C++ static pointer to function

Question

I would like to have a private static pointer to a function in my class. Basically, it would look like this:

//file.h
class X {
private:
    static int (*staticFunc)(const X&);
    ...
public:
    void f();

};


//file.cpp
void X::f()
{
    staticFunc(*this);
}

This gives me an "unresolved external symbol" error. I know that static members must be initialized in the .cpp too, I've tried this:

int (X::*staticFunc)(const X&) = NULL;

but this gives me an "initializing a function" error. It gives me an uglier error if I try to initialize it with an existing function. Without "= NULL", I get the same error.

Thanks.

Did you try staticFunc = NULL; in the .cpp file? (Note: This is me forgetting if you have to redeclare the type in the .cpp file, sorry) — Platinum Azure
– Platinum Azure, Commented Nov 30, 2010 at 18:16
Sorry, I wrote the code on the run. I corrected it now, of course I was passing a parameter to staticFunc when using it. And I was actually writing in the .cpp file "int (X::*staticFunc)(const X&)" instead of "int (*X::staticFunc)(const X&)". Thanks a lot for your answers, it was a stupid mistake. — stonecup64
– stonecup64, Commented Nov 30, 2010 at 18:39

TonyK · Accepted Answer · 2010-11-30 21:12:47Z

4

//file.cpp  
int (*X::staticFunc)(const X&);

void X::f()  
{  
staticFunc(*this);  
}

edited Nov 30, 2010 at 21:12

answered Nov 30, 2010 at 18:20

TonyK

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3 Comments

j_random_hacker Over a year ago

Yep. Spelling it out, a static data member declaration in a class is just a declaration -- it also needs a separate definition outside of the class, in exactly 1 translation unit (source code file).

aschepler Over a year ago

You can add an initializer = NULL or = &someFunc to the X::staticFunc definition. The name of the static member you want to initialize is X::staticFunc, so you need to qualify it that way. Without the X::, C++ thought you were creating a different declaration.

TonyK Over a year ago

'= NULL' is redundant. This is guaranteed.

EboMike · Accepted Answer · 2010-11-30 18:21:55Z

2

It's a member of X, so you need to say

int (*X::staticFunc)(const X&) = NULL;

Otherwise, you'd just create a global variable called staticFunc which is not related to that static member of X.

answered Nov 30, 2010 at 18:21

EboMike

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Comments

Edward Strange · Accepted Answer · 2010-11-30 18:22:23Z

0

Couple problems here.

First error is that you're not passing a parameter in your attempt to use staticFunc. This should cause a compiler error that you're not reporting.

Second issue is that your syntax is wrong. TonyK got that one.

answered Nov 30, 2010 at 18:22

Edward Strange

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