undefined reference error due to use of static var

Question

This question already has an answer here:

• static variable link error 2 answers

I asked a question earlier today about singletons, and I'm having some difficulties understanding some errors I encountered. I have the following code:

Timing.h

class Timing {

public:
    static Timing *GetInstance();
private:
    Timing();
    static Timing *_singleInstance;
};

Timing.cpp

 #include "Timing.h"

 static Timing *Timing::GetInstance() {  //the first error
    if (!_singleInstance) {
        _singleInstance = new Timing();  //the second error
    }
    return _singleInstance;
}

There are two errors in this code which I can't figure out.

1. The method GetInstance() is declared in the header as static. Why in the cpp file do I have to omit the word static? It gives the error: "cannot declare member function ‘static Timing* Timing::GetInstance()’ to have static linkage". The correct way to write it is:

   Timing *Timing::GetInstance() { ... }  
   

2. Why can't I write _singleInstance = new Timing();? It gives the error: "undefined reference to Timing::_singleInstance". I solved this error by defining _singleInstance as a global var in the cpp file.

Answer 1

1: static means "local linkage" when used for a function declaration/definition outside a class-declaration.

Local linkage means that the particular function can only be referenced from code inside this particular file, and that doesn't make much sense with a method in a class.

2: Since your class declaration can be included multiple times, the actual storage for the static member should be defined in the cpp-file:

#include "Timing.h"

Timing* Timing::_singleInstance;

Timing *Timing::GetInstance() {  //the first error
    if (!_singleInstance) {
        _singleInstance = new Timing();  //the second error
    }
    return _singleInstance;
}

Answer 2

Referencing to question 2: You need to specify the static variable at the top of your cpp-file:

Timing* Timing::_singleInstance = NULL;

Answer 3

In the definition, you need to omit the static keyword. Its because that's teh syntax of C++. Nothing big.

Once you fix error number 1, error number 2 will be fixed automatically.

Answer 4

1. static within a class means something completely different than static outside of it. Yeah, not the greatest design decision of C++, but, we have to live with it.

2. I imagine the whining comes from the linker, and it's because you have declared that variable but never defined it, making it an undefined references. Just add in your .cpp file a line like:

   Timing* Timing::_singleInstance;
   

Answer 5

1. yes, you have to omit the static in the .cpp file

2. You'll have to 'reserve memory' for _singleInstance somewhere, e.g. by writing the following in the .cpp file:

   Timing *Timing::_singleInstance = NULL;

(outside the definition of the member functions)