undefined reference error due to use of static variables [duplicate] - c++

Question

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static variable link error [duplicate]
(2 answers)
Closed 8 years ago.
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.
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() { ... }
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

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.
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 4

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

Answer 5

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.