateId;
    int eventId;
public:
    typedef string (Actions::*functionPtr)(int previousState, int currentState, int eventId);
    static functionPtr _ptrAction1;
    int doAction(int previousState, int currentState, int eventId);
    string function1(int previousState, int currentState, int eventId);
};

Actions.c:

#include "stdafx.h"
#include "Actions.h"
Actions::Actions(int actionId, int stateId, int eventId){
    this->actionId = actionId;
    this->stateId = stateId;
    this->eventId = eventId;
    _ptrAction1 = &Actions::function1;
}

int doAction(int actionId, int previousState, int currentState, int eventId){
int functionId = 0;
string message;

switch(actionId){
   case 1:
       message = (*_ptrAction1)(previousState, currentState, eventId);
       break;
   default:
       break;
   }
}

string Actions::function1(int previousState, int currentState, int eventId){
    return "this is an example for now";
}

The error I'm receiving is in Actions.c on line

message = (*_ptrAction1)(previousState, currentState, eventId);

The error is as follows:

1>Actions.cpp(37): error C2065: '_ptrAction1' : undeclared identifier

I thought perhaps I was referencing it incorrectly so I changed the above line to be as follows:

message = (*Actions::_ptrAction1)(previousState, currentState, eventId);

But now I receive a different error:

Actions.cpp(37): error C2064: term does not evaluate to a function taking 3 arguments

For reference I have read C++ calling static function pointer but it is not the same issue that I am having. I really hope you can help and I thank you for your time, I greatly appreciate it!

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2
  • 2
    Is that the complete code? You don't implement function1, only define it. Commented Jun 30, 2015 at 20:54
  • Sorry yes this is not the complete code just a snippit I'll include function1 for reference. Commented Jun 30, 2015 at 20:58

2 Answers 2

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ptrAction1 is static pointer to an object method. function1 is a method on which your pointer points to, so you must call it in an object context.

Suppose that you want to call it on the current object, then *this.*_ptrAction1 is the way to call it in a method.

Your doAction function doesn't use any Actions instance, you then cannot call the method... Define doAction(4 params) as a method of the class or pass a instance as a parameter.

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3 Comments

The relative precedence of * and .* is quite surprising, I almost mistakenly edited to add parentheses.
*this.* .. or this->*
Thank you very much for your concise explanation Jean-Baptiste! I hadn't realised I'd left the Actions:: off of the doAction method (and I feel a little bit silly now)!
0

There's a couple of things going on with your code you have posted:

First, you need to properly declare your Actions::doAction method to match the signature of your other doAction (or vice versa).

Next, since you've declared _ptrAction1 as a static member, you'll need to re-declare for linkage outside of the class (otherwise you'll get undefined reference errors).

After that, then you can call it like so: (this->*_ptrAction1)(...) when you are within an Actions context (that is, you can call this->* when you are in another member function of Actions).

The reason is you need an object instance to call a pointer-to-member function, so you don't explicitly need a this if you have a valid object of an Actions (see main function):

#include <iostream>
#include <string>

class Actions{
private:
    int actionId;
    int stateId;
    int eventId;
public:
    typedef std::string (Actions::*functionPtr)(int previousState, int currentState, int eventId);
    static functionPtr _ptrAction1;

    Actions(int actionId, int stateId, int eventId)
    {
        this->actionId = actionId;
        this->stateId = stateId;
        this->eventId = eventId;
        Actions::_ptrAction1 = &Actions::function1;
    }
    int doAction(int act, int previousState, int currentState, int eventId);
    std::string function1(int previousState, int currentState, int eventId);
};

// linker
Actions::functionPtr Actions::_ptrAction1;

std::string Actions::function1(int previousState, int currentState, int eventId){
    return "this is an example for now";
}

int Actions::doAction(int act, int previousState, int currentState, int eventId)
{
    int functionId = 0;
    std::string message;

    switch(act){
       case 1:
           message = (this->*_ptrAction1)(previousState, currentState, eventId);
           break;
       default:
           break;
       }
}

int main(int argc, char* argv[])
{
    Actions* act = new Actions(1,2,3);
    // have to say Actions::_ptrAction1 since it's a static member, but
    // we still need a valid Actions object to act on (for the hidden this)
    std::cout << (act->*Actions::_ptrAction1)(3,2,1) << std::endl;
    delete act;
    return 0;
}
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1 Comment

Thank you txtechhelp that is very helpful especially your explanation on re-declaration and * precedence. I will look to revise my code with your suggestions I'm afraid I don't have the rep to upvote otherwise I would do so! :)

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