Why Does 'undefined reference to WindowsTimer::_frequency' Occur and How Is It Fixed?

Undefined Reference to a Static Member

This error occurs when the compiler cannot find the definition of a static member variable. In this case, it pertains to the static member variable _frequency of the WindowsTimer class.

Understanding the Code

The code presented is:

class WindowsTimer {
public:
  WindowsTimer() {
    _frequency.QuadPart = 0ull;
  }
private:
  static LARGE_INTEGER _frequency;
};

Here, _frequency is a static member variable of type LARGE_INTEGER, which is declared but not defined within the class.

Resolving the Error

The error message "undefined reference to WindowsTimer::_frequency" indicates that the compiler cannot find the definition of _frequency. Static member variables must be defined outside the class declaration, typically in the implementation (.cpp) file.

To resolve the error, add the following definition to the .cpp file:

LARGE_INTEGER WindowsTimer::_frequency;

This will create a global variable of type LARGE_INTEGER named _frequency that is linked to the WindowsTimer class.

Why the Other Changes Didn't Work

• Changing _frequency to _frequency.QuadPart would only define the QuadPart member of the LARGE_INTEGER struct, but not the _frequency itself.
• Changing _frequency to static LARGE_INTEGER _frequency.QuadPart would attempt to redefine _frequency as a static variable with type LARGE_INTEGER.QuadPart, which is incorrect.

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