Singleton in C++

Question

I'm learning about Singleton design pattern. I have the following code example:

//singleton.hpp
#ifndef SINGLETON_HPP
#define SINGLETON_HPP

class Singleton {
public:
    static Singleton& Instance();
private:
    Singleton();
    static Singleton* instance_;
};

#endif

and:

//singleton.cpp
#include "singleton.hpp"

Singleton* Singleton::instance_ = 0;

Singleton& Singleton::Instance() {
    if (instance_ == 0) {
        instance_ = new Singleton();
    }
    return *instance_;
}

Singleton::Singleton() {

}

What I don't understand is the line:

Singleton* Singleton::instance_ = 0;

What does this line do and how? I have never seen something like this.

It's just a pointer, initialised to NULL. Or is that not the part you don't understand? — Mr Lister, May 10 '13 at 21:08
I'm just curious how can you access a private member like that? — darxsys, May 10 '13 at 21:09
Make sure you google "Thread Safe Singleton C++" somewhere in your quest. — granadaCoder, May 10 '13 at 21:10
It is a _definition_ of static variable, _declared_ in your class. — Lol4t0, May 10 '13 at 21:11
Off topic a little, just be aware when you understand it that the Singleton pattern is very frequently used inappropriately — PeteH, May 10 '13 at 21:14
See [Singletons: Good Design or Crutch](http://stackoverflow.com/q/11831/1084416) and [What is so bad about Singletons?](http://stackoverflow.com/q/137975/1084416), amongst many. — Peter Wood, May 10 '13 at 21:20
[Singleton: How should it be used](http://stackoverflow.com/questions/86582/) — fredoverflow, May 11 '13 at 01:25

score 0 · Answer 1 · answered May 10 '13 at 21:09

that is the same as

Singleton* Singleton::instance_ = NULL;

It's just setting the instance to null at the beginning, so that the first time you grab the Singleton it will new up the Singleton object.

Then from there on out, anytime you try to get the Singleton it will give you the first instantiated object.

score 0 · Accepted Answer · answered May 10 '13 at 21:10

0

Singleton* Singleton::instance_ = 0;

just means

Singleton* Singleton::instance_ = NULL;

Because it is static variable, you need to declare it in .h file and define it in .cpp file

answered May 10 '13 at 21:10

diwatu

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Oh, I didn't really check up on static variables enough. That's the answer I'm looking for. Thanks. – darxsys May 10 '13 at 21:17

Singleton in C++

2 Answers2