C++ initialization of non constant static member variable?

Question

I got a qualification error of the member variable 'objectCount'. The compiler also returns 'ISO C++ forbids in-class intialization of non-const static member'. This is the main class:

#include <iostream>
#include "Tree.h"
using namespace std;

int main()
{
    Tree oak;
    Tree elm;
    Tree pine;

    cout << "**********\noak: " << oak.getObjectCount()<< endl;
    cout << "**********\nelm: " << elm.getObjectCount()<< endl;
    cout << "**********\npine: " << pine.getObjectCount()<< endl;
}

This is the tree class which contains the non-const static objectCount:

#ifndef TREE_H_INCLUDED
#define TREE_H_INCLUDED

class Tree
{
    private:
        static int objectCount;
    public:
        Tree()
        {
            objectCount++;
        }
        int getObjectCount() const
        {
            return objectCount;
        }
    int Tree::objectCount = 0;
}
#endif // TREE_H_INCLUDED

There is another alternative that wasn't mentioned in any of the suggested answers at the time of this writing, which allows you to **keep** everything in a **single header**. See the example in [this SO answer](http://stackoverflow.com/a/33618854/3041008), it maps perfectly to your example. — mucaho, Nov 09 '15 at 21:53

score 18 · Accepted Answer · answered Jul 16 '11 at 15:55

18

You have to define the static variable in the source file that includes this header.

#include "Tree.h"

int Tree::objectCount = 0;  // This definition should not be in the header file.
                            // Definition resides in another source file.
                            // In this case it is main.cpp

answered Jul 16 '11 at 15:55

Mahesh

33,625
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there is still the following error 'two or more data types in declaration of objectCount' – kifcaliph Jul 16 '11 at 22:16
I forgot to put this line int `Tree::objectCount = 0;` outside the class and I forgot to end the header class with semicolon thank you – kifcaliph Jul 16 '11 at 22:35
Why does this variable have to be declared outside of the class? I am trying to define and initialize a private variable, and I am getting this same error. How do I do this outside the class? – dangerChihuahua007 Feb 01 '12 at 03:31
@DavidFaux Show what are you trying to do. Only static const integral data members can be initialized within a class. – Mahesh Feb 01 '12 at 03:55

Nawaz · Answer 2 · 2011-07-16T16:01:17.290

5

int Tree::objectCount = 0;

The above line should be outside the class, and in .cpp file, as shown below:

//Tree.cpp 
#include "Tree.h"

int Tree::objectCount = 0;

edited Jul 16 '11 at 16:01

answered Jul 16 '11 at 15:55

Nawaz

341,464
111
648
831

score 3 · Answer 3 · answered Jul 16 '11 at 15:55

You need to define it outside the scope in a single C++ file, not in the header.

int Tree::objectCount = 0;
int main()
{
    Tree oak;
    Tree elm;
    Tree pine;

    cout << "**********\noak: " << oak.getObjectCount()<< endl;
    cout << "**********\nelm: " << elm.getObjectCount()<< endl;
    cout << "**********\npine: " << pine.getObjectCount()<< endl;
}

#ifndef TREE_H_INCLUDED
#define TREE_H_INCLUDED

class Tree
{
    private:
        static int objectCount;
    public:
        Tree()
        {
            objectCount++;
        }
        int getObjectCount() const
        {
            return objectCount;
        }
}
#endif // TREE_H_INCLUDED

C++ initialization of non constant static member variable?

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