Unresolved static symbols in constructor while using another class's static member function.

Question

I have two classes, Foo and Bar.

Foo.h

class Foo
{
public:
    static void Setm(int i) { m = i; }
private:
    static int m;
};

Bar.h

class Foo;
class Bar
{
 public:
     Bar(int m) {Foo::Setm(m);}
};

main.cpp

#include "Foo.h"
#include "Bar.h"
void main() {
   Bar bar(5);
}

When I compile I get these errors: 1>Foo.obj : error LNK2001: unresolved external symbol "private: static int Foo::m" (?m@Foo@@0HA) 1>Bar.obj : error LNK2001: unresolved external symbol "private: static int Foo::m" (?m@Fool@@0HA)

UpAndAdam · Accepted Answer · 2013-03-14 15:46:10Z

2

A static class member variable for any non integral/enum type must be initialized outside of its declaration/definition.

One such place can be the global scope from main.

i.e. in main.cpp:

 int Foo::m = 0;

edited Mar 14, 2013 at 15:46

UpAndAdam

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answered Mar 13, 2013 at 16:40

suspectus

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Comments

UpAndAdam · Accepted Answer · 2013-08-27 20:38:32Z

Couple of things.

You should include header guards:

Example:

//Foo.H
#ifndef __FOO_H__
#define __FOO_H__
...
...
#endif

Bar.H should be including Foo.H if its implementing a constructor that requires knowledge of Foo's implementation. (You are)
- This is another problem you will likely run into later.
The static member does NOT have to be initialized in global scope, it has to be defined once and only once.
- See below for further explanation. This answer is abbreviated from http://www.parashift.com/c++-faq/link-errs-static-data-mems.html where your exact problem is roughly described. and see http://www.parashift.com/c++-faq/static-const-with-initializers.html for slightly more information.

Despite what the other answers state, main is a horrible place to do this. Doing so requires everyone who ever uses this class to have know to do this in their main. How the heck would I know to define static constants in my main when I want to use your header (or even another developer on your team or for another block of code) ? The answer is I wouldn't. It's a lazy practice that will form a bad habit. And don't even get me started on static initialization/destruction fun.

You can't put it in Foo.H because doing so would result in linker errors from the redefinition by every compilation unit that includes your header.

A far better place to do this is in a newly created Foo.[c|.cxx|.cpp|.cc] (whatever suffix you use). Does main work? Yes it works... for you right now. Will it cause issues later? Probably. Is it good form and a solution to be remembered? Absolutely not.

Best Answer:

//Foo.C (NOT MAIN)  
#include<Foo.h>  
int Foo::m = 0;

What you should also notice is nothing about my answer has anything do with a second class using it in its constructor. Were I to have the points, I would edit the title of your question because it has nothing to do with constructors or class member functions. You are talking about static class methods/functions not member functions/methods.

masoud · Accepted Answer · 2013-03-13 16:40:33Z

0

Put

int FOO::m = 0;

Outside the class definition in a implementation file.

answered Mar 13, 2013 at 16:40

masoud

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Comments

4pie0 · Accepted Answer · 2013-03-13 16:56:21Z

0

you have to initialize static variable:

class Foo
{
public:
    static int m;
static void Setm(int i) { m = i; }
private:
};
int Foo::m=2;

class Foo;
class Bar
{
 public:
 Bar(int m) {Foo::Setm(m);}
};


int main(int argc, char** argv) {
    Foo::Setm(5);
    Bar b(5);
    return 0;
}

answered Mar 13, 2013 at 16:56

4pie0

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