c ++中对静态函数指针成员的未定义引用，我做错了什么？

Question

please consider these files:

ph:

#ifndef _p_h_
#define _p_h_

class p{
public:    
    static void set_func(int(*)());

private:
    static int (*sf)();

};
#endif

p.cpp:

#include "p.h"
#include <cstdio>

int (p::*sf)() = NULL;    //defining the function pointer

void p::set_func(int(*f)()){
    sf = f;
}

main.cpp:

#include "p.h"
#include <iostream>

int function_x(){
        std::cout << "I'm function_x()" << std::endl;
        return 1234;
}

int main(){
        p::set_func(function_x);
}

when compiling, I get this:

$ g++ -o pp main.cpp p.cpp
/tmp/ccIs0M7r.o:p.cpp:(.text+0x7): undefined reference to `p::sf'
collect2: ld returned 1 exit status

but:

$ g++ -c -o pp p.cpp

compiles right.

What's wrong with the code? I just can't find where the problem is, please your help will be more than appreciated.

Thanks.

Answer 1

Your attempt at defining p::sf is incorrect – yours is a definition of a global variable named sf that is of type int (p::*)() , ie a pointer to a member function. Consequently p::sf remains undefined, hence the linker error.

Try this instead:

int (*p::sf)() = 0;

// or,

typedef int (*p_sf_t)();
p_sf_t p::sf = 0;

Answer 2

The difference is because error only occurs when you actually link the program. The problem is in your declaration of the static function pointer. The correct syntax is:

int (*p::sf)() = NULL;    //defining the function pointer

Answer 3

You define a member function pointer and not a function pointer. I'm not sure what the correct syntax is, but I would have tried something like this:

int (*p::sf)() = NULL; 

Answer 4

I will not give another answer (ildjarn answer is correct) but i will suggest you another way of achieving the same without static initialization (and the burdens it implies)

class p{
public:  
    typedef int (*func_t)();  
    static void set_func(func_t v) { 
      func_t& f = getFuncRef();
      f = v;
    }

    static void call_func() {
      func_t& f = getFuncRef();
      assert( f != 0);
      f();
    }

private:

    static func_t& getFuncRef() {
     static func_t sf = 0;
     return sf;
    }

};

in this way you delegate the static initialization to a static function variable, which doesn't have the initialization order problems that affect static data variables, and is lazy-initialised