error using static variable unresolved external symbol / undefined reference

Question

I am using static variable. After referring to Unresolved external symbol on static class members, I modified the program with Abc::ct

#include <iostream>

class Abc
{
private:
    static unsigned int ct;

public:
    void f1()
    {
        for (int i = 0; i < 5; ++i)
            f2();
    }

    void f2() {
        Abc::ct = 0;
        if (Abc::ct == 0)
            std::cout << "Zero iteration\n";

        std::cout << Abc::ct << "\t";
        ++Abc::ct;
    }
};



int main()
{
    Abc obj;
    obj.f1();
}

but getting error as error LNK2001: unresolved external symbol "private: static unsigned int Abc::ct" in MSVC or undefined reference to Abc::ct in g++. How can I define static variable in class Abc?

The `Abc::ct` must be defined outside your class, above `main`. — Robert Andrzejuk, Jul 24 '19 at 23:55
Any way to define `static` variable only in class? (without `const` of course) — ewr3243, Jul 24 '19 at 23:56
From your sample, instance data seems more appropriated that static data BTW — Jarod42, Jul 24 '19 at 23:57
@ewr3243 If C++17 or above is used then: `inline static unsigned int ct;` — Robert Andrzejuk, Jul 25 '19 at 08:59

Fureeish · Answer 1 · 2019-07-25T00:00:25.983

0

You declared your static variable, but you did not define and initialize it. Above main(), but outside of your class, add the following line:

unsigned int Abc::ct = 0;

or, if you are using C++17, you can change your:

static unsigned int ct;

to:

static inline unsigned int ct = 0;

edited Jul 25 '19 at 00:00

answered Jul 24 '19 at 23:54

Fureeish

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Jarod42 · Answer 2 · 2019-07-25T00:01:37.000

0

You have to define it:

unsigned int Abc::ct = 0;

Demo

edited Jul 25 '19 at 00:01

answered Jul 24 '19 at 23:55

Jarod42

190,553
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What do you mean? Where to define? What about `static`? – ewr3243 Jul 24 '19 at 23:58

error using static variable unresolved external symbol / undefined reference

2 Answers2