C++: Using function pointers with member functions

Question

I'm trying to pass a function to another function as a parameter, and they both happen to be member functions of the same class.

I'm getting a weird error and I can't figure out what the problem is.

Here are my functions:

void myClass::functionToPass()
{
   // does something
}

void myClass::function1(void (*passedFunction)())
{
   (*passedFunction)();
}

void myClass::function2()
{
   function1( &myClass::functionToPass );
}

However, I'm getting the following error:

   cannot convert parameter 1 from 'void(__thiscall myClass::*) (void)' 
   to 'void(__cdecl*)(void)'

So what gives? I feel like I've tried every variation to try to get this to work. Can you even pass function pointers for member functions? How can I get this to work?

Note: Making functionToPass static isn't really a valid option.

The error message gives you all necessary info to fix the problem. — Kirill V. Lyadvinsky
– Kirill V. Lyadvinsky, Commented Aug 16, 2011 at 4:28
Using boost::function may be easier than using function pointers. You'd have to check whether that should be boost::function<void(void)> (in which case function2 should bind this) or boost::function<void(MyClass&)> (in which case function1 should pass *this) — MSalters
– MSalters, Commented Aug 16, 2011 at 8:52

john · Accepted Answer · 2011-08-16 04:11:53Z

11

You can pass function pointers to member functions. But that is not what your code is doing. You are confused between regular function pointers (void (*passedFunction)() is a regular function pointer) and pointers to member functions (&myClass::functionToPass is a pointer to a member function). They are not the same thing and they are not compatible.

You can rewrite your code like this

void myClass::function1(void (myClass::*passedFunction)())
{
   (this->*passedFunction)();
}

Now your code is using pointers to member functions, but of course this means you won't be able to pass a regular function pointer.

answered Aug 16, 2011 at 4:11

john

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1 Comment

Casey Patton Over a year ago

Makes sense! I don't have a chance to test this out until tomorrow but if it works out I'll definitely give an accept.

Dima · Accepted Answer · 2011-08-16 04:15:56Z

1

As pointed out by others, your mistake is in the type of the function pointer being passed. It should be void (myClass::*passedFunction)().

Here is a good tutorial on using pointers to member functions in C++.

edited Aug 16, 2011 at 4:15

answered Aug 16, 2011 at 4:07

Dima

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1 Comment

Dima Over a year ago

Sorry, I missed the fact that you were passing the function pointer to a member function of the same class. I thought you were passing it to some other function, in which case you would need an object of myClass to actually call the function pointed to by passedFunction.

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