Using function pointers?

Question

I'm not yet very familiar with these but maybe someone can shine light on this example.

Imagine I have class CFoo and it will have a function to add a handler and a function which is a function pointer.

So something like this:

class CFoo {

int *pointedFunc(int a, int b) = 0;

void setFunc(int *func(int a, int b))
{
    pointedFunc = func;
}
};

Given the above context, I want to know the proper way of doing this. I don't think I have done it properly. Also, how would I go about calling pointedFunc?

Thanks

Not "a function which is a function pointer", but "a member variable (known in OO as a field) which is a function pointer". — Ben Voigt
– Ben Voigt, Commented Oct 1, 2010 at 0:39
Please avoid function pointers in C++ code. Define an interface and create types that implement that interface. You can then store objects neatly without having to resort to C hocks-pockus of function pointers. — Loki Astari
– Loki Astari, Commented Oct 1, 2010 at 1:04
@Martin: Sometimes an interface is the right approach, sometimes templated (and duck-typed) functors, and sometimes function pointers are best. Especially for dynamic linking, function pointers are needed. — Ben Voigt
– Ben Voigt, Commented Oct 1, 2010 at 5:20

Community · Accepted Answer · 2017-05-23 11:48:33Z

8

Right now you have a member function returning int *, not a pointer to a function returning int. A set of parenthesis would fix that.

int (*pointedFunc)(int a, int b);

void setFunc(int (*pfunc)(int a, int b))
{
    pointedFunc = pfunc;
}

Also, member variables get initialized in the constructor ctor-initializer-list, like

CFoo::CFoo() : pointedFunc(0) {}

not like you did. Your = 0 was actually a pure-specifier (and it won't work unless the member function is virtual), when you fix the pointer-return-type vs pointer-to-function issue you'll find that the compiler also complains about your attempt to initialize it.

Using a typedef as Graeme suggests is the easiest and best way to keep track of function-pointer types.

edited May 23, 2017 at 11:48

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answered Oct 1, 2010 at 0:35

Ben Voigt

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5 Comments

Ben Voigt Over a year ago

It's not so much that it's only for virtual functions, as that = 0 means something completely different (cue Monty Python) when it appears in a function declaration as when it appears in a data declaration. Actually, int (*pointedFunc)(int a, int b) = 0; would be perfectly OK as a local variable, which can be initialized, but member variables get initialized in the constructor and not in the declaration.

Bryan Over a year ago

While you're certainly correct about setting a function itself = 0 being a pure-specifier, I'm under the impression you can set a function pointer to NULL with no issues, which I believe was the original intent.

Ben Voigt Over a year ago

@Bryan: absolutely you can set it (the variable with function-pointer type) to NULL. But not in the declaration of a non-static member variable. Non-static member variables get initialized in a ctor-initializer-list.

Ben Voigt Over a year ago

Oh... I submitted my answer partly written so I could check what his class name was, I guess you saw that before I added the ctor-initializer-list example.

Bryan Over a year ago

@Ben: Oh... duh. I knew that. Hah, thanks for keeping me straight.

Graeme Perrow · Accepted Answer · 2010-10-01 00:38:49Z

3

In your example, pointedFunc is a member function that returns an int *. To make it a function pointer, you need parens around pointedFunc, like:

int (*pointedFunc)( int a, int b );

A typedef might make it clearer:

class CFoo {
    CFoo() : pointedFunc( NULL ) {}
    typedef int (*funcType)(int, int);
    funcType pointedFunc;

    void setFunc( funcType f ) {
         pointedFunc = f;
    }
};

To call the function, you can use either pointedFunc( 1, 2 ) or (*pointedFunc)(1, 2). I tend to use the latter to make it clear that you are going through a function pointer, but either will work.

edited Oct 1, 2010 at 0:38

answered Oct 1, 2010 at 0:33

Graeme Perrow

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1 Comment

Ben Voigt Over a year ago

No it's not fine, right now he has a member function named pointedFunc and the return type is int*.

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