How do you pass a member function pointer?

Question

I am trying to pass a member function within a class to a function that takes a member function class pointer. The problem I am having is that I am not sure how to properly do this within the class using the this pointer. Does anyone have suggestions?

Here is a copy of the class that is passing the member function:

class testMenu : public MenuScreen{
public:

bool draw;

MenuButton<testMenu> x;

testMenu():MenuScreen("testMenu"){
    x.SetButton(100,100,TEXT("buttonNormal.png"),TEXT("buttonHover.png"),TEXT("buttonPressed.png"),100,40,&this->test2);

    draw = false;
}

void test2(){
    draw = true;
}
};

The function x.SetButton(...) is contained in another class, where "object" is a template.

void SetButton(int xPos, int yPos, LPCWSTR normalFilePath, LPCWSTR hoverFilePath, LPCWSTR pressedFilePath, int Width, int Height, void (object::*ButtonFunc)()) {

    BUTTON::SetButton(xPos, yPos, normalFilePath, hoverFilePath, pressedFilePath, Width, Height);

    this->ButtonFunc = &ButtonFunc;
}

If anyone has any advice on how I can properly send this function so that I can use it later.

Solution 1

To call a member function by pointer, you need two things: A pointer to the object and a pointer to the function. You need both in MenuButton::SetButton()

template <class object>
void MenuButton::SetButton(int xPos, int yPos, LPCWSTR normalFilePath,
        LPCWSTR hoverFilePath, LPCWSTR pressedFilePath,
        int Width, int Height, object *ButtonObj, void (object::*ButtonFunc)())
{
  BUTTON::SetButton(xPos, yPos, normalFilePath, hoverFilePath, pressedFilePath, Width, Height);

  this->ButtonObj = ButtonObj;
  this->ButtonFunc = ButtonFunc;
}

Then you can invoke the function using both pointers:

((ButtonObj)->*(ButtonFunc))();

Don't forget to pass the pointer to your object to MenuButton::SetButton():

testMenu::testMenu()
  :MenuScreen("testMenu")
{
  x.SetButton(100,100,TEXT("buttonNormal.png"), TEXT("buttonHover.png"),
        TEXT("buttonPressed.png"), 100, 40, this, test2);
  draw = false;
}

Solution 2

I'd strongly recommend boost::bind and boost::function for anything like this.

See Pass and call a member function (boost::bind / boost::function?)

Solution 3

I know this is a quite old topic. But there is an elegant way to handle this with c++11

#include <functional>

declare your function pointer like this

typedef std::function<int(int,int) > Max;

declare your the function your pass this thing into

void SetHandler(Max Handler);

suppose you pass a normal function to it you can use it like normal

SetHandler(&some function);

suppose you have a member function

class test{
public:
  int GetMax(int a, int b);
...
}

in your code you can pass it using std::placeholders like this

test t;
Max Handler = std::bind(&test::GetMax,&t,std::placeholders::_1,std::placeholders::_2);
some object.SetHandler(Handler);

Solution 4

Would you not be better served to use standard OO. Define a contract (virtual class) and implement that in your own class, then just pass a reference to your own class and let the receiver call the contract function.

Using your example (I've renamed the 'test2' method to 'buttonAction'):

class ButtonContract
{
  public:
    virtual void buttonAction();
}


class testMenu : public MenuScreen, public virtual ButtonContract
{
  public:
    bool draw;
    MenuButton<testMenu> x;

    testMenu():MenuScreen("testMenu")
    {
      x.SetButton(100,100,TEXT("buttonNormal.png"), 
              TEXT("buttonHover.png"), 
              TEXT("buttonPressed.png"), 
              100, 40, &this);
      draw = false;
    }

    //Implementation of the ButtonContract method!
    void buttonAction()
    {
      draw = true;
    }
};

In the receiver method, you store the reference to a ButtonContract, then when you want to perform the button's action just call the 'buttonAction' method of that stored ButtonContract object.

Solution 5

Others have told you how to do it correctly. But I'm surprised no-one told you this code is actually dangerous:

this->ButtonFunc = &ButtonFunc;

Since ButtonFunc is a parameter, it will go out of scope when the function returns. You are taking its address. You will get a value of type void (object::**ButtonFunc)() (pointer to a pointer to a member function) and assign it to this->ButtonFunc. At the time you would try to use this->ButtonFunc you would try to access the storage of the (now not existing anymore) local parameter, and your program would probably crash.

I agree with Commodore's solution. But you have to change his line to

((ButtonObj)->*(ButtonFunc))();

since ButtonObj is a pointer to object.

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