Singleton Classes

[Fizion]

A tutorial I came across more recently reads:

One problem with public constructors is that they do not provide any way to control how many of a particular class may be created. If a public constructor exists, it can be used to instantiate as many class objects as the user desires. Often it is useful to restrict users to being able to create only one instance of a particular class. Classes that can only be instantiated once are called singletons. There are many ways to implement singletons, but most of them involve use of a private (or protected) constructor to prevent users from instantiating as many of the class as they want

The author stopped there and did not speak on it any further. I'd like to know more about this subject.

How can I do this? Please, use as much detail as you can, and thanks.

[vart]

You make public function getInstance

and static member _instance

getInstance will check if _instance is nullPrt - it will call private constructor, assign result to _instance and return it - otherwise just returns _instance

It can also implement refCounter - to check when the _instance could be freed... But with Singleton the object could be left to live till the end of program even it is not used...

[Fizion]

You make public function getInstance

and static member _instance

getInstance will check if _instance is nullPrt - it will call private constructor, assign result to _instance and return it - otherwise just returns _instance

It can also implement refCounter - to check when the _instance could be freed... But with Singleton the object could be left to live till the end of program even it is not used...

So you're suggesting I just use a variable that indicates whether an instance exists or not? An example of what I think you mean, below:

class instance {
    bool instanced;


    void initiate() {
        // do initiation programming...
    }


    public:


        instance() {
            if(!instanced) {
                instanced = true;
                initiate();
            }
        }


        ~instance() {
            instanced = false;
        }
};

[SirPrattlepod]

You probably want

static bool instanced;

Instead of

bool instanced;

[Fizion]

You probably want

static bool instanced;

Instead of

bool instanced;

I've tried that and it doesn't work; I get an error, as seen below:

1>Third.obj : error LNK2001: unresolved external symbol "private: static bool instance::instanced" (?instanced@instance@@0_NA)

[SirPrattlepod]

In the .cpp file you have to "give it memory":

bool instance::instanced;

[vart]

with such approach you can not prevent creating new instance

What i mean is

class st
{
   public:
     static st* getInstance()
     {
        if(_st != nullptr) return _st;
        _st = new st();
        return _st;
     }
private:
    static st* _st;
    st() {};
    st(const st&);
    st& operator= (const st&);
};
st* st::_st = nullptr;

int main()
{
    {
        //first function
        st* singletonvar = st::getInstance();
    }

    {
        //second function
        st* singletonvar2 = st::getInstance();
    }
    return 0;
}