why singleton instance has to intialize in following code [duplicate]

Question

#include <iostream> using namespace std; class Singleton { public: static Singleton *getInstance(); private: Singleton(){} static Singleton* instance; }; Singleton* Singleton::instance = 0; Singleton* Singleton::getInstance() { if(!instance) { instance = new Singleton(); cout << "getInstance(): First instance\n"; return instance; } else { cout << "getInstance(): previous instance\n"; return instance; } } int main() { Singleton *s1 = Singleton::getInstance(); Singleton *s2 = Singleton::getInstance(); return 0; }

i didn't understand why should singleton instance variable should be initilized with 0 in the following line. Singleton* Singleton::instance = 0; Because when i forgot to initilize the instance i got error as

singleton.cpp:(.text+0xc): undefined reference to `Singleton::instance' singleton.cpp:(.text+0x2d): undefined reference to `Singleton::instance' singleton.cpp:(.text+0x43): undefined reference to `Singleton::instance' singleton.cpp:(.text+0x5b): undefined reference to `Singleton::instance' collect2: error: ld returned 1 exit status

Because it's the rules of C++. static Singleton* instance; is a declaration, but variables have to be defined as well, Singleton* Singleton::instance = 0; is a definition. — john
– john, Commented Jan 21, 2019 at 7:47

Nadir · Accepted Answer · 2019-01-21 07:48:32Z

Besides the procedure to initialize a static member in C++, the only reason to initialize a singleton to null is because you explicitly need to initialize at a later stage in your application for some reason (like waiting for other resources to be allocated and/or initialized). Otherwise, it can be initialized during the static declaration. Furthermore, you dont need to deal with pointers to create a Singleton.

class Singleton { public: static Singleton & getInstance(); private: Singleton(){} static Singleton instance; }; Singleton Singleton::instance; Singleton & Singleton::getInstance() { return instance; }

EDIT: Nevermind, I see your question was oriented towards the static initialization procedure rather than the pattern itself

Holt · Accepted Answer · 2019-01-21 08:12:27Z

static Singleton* instance; is a declaration and not a definition.

As per [class.static.data]/2

The declaration of a non-inline static data member in its class definition is not a definition...

So without the below line of code,

Singleton* Singleton::instance = 0;`

instance is not defined in the code. And trying to use it in getInstance will result in undefined reference errors.

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