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function pointer passed from within a class

General and Gameplay Programming Programming

Started by Funkymunky June 28, 2008 01:01 PM

9 comments, last by Funkymunky 15 years, 9 months ago

Funkymunky

1,420

Author

June 28, 2008 01:01 PM

I'm a little new to function pointers (C++), and this part is confusing me. I have a global function for Keypress callbacks, like so:


void KeyPressCallback(int k, void (*p)());

I have a class which wants to use this function. Except I can't figure out how to pass one of its member functions ("void AKey();") as the parameter. I've tried these from within the class's initialization function:


KeyPressCallback(keyA, this->AKey);
KeyPressCallback(keyA, &ManagerClass::AKey);

both spur errors. I also tried this from the function which calls the class's initialization function, after that call:


KeyPressCallback(keyA, gManager.AKey);

and still no go. So, what would be the correct syntax/method for doing what I'm trying to do?

Gage64

1,235

June 28, 2008 01:06 PM

You are getting errors because the function expects a pointer to a free (i.e., non-member) function, and you're trying to pass it a pointer to a member function.

If you want the function to accept a pointer to a member function, the signature should look like this:

void KeyPressCallback(int k, void (MyClass::*p)());

BTW, of the 3 function calls you listed, the second one is the correct one.

EDIT: You also need to pass a pointer to the object that the function is to be invoked on. See here and here for more info on pointers to members.

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fpsgamer

856

June 28, 2008 01:10 PM

It is not enough to just supply a member function pointer. You must also supply an instance to invoke the function on.

nullsquared

128

June 28, 2008 01:12 PM

Except now you'll need an instance of the class to call the member function pointer on:

someInstance->(*memberFunctionPointer)(parameters);

Otherwise you'll want to use static functions.

www.nullular.com

Funkymunky

1,420

Author

June 28, 2008 01:32 PM

thorough and near-instantaneous replies, thank you

Funkymunky

1,420

Author

June 28, 2008 03:02 PM

hmmm... can I do it with a class member function that is inherited from a pure virtual class? like this:

class Manager{public:    virtual void AKey() = 0;};class WorldManager : public Manager{public:    void Init();    void AKey() { // do something }};void KeyPressCallback(int k, Manager *M, void (Manager::*p)());void WorldManager::Init(){    KeyPressCallback(keyA, this, &WorldManager::AKey);}

This gives me an error that it can't covert the function pointer namespace from WorldManager to Manager... is there a way to?

[Edited by - Funkymunky on June 28, 2008 3:02:58 PM]

fpsgamer

856

June 28, 2008 03:06 PM

Quote:Original post by Funkymunky
This gives me an error that it can't covert the function pointer namespace from WorldManager to Manager... is there a way to?

It might have something to do with the fact that you forgot to have WorldManager inherit from Manager.

class WorldManager : public Manager{public:    void Init();    void AKey() { // do something }};

But why use a function pointer and a free function in the first place? Why not just utilize polymorphsim ?

void handleKeyEvent(Manager& man){     man->AKey(); // All managers define this function}

Supplying an instance and a function pointer is redundant. You're trying to apply C style callback idioms to C++ when they aren't really necessary.

Antheus

2,410

June 28, 2008 03:25 PM

Quote:Original post by Funkymunky
hmmm... can I do it with a class member function that is inherited from a pure virtual class? like this:

This gives me an error that it can't covert the function pointer namespace from WorldManager to Manager... is there a way to?

So you have control over callback design?

Why use such way in the first place then? This approach is usually used for legacy C API, which relied on function pointers.

Under C++, you have choice of polymorphism or boost::function (and equivalent solutions).

Funkymunky

1,420

Author

June 28, 2008 04:02 PM

oops, the code is just a simple representation of what I'm attempting, I edited it such that WorldManager inherits from Manager (which I have in my full code already)

I'm essentially attempting to have a generic callback method for keypresses, such that any class which inherits from Manager can specify a member function of its own that should be called when a key is pressed.

fpsgamer

856

June 28, 2008 04:09 PM

Quote:Original post by Funkymunky
oops, the code is just a simple representation of what I'm attempting, I edited it such that WorldManager inherits from Manager (which I have in my full code already)

I'm essentially attempting to have a generic callback method for keypresses, such that any class which inherits from Manager can specify a member function of its own that should be called when a key is pressed.

boost::function and boost::bind

function pointer passed from within a class

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