Why won't my C++ program link when my class has static members?

Question

I have a little class called Stuff that I want to store things in. These things are a list of type int. Throughout my code in whatever classes I use I want to be able to access these things inside the Stuff class.

Main.cpp:

#include "Stuff.h"

int main()
{
    Stuff::things.push_back(123);
    return 0;
}

Stuff.h:

#include <list>

class Stuff
{
public:
    static list<int> things;
};

but I get some build errors with this code:

error LNK2001: unresolved external symbol "public: static class std::list<int,class std::allocator<int> > Stuff::things" (?things@Stuff@@2V?$list@HV?$allocator@H@std@@@std@@A) Main.obj CSandbox

fatal error LNK1120: 1 unresolved externals C:\Stuff\Projects\CSandbox\Debug\CSandbox.exe CSandbox

I am a C# guy, and I am trying to learn C++ for a side project. I think that I don't understand how C++ treats static members. So please explain what I have got wrong here.

Pattani

Might I politly suggest that you try to format your code sample? :-) – Eberta

I would, if I knew how, though I notice that Greg was kind enough to do it for me – Pattani

I was going to put my answers and...2 new answers. Just because I knew it. – Minnesota

Answer 1

Mentioning a static member in a class declaration is a declaration only. You must include one definition of the static member for the linker to hook everything up properly. Normally you would include something like the following in a Stuff.cpp file:

#include "Stuff.h"

list<int> Stuff::things;

Be sure to include Stuff.cpp in your program along with Main.cpp.

Dyna

@Greg - that should be list<int> Stuff::things; – Roundish

Thanks, just noticed and fixed that :) – Dyna

Answer 2

Static data members have to be defined outside class declarations, much like methods.

For example:

class X {
    public:
        static int i;
};

Must also have the following:

int X::i = 0; // definition outside class declaration

Yoshi

Answer 3

Stuff::things is only declared, but it is not defined.

please use:

// Stuff.cpp
#include "Stuff.h"

std::list<int> Stuff::things;

Added: it is also a good practice to protect your header files against multiple inclusion:

// Stuff.h
#ifndef STUFF_H_
#define STUFF_H_

#include <list>

class Stuff {
    public:
       static std::list<int> things;
};

#endif

Cenac

Answer 4

Just For your Information, why this works is that in C++ all global variables (including static global) are created before the execution of the main function begins.

Sava

Answer 5

Static member have to be declared in the class but defined in the unit (cpp file) where it is really located.

The only exception is in case the class is template: in this case you have to define the member outside the class, but you have to provide it too with the class declaration, in the header file.

Abbotson