c++ - How to use memcpy correctly with different types of arrays

Question

I have a templated class named Stack that has a private method resize.

Whenever I use the int or double template version of this class, it works as desired. But, whenever I use the float or string versions of the template, it crashes. I believe that it's a problem with my memcpy function call. How am I supposed to use it correctly?

template <class Type>

void Stack<Type>::resize(int capacity) {
    if(capacity >= MAX_SIZE)
        capacity = MAX_SIZE;

    Type* copy = new Type[capacity];

    for (int i = 0; i < N; i++) {
        copy[i] = s[i];
    }

    s = new Type[capacity];

    memcpy(s, copy, sizeof(Type) * capacity);

    size = capacity;

    delete copy;
}

s is a heap allocated member variable array of type Type.

Answer 1

First of all, it is incorrect to use memcpy to copy non-POD types. Just use a for loop, or std::copy.

Second, you are doing more work than necessary (and you have a memory leak).

void Stack<Type>::resize(int capacity) {
    if(capacity >= MAX_SIZE)
        capacity = MAX_SIZE;

    Type* copy = new Type[capacity];

    for (int i = 0; i < N; i++) {
        copy[i] = s[i];
    }

Up to this point, you're okay. You've allocated a new array, and assigned over the elements from the old array. I'm assuming N is the number of valid elements.

s = new Type[capacity];

Assuming that s previously pointed to an allocated array, this is a memory leak. First, you need to delete the previous data.

delete [] s;

Then, you don't need to allocate another array. Use the one you just allocated.

s = copy;

All together, the function now looks like this:

template <class Type>
void Stack<Type>::resize(int capacity) {
    if(capacity >= MAX_SIZE)
        capacity = MAX_SIZE;

    Type* copy = new Type[capacity];

    for (int i = 0; i < N; i++) {
        copy[i] = s[i];
    }

    delete [] s;
    s = copy;
    size = capacity;
}

If there are still problems, some other part of your code is broken.