CODEWITHSUNDEEP
c++singleton
Morgan Wilde
Morgan Wilde

Reputation: 17295

How to properly create a singleton object and use it in C++?

Background

Most of my experience with OOP comes from Objective-C. In that language there's a clear distinction between instance and class methods. As a result it is fairly easy to work with a singleton without any side effects.

In C++ I haven't been that lucky and I can't seem to avoid objects being created out of my control.

Code

I have the following object

class Window
{
private:
    Window();
    Window(Window const &windowCopy);
    void operator=(Window const &windowRight);

public:
    ~Window();
    static Window getSingleton();
};

This is the .h. Most of the implementation is just me using cout to print messages when each method in the .h is being called. Except for the getSingleton() method.

Window Window::getSingleton()
{
    static Window singleton;
    return singleton;
}

And here's my main

int main(int argc, char *argv[])
{
    Window::getSingleton();
    Window::getSingleton();

    std::cout << "Stack is being removed" << std::endl;

    return 0;
}

Result

After running this I get the following output

Window created   -> 0x10c9bf0e0 // This is the static singleton
Window copied    -> 0x7fff53242bb8 <- from 0x10c9bf0e0 // What is this?
Window destroyed -> 0x7fff53242bb8 // And what is it's scope?
Window copied    -> 0x7fff53242bb0 <- from 0x10c9bf0e0
Window destroyed -> 0x7fff53242bb0
Stack is being removed
Window destroyed -> 0x10c9bf0e0

Question

For some reason whenever I call my singleton method, a new object appears and the singleton gets assigned to itself. Why? And how can I change this so that there's only one Window object for the entire duration of the app?

Upvotes: 1

Views: 121

Answers (1)

Marco A.
Marco A.

Reputation: 43662

You're passing the object by value

Window Window::getSingleton()
{
    static Window singleton;
    return singleton;
}

you should return it by reference (or a pointer to it) instead

Window& Window::getSingleton()
{
    static Window singleton;
    return singleton;
}

That is usually the intended behavior of the singleton pattern.

Upvotes: 5

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