Pointer to static member variable

Question

Possible Duplicate:
C++: undefined reference to static class member

The following C++ code compiles well (using g++ -c) but it doesn't link giving the error: undefined reference toAbc::X'`

#include <iostream>

using namespace std;

class Abc {

public:
    const static int X = 99;
};

int main()
{
    Abc a1;
    cout << &(Abc::X) << endl;
}

I want to know why this is not allowed?

YePhIcK · Answer 1 · 2018-09-28T23:14:52.253

4

You need to have that static member actually defined, not just declared...

Add this line before your main():

const int Abc::X = 99;

As of C++17 you can also do an inline static, in which case the above additional line of code in a .cpp file is not needed:

class Abc {

public:
    inline const static int X = 99; // <-- "inline"
};

edited Sep 28 '18 at 23:14

answered Jul 15 '12 at 07:15

YePhIcK

5,816
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1

Also change the statement inside class to this: const static int X; – Zohaib Jul 15 '12 at 07:18
1

I was hoping Ravi would do that on his own as an additional credit ;-) – YePhIcK Jul 15 '12 at 07:20

score 1 · Answer 2 · answered Jul 15 '12 at 07:24

1

If you don't like to think about translation units, static initialization order and stuff like that, just change your static constants into methods.

#include <iostream>
using namespace std;

class Abc {

public:
    inline static const int& X(){ 
      static int x=99;
      return x; 
    }
};

int main()
{
//    Abc a1;
    cout << &(Abc::X()) << endl;
}

answered Jul 15 '12 at 07:24

Johan Lundberg

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In fact not looking for a quick solution. Would be glad to read detailed explanation (or links to explanations) about "translation units, static initialization order and stuff like that". – Xolve Jul 15 '12 at 07:47
Right. There's a lot about those words on SO and google. Or even better, any entry-level book from this list: http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list, perhaps I'll add more later. – Johan Lundberg Jul 15 '12 at 08:12

score 1 · Accepted Answer · answered Jul 15 '12 at 10:24

1

If the static member is used in a way which requires an lvalue (i.e. in a way that requires it to have an address) then it must have a definition. See the explanation at the GCC wiki, which includes references to the standard and how to fix it.

answered Jul 15 '12 at 10:24

Jonathan Wakely

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Pointer to static member variable

3 Answers3