Compilation failed, c++ program with static variable as private member variable

Question

Possible Duplicate:
undefined reference to static member variable
What is an undefined reference/unresolved external symbol error and how do I fix it?

#include<iostream>
using namespace std;

class abc {

    private:
    static int a ;

    public:

    abc(int x) {
        a = x;
    }

    void showData() {
        cout<<"A = "<<a<<endl;
    }
};

int main() {
    abc a1(4);
    abc a2(5);

    a1.showData();
    a2.showData();

    return 0;
}

When I try to compile this function on Ubuntu with GCC compiler. I get the following error.

/tmp/ccCKK2YN.o: In function `main':
static1.cpp:(.text+0xb): undefined reference to `Something::s_nValue'
static1.cpp:(.text+0x14): undefined reference to `Something::s_nValue'
collect2: ld returned 1 exit status
Compilation failed.

Where as the following code runs fine

#include<iostream>
using namespace std;

class Something
{
public:
    static int s_nValue;
};

int Something::s_nValue = 1;

int main()
{
    Something cFirst;
    cFirst.s_nValue = 2;

    Something cSecond;
    std::cout << cSecond.s_nValue;

    return 0;
}

Is this because Static member variables needs to initialized explicitly before accessing them via objects.Why so ?

See http://stackoverflow.com/questions/12573816/what-is-an-undefined-reference-unresolved-external-symbol-error-and-how-do-i-fix/12574407#12574407 - more specifically "static data members must be defined outside the class in a single translation unit" — Luchian Grigore, Oct 29 '12 at 16:26

score 1 · Accepted Answer · answered Oct 29 '12 at 16:30

1

static int s_nValue; doesn't allocate any storage to store the int, it just declares it.

You allocate somewhere in memory to store the variable with:

int Something::a=0;

answered Oct 29 '12 at 16:30

Douglas Leeder

52,368
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What would just `int Something::a;` do? – Luchian Grigore Oct 29 '12 at 16:31
Not sure if static members get auto-initialised or not: either undefined value, or 0 depending. – Douglas Leeder Oct 29 '12 at 16:34
In general, all static storage variables (globals and the likes - including static members) are value-initialized - so to `0` in this case. – Luchian Grigore Oct 29 '12 at 16:35
But I used constructor to initialize the static variable. Whats wrong with that ? – hsinxh Oct 29 '12 at 16:35
You don't have any constructors in your code. – Douglas Leeder Oct 30 '12 at 17:55

score 0 · Answer 2 · edited May 23 '17 at 10:26

0

The declaration of a static data member in the member list of a class is not a definition. You must define the static member outside of the class declaration, in namespace scope.

See this thread.

In short, the static member needs to be initialized somewhere in a .cpp file so that the compiler allocates space for it. The declaration would look like this:

int abc::a = 0;

edited May 23 '17 at 10:26

Community

1
1

answered Oct 29 '12 at 16:30

Matt Kline

10,149
7
50
87

But I used constructor to initialize the static variable. Whats wrong with that ? – hsinxh Oct 29 '12 at 16:36
You can assign values to static variable in your constructor like you did, but you still need to define it in a `.cpp` file. – Matt Kline Oct 29 '12 at 16:38

score 0 · Answer 3 · answered Oct 29 '12 at 16:31

That happens because since static members are shared between all instances of a class, they need to be declared in one single place.

If you define the static variable inside the class declaration then each include to that file would have a definition to that variable (which is against to the static meaning).

Because of that you have to define the static members in the .cpp.

Compilation failed, c++ program with static variable as private member variable

3 Answers3

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