CODEWITHSUNDEEP
c++c++11singleton
Osman
Osman

Reputation: 76

What's wrong with my singleton class so that I can't return the unique_ptr?

I'm studying C++. To practice, I tried to implement a very simple singleton class, but I can't understand why this causes an error.

When I return a reference to it (which I think I should use it like that), like static unique_ptr<Singleton>& getInstance(), it is accepted.

So, what's wrong here?

class Singleton
{
private:
    static unique_ptr<Singleton> obj;
    Singleton()
    {
        cout << "it's created!!\n";
    }

public:
    ~Singleton()
    {
        cout << "it's desctructed!!\n";
    }
    static unique_ptr<Singleton> getInstance()
    {
        if (obj.get() == nullptr)
            obj.reset(new Singleton());
        return obj;
    }
};
unique_ptr<Singleton> Singleton::obj{nullptr};


int main(){
  
  unique_ptr<Singleton> ss = Singleton::getInstance();
  return 0
}

/home/rt7/Desktop/ccProjects/cppTutorial/main.cc: In static member function ‘static std::unique_ptr<Singleton> Singleton::getInstance()’:
/home/rt7/Desktop/ccProjects/cppTutorial/main.cc:25:16: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = Singleton; _Dp = std::default_delete<Singleton>]’
   25 |         return obj;
      |                ^~~
In file included from /usr/include/c++/11/bits/locale_conv.h:41,
                 from /usr/include/c++/11/locale:43,
                 from /usr/include/c++/11/iomanip:43,
                 from /usr/include/x86_64-linux-gnu/c++/11/bits/stdc++.h:72,
                 from /home/rt7/Desktop/ccProjects/cppTutorial/main.cc:1:
/usr/include/c++/11/bits/unique_ptr.h:468:7: note: declared here
  468 |       unique_ptr(const unique_ptr&) = delete;

Upvotes: 0

Views: 198

Answers (1)

3CxEZiVlQ
3CxEZiVlQ

Reputation: 38549

The singleton is the object owner. Usually a raw pointer is returned if the ownership is not transfered.

static Singleton* getInstance()
{
    if (!obj)  // Simpler than obj.get() == nullptr
        obj.reset(new Singleton());
    return obj.get();
}

Upvotes: 2

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