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[c++] Passing a function ptr to a member function

Started by VanillaSnake21 Sep 19, 2009 at 6:57 PM 7 replies 1.4k views

Original Post

Sep 19, 2009 at 6:57 PM

Hi, I'm having trouble finding info on how to pass a function as a pointer to a member function, this example will explain it more clearly:


class Foo
{
public:

void SetRenderCallback(int (*SomeFunc)() )
{
    m_Callback = SomeFunc;
}

void Render()
{
   //call the callback
   m_Callback();
}

private:

//somehow declaring a pointer to a function;
int (*m_Callback) ();
};

// another file
Foo FooClass;
int MyCallback();
FooClass.SetRenderCallback(&MyCallback);
FooClass.Render();

What would be the correct way to accomplish that? Thanks.

Denzin

Sep 19, 2009 at 7:07 PM

whats the error your getting?

VanillaSnake21

Sep 19, 2009 at 7:09 PM

Quote:
Original post by Denzin
whats the error your getting?

Well actually I havent really tried to compile it because I wanted to be at least 50% sure of what I'm doing. I just threw together the above code from how I think it might look.

FReY

Sep 19, 2009 at 7:28 PM

This should do it:

typedef int (*RenderCB)();class Foo{public:void SetRenderCallback(RenderCB renderCB ){    m_renderCB = renderCB;}void Render(){   //call the callback   m_renderCB();}private:   RenderCB m_renderCB;};// another fileFoo FooClass;int MyCallback();FooClass.SetRenderCallback(MyCallback);FooClass.Render();

VanillaSnake21

Sep 19, 2009 at 7:48 PM

Thanks FReY! Everything seems to compile perfectly except for the part where I pass my function as the pointer:

Foo class;
class.SetCallback(&MyClass::MyCallback);

I forgot to mention that I will be calling the SetCallback from another class, so technically MyCallback is a member function. How would I pass it? Right now I'm getting this error:

Foo::SetRenderCallback' : cannot convert parameter 1 from 'int (__thiscall MyClass::* )(void)' to 'RenderCallback'

FReY

Sep 19, 2009 at 8:58 PM

Don't use &MyClass::MyCallback, ie. just do this:

Foo class;class.SetCallback(MyClass::MyCallback);

Also, you'll need to make sure MyClass::MyCallback is a static class function.

ie.
class MyClass
{
static int MyCallback();
};

nobodynews

Sep 19, 2009 at 9:06 PM

Read this for more information on why you can't use function pointers with member functions as well as ways around this limitation.

VanillaSnake21

Sep 19, 2009 at 9:42 PM

Thanks guys, but while I was waiting for a response I managed to think my way around using a function ptr. I came up with this:

class CallbackFunction{public:~virtual CallbackFunction() {}virtual Function() = 0;};class SuperClass{private: class MyCallback : public CallbackFunction {   Function() { ... } };}m_RenderCallback;

Then I just pass the m_RenderCallback to a function.

Thanks everyone :)

Zahlman

Sep 20, 2009 at 12:05 AM

Yes, that is a good technique. But in C++, don't use a named member function for this; instead, overload the operator(). That allows you to treat the variable name for the instance as a "function name".

More here, especially near the end.

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