从一个动态分配的数组复制到另一个C ++

Question

This seems like it should have a super easy solution, but I just can't figure it out. I am simply creating a resized array and trying to copy all the original values over, and then finally deleting the old array to free the memory.

void ResizeArray(int *orig, int size) {
    int *resized = new int[size * 2]; 
    for (int i = 0; i < size; i ++)
        resized[i] = orig[i];
    delete [] orig;
    orig = resized;
}

What seems to be happening here is that resized[i] = orig[i] is copying values by reference rather than value, as printing orig after it gets resized returns a bunch of junk values unless I comment out delete [] orig . How can I make a deep copy from orig to resized, or is there some other problem that I am facing? I do not want to use std::vector.

Answer 1

Remember, parameters in C++ are passed by value. You are assigning resized to a copy of the pointer that was passed to you, the pointer outside the function remains the same.

You should either use a double indirection (or a "double pointer", ie a pointer to a pointer to int ):

void ResizeArray(int **orig, int size) {
    int *resized = new int[size * 2]; 
    for (int i = 0; i < size; i ++)
        resized[i] = (*orig)[i];
    delete [] *orig;
    *orig = resized;
}

or a reference to the pointer:

void ResizeArray(int *&orig, int size) {
    int *resized = new int[size * 2]; 
    for (int i = 0; i < size; i ++)
        resized[i] = orig[i];
    delete [] orig;
    orig = resized;
}

By the way, for array sizes you should use the type std::size_t from <cstddef> - it is guaranteed to hold the size for any object and makes clear that we are dealing with the size of an object.

Answer 2

I highly suggest replacing the arrays with std::vector<int> . This data structure will resize as needed and the resizing has already been tested.

Answer 3

orig must be a pointer to a pointer to assign it to resized :

int **orig;
*orig = resized;