Delete objects from a singleton class

Question

I want to create a simple Singleton class in C++ which contains pointers to other types.

The singleton will become the owner over that pointer at some point.

So it's this a good way to delete the object of type Test and release it's memory on Singleton destructor?.

 class Test{...};
 class Singleton
 {
    public:
    static Singleton& getInstance()
    {
        static std::unique_ptr<Singleton> instance(new Singleton());
        return *instance.get();
    }

    void SetTest(Test* test);
    Test* GetTest() const;
    ...
    private:
      Singleton(){}
      ~Singleton(){ if(test) delete test;} // ??
      Test* test;
     ...
 };

PS: I can't use smart pointers for the Test object.

i guess you meant "pointer to objects of other type", afaik there is no pointer to a type — 463035818_is_not_an_ai, Oct 09 '17 at 11:27
You should probably initialize `test` in the constructor. Otherwise if the user never calls `SetTest` then `GetTest` will return an invalid (and most likely non-null) pointer, and the destructor will attempt to delete something which wasn't allocated. — Some programmer dude, Oct 09 '17 at 11:28
Strictly speaking, a singleton is anti-pattern. And anything you need it to own is probably indicative of greater anti-patterns in your code. — StoryTeller - Unslander Monica, Oct 09 '17 at 11:29
As for your problem, perhaps using pointers for the member variables is not the correct thing to do? Perhaps use plain objects instead? And if you need to use pointers (due to some other requirement), then perhaps use `std::unique_ptr` for them too? — Some programmer dude, Oct 09 '17 at 11:29
Test is forwarded declared so that's why I can't use smart pointers — CrSe, Oct 09 '17 at 11:31
With a custom deleter it's still possible to use smart pointers. — Some programmer dude, Oct 09 '17 at 11:43
shared_ptr<> does not need a complete type ( but shared_ptr dtor does ), no need of a custom deleter — Massimiliano Janes, Oct 09 '17 at 12:50
moreover, "if(test) delete test;" is useless ( you can delete a null pointer ) ... — Massimiliano Janes, Oct 09 '17 at 12:51
I don't understand. Usually, a singleton lasts forever. The memory allocated to a singleton will be recovered after the program exits. Why would you need to delete objects inside a singleton? — Thomas Matthews, Oct 09 '17 at 14:50

score 1 · Accepted Answer · answered Oct 09 '17 at 15:49

To give this question some conclusion: Yes, it generally fine to delete the object like you propose. However, please mind the following caveats, that have already been pointed out in the comments:

Make sure to initialize test to nullptr in the constructor of the singleton. Otherwise you might end up trying to delete a random memory address. (Some programmer dude)
You don't need to check for nullptr in the destructor. Deleting a nullptr is generally safe and won't do anything. (Massimiliano Janes and here)
It is probably possible to use a smart pointer in your case, which would be preferable. (Some programmer dude, Massimiliano Janes)

Delete objects from a singleton class

1 Answers1