memory leaks - C++ - Is my object being deleted twice if I use a map?

Saturday, 24 September 2016

memory leaks - C++ - Is my object being deleted twice if I use a map?

I've been working on a Qt project and I encountered a problem with deleting objects that are being hold in a map. I prepared a simple C++ code to show my problem:

   #include 
#include 
#include 

using namespace std;

    class A 

    {
    public:
        int *tab;

        A()
        {
            tab = NULL;
        }
        ~A()
        {

            if (tab != NULL)
            {
                delete[] tab;
            }
        }
    };

        int main()
        {
            map mapa;


            string name = "MyArray";

            A *a = new A;
            a->tab = new int[3];
            a->tab[0] = 1;
            a->tab[1] = 2;
            a->tab[2] = 3;

            mapa[name] = *a;


            delete a;


            system("PAUSE");
            return 0;
        }

After closing the program I get:

Debug Assertion Failed!

_BLOCK_TYPE_IS_VALID
etc..

My question is: why is that? The reason is probably that map gets deleted after I quit the program and it holds an A object (a) that is being deleted before I close the program. However, I passed a value, not an address, so what's the problem?

Isn't that value just copied into the map and held in some different address?

Answer

Yes, you're double-deleting.

Maps copy their elements, and your type has no copy constructor. You're actually copying the A yourself with mapa[name] = *a anyway.

The ownership of the member pointer is also very unclear.

Abide by the rule of three. Ideally avoid the manual dynamic allocation altogether; A should have a std::vector instead.

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Saturday, 24 September 2016

memory leaks - C++ - Is my object being deleted twice if I use a map?

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