Using a static member in a class

[feraudyh]

Hello,
This is just about C++ and it's very simple.
I'm using QtCreator and MingW .
I define a new console project with the usual main.cpp and a class called myclass. The header is as follows:

class MyClass
{
public:
MyClass();
static int getInstanceCount(){return _instanceCount;}
private:
static int _instanceCount;
};

The code is as follows

#include "myclass.h"

MyClass::MyClass()
{
_instanceCount++;
}

The code of main is as follows:

#include <QtCore/QCoreApplication>
#include <iostream>
using namespace std;

#include "myclass.h"
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
MyClass foo;
cout<<MyClass::getInstanceCount();
return a.exec();
}

The trouble is that this fails at link time with the repeated message that there is an
undefined reference to MyClass::_instanceCount

What have I done wrong and what can I do to fix it?
Thankyou.

[psih128]

your static member has been declared, but not initialized.

Put this in your .cpp file:

int MyClass::_instanceCount = 0;

int MyClass::_instanceCount = 0;

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Or change the declaration in the .h file:

static int _instanceCount = 0;

static int _instanceCount = 0;

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BTW, this is not related to Qt in any way

[feraudyh]

I realise that this is not related to Qt in any way. It was just not obvious to find a thread to ask about it and it was pretty easy to answer.

Thank you.

[squidge]

So a better forum would be General programming, rather than Qt programming, which clearly states "For general, non-Qt programming issues."

[feraudyh]

Yes, how embarassing.