Singleton Class in C++

Question

I have used singleton calss following the example:

singleton class

But i get the error as \"Unresolved external symbols\"

this is th

Answer 1

For a start, I think:

singleton();

should be:

instance = new singleton();

The way you have it, you're not actually storing the newly instantiated object so instance will always be null.

It's also good form to explicitly set statics with:

singleton *singleton::instance = 0;

(outside the class definition).

In fact, it's possibly better to start with the baseline singleton code and work your way up from there. This is a standard-form pointer version:

#include 

class singleton {
    protected:
        static singleton *instance;
        singleton() { }
    public:
        static singleton *getInstance() {
            if (instance == 0)
                instance = new singleton();
            return instance;
        }
};
singleton *singleton::instance = 0;

int main() {
    singleton *s1 = singleton::getInstance();
    singleton *s2 = singleton::getInstance();
    std::cout << s1 << '\n';
    std::cout << s2 << '\n';
    return 0;
}

You can see that both pointers are the same from the output:

0xbc0358
0xbc0358

Or the reference version, since that seems to be what you're aiming for:

#include 

class singleton {
    protected:
        static singleton *instance;
        singleton() { }
    public:
        static singleton& getInstance() {
            if (instance == 0)
                instance = new singleton();
            return *instance;
        }
};
singleton *singleton::instance = 0;

int main() {
    singleton &s1 = singleton::getInstance();
    singleton &s2 = singleton::getInstance();
    std::cout << &s1 << '\n';
    std::cout << &s2 << '\n';
    return 0;
}