Question

I have

int * array=new int[2];

and I would like to free the memory of the last element, thus reducing the allocated memory to only 1 element. I tried to call

delete array+1;

but it gives error

*** glibc detected *** skuska:
free(): invalid pointer: 0x000000000065a020 *

Can this be done in C++03 without explicit reallocation?

Note: If I wanted to use a class instead a primitive datatype (like int), how can I free the memory so that the destructor of the class is called too?

Note2: I am trying to implement vector::pop_back

Answer 1

Don’t use new[] expression for this. That’s not how vector works. What you do is allocate a chunk of raw memory. You could use malloc for this, or you could use operator new, which is different from the new expression. This is essentially what the reserve() member function of std::vector does, assuming you’ve used the default allocator. It doesn’t create any actual objects the way the new[] expression does.

When you want to construct an element, you use placement new, passing it a location somewhere in the raw memory you’ve allocated. When you want to destoy an element, you call its destructor directly. When you are done, instead of using the delete[] expression, you use operator delete if you used operator new, or you use free() if you used malloc.

Here’s an example creating 10 objects, and destoying them in reverse order. I could destroy them in any order, but this is how you would do it in a vector implementation.

int main()
{
    void * storage = malloc(sizeof(MyClass) * 10);

    for (int i=0; i<10; ++i)
    {
        // this is placement new
       new ((MyClass*)storage + i) MyClass;
    }

    for (int i=9; i>=0; --i)
    {
        // calling the destructor directly
        ((MyClass*)storage + i)->~MyClass();
    }

    free(storage);
}

pop_back would be implemented by simply calling the destructor of the last element, and decrementing the size member variable by 1. It wouldn’t, shouldn’t (and couldn’t, without making a bunch of unnecessary copies) free any memory.