Why Do I Get 'Undefined Reference to Static Member' Errors During Cross Compilation?

Unresolved Reference to a Static Member in Cross Compilation

When working with cross compilers, it's common to encounter errors related to unresolved references to static members. This occurs when a static variable is declared in a class header but not defined in the corresponding .cpp file.

Problem Explanation:

Consider the provided code example:

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

The static variable _frequency is declared within the class definition but is not defined. When attempting to build the code, the linker fails to resolve the reference to _frequency, resulting in an undefined reference error.

Solution:

To resolve this issue, the static variable must be defined in the corresponding .cpp file. This can be done as follows:

// WindowsTimer.cpp
LARGE_INTEGER WindowsTimer::_frequency;

Reasoning:

Static variables, unlike instance variables, are shared among all instances of a class. Therefore, they require a single definition in the program. By defining the static variable in the .cpp file, the linker can successfully locate and resolve the reference to it.

Additional Notes:

• The error message "undefined reference to WindowsTimer::_frequency'" explicitly indicates that the linker is unable to find a definition for the variable _frequency in the context of the class WindowsTimer`.
• Modifying the declaration of _frequency as LARGE_INTEGER _frequency.QuadPart = 0ull; or static LARGE_INTEGER _frequency.QuadPart = 0ull; will not solve the issue, as these declarations only apply to the individual instance of _frequency within the object and not to the static definition.
• It's important to ensure that the definition of the static variable in the .cpp file matches the declaration in the header file. Mismatches can lead to unexpected behavior or compilation errors.

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