Friday, 27 October 2017

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            A> 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?


class="post-text" itemprop="text">
class="normal">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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