Why Am I Getting an 'Undefined Reference to Static Variable' Error in C ?

Resolving "Undefined Reference to Static Variable" Error in C

When working with static class variables, you may encounter the "undefined reference to static variable" error in C . This article will explore the issue and provide a solution.

Consider the following code as an example:

class Helloworld {
  public:
    static int x;
    void foo();
};

void Helloworld::foo() {
  Helloworld::x = 10;
}

Upon compiling this code, you may receive the error "undefined reference to 'Helloworld::x'". This is because you have not provided a definition for the static variable x.

In C , static class variables must be defined outside the class definition. Simply adding a definition for x in the global scope will resolve the error:

int Helloworld::x; // Define the static variable

You can initialize x with any appropriate value. If no initializer is provided, it will be zero-initialized by default.

Therefore, the modified code will look like this:

class Helloworld {
  public:
    static int x;
    void foo();
};

int Helloworld::x = 0; // Initialize the static variable

void Helloworld::foo() {
  Helloworld::x = 10;
}

Now, the compiler will be able to correctly locate the definition of x and resolve the error. Remember, even though the foo() function is not static, it can still access the static variable x of the Helloworld class.

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