C++ vector dynamic memory deallocation/delete

Question

When I try and delete dynamic memory elements of a dynamic vector of objects, I have to iterate over the entire size of the vector more than once to ensure complete deallocation.

class CV{
    public:
            float x;
            float y;

            CV();
            CV(float, float);
            ~CV();
 };


int main(){

    vector<CV*>* cars;
    cars = new vector<CV*>;


    //create objects with new, push into vetor
    for(int j=0;j<4;j++){
            cars->push_back( new CV(10.0+j, 11.99+j) );
    }

    while( cars->size() > 0   ){
            for(int i=0;i<cars->size();i++){
                    delete (*cars)[i];
                    cars->erase( cars->begin()+i);
            }
            cout << "size:"<< cars->size() << endl;
    }

    delete cars;
    return 0;

}

This will output:

  size:2
  size:1
  size:0

The vector seems to iterate over every second element when I try to delete, as I require the extra while loop to ensure total deallocation.

I seem to be missing something about the internal workings of vector, I've tried reading the vector c++ reference and I understand that vectors store elements in a contiguous location and that they allocate extra storage for possible growth, but I fail to understand the behaviour of this code.

Solution 1

You can write a generic function to handle both deallocation and erasure of the vector's elements

template<typename T>
void destroy_vector(std::vector<T*> &v)
{
    while(!v.empty()) {
        delete v.back();
        v.pop_back();
    }
}

Some remarks

• Always check with empty for empty container
• Store your vector<T*>* in a smart pointer to avoid memory leaks

Solution 2

When you erase an element from vector, elements after that are moved by one place. When you erase the element in index 0, the element in index 1 will be moved to index 0 and will not be erased in the next iteration.

Take the erase from the loop. Elements will be erased in the vectors destructor anyway.

Comments

[Wade G]

Ok, I understand the elements have to shift when a specific element is erased or removed. I think I can deallocate all of the elements and then just call vector::clear once to make the vector size 0 and empty.

[eerorika]

@WadeG You can use clear to empty the vector, but it is redundant if you delete the vector right after that.