Question

Possible Duplicate:
What is The Rule of Three?

How exactly does std::pair call destructors for its components? I am trying to add instances of a class to an std::map, but I am getting errors regarding the destructor of my class.

I have narrowed down my question/problem to the following extremely simple example.

Below, my_class merely creates an int array at construction, and deletes it at destruction. Somehow I am getting a “double delete” error:

//my_class.h
class my_class {
  public:
    int an_int;
    int *array;

    //constructors:
    my_class()
    {
      array = new int[2];
    }
    my_class(int new_int) : an_int(new_int)
    {
      array = new int[2];
    }

    //destructor:
    ~my_class()
    {
      delete[] array;
    }
};  //end of my_class

Meanwhile, over in main.cpp…

//main.cpp
int main(int argc, char* argv[])
{
  std::map<int, my_class>   my_map;

  my_map.insert( std::make_pair<int, my_class> (1, my_class(71) ) );

  return 0;
} // end main

Compilation goes fine, but this generates the following runtime error:

*** glibc detected *** ./experimental_code: double free or corruption (fasttop):

Or, with valgrind:

==15258== Invalid free() / delete / delete[] / realloc()
==15258==    at 0x40249D7: operator delete[](void*) (vg_replace_malloc.c:490)
==15258==    by 0x8048B99: main (my_class.h:38)
==15258==  Address 0x42d6028 is 0 bytes inside a block of size 8 free'd
==15258==    at 0x40249D7: operator delete[](void*) (vg_replace_malloc.c:490)
==15258==    by 0x8048B91: main (my_class.h:38)

(line numbers are off because I cut out comments and stuff)

I must be missing something about std::pair…?

Thanks to all in advance!

Answer 1

When you add my_class to stl containers the copy constructor is called. As you don’t define one it does a memberwise copy and two my_class objects are created that point to the same int array, When these are deleted the same int array might be deleted twice

Please take a look at Rule of three

In C++11 also look at move constructor if you are worried about the efficiency.