Change array size C++

To resize an array you have to allocate a new array and copy the old elements to the new array, then delete the old array.

T * p_bag;
p_bag = new T[old_size];
//...
T * p_expanded_bag = new T[new_size];
for (unsigned int i = 0; i < old_size; ++i)
{
  p_expanded_bag[i] = p_bag[i];
}
delete[] p_bag;
p_bag = p_expanded_bag;

You could use std::copy instead of the for loop.

Memory, allocated by new, would not disappear after your method ends.

You can return pointer to a new array by using a reference: void f(int *&ptr, size_t &size).

Also, be aware, that you need to clear memory manually after you use it. For example:

int* newArray = new int[newSize];
// copying from old array:
int* temp = oldArray;
oldArray = newArray;
delete[] temp;

The thing you need can do the following things

• Automatically handle the resizing when requested size is larger than current array size.

• When resizing, they can copy the original content to the new space, then drop the old allocation immediately .

• There is a non-global-variable way mechanism they can track the array pointer and the current size.

The thing is very similar to std::vector. If it is not allowed to use, you may need manage a dynamic allocated resource like std::vector on your own. You can reference the implementation in that answer link.

If eventually you need to wrap it in a class, make sure to follow the big 3 rules (5 rules in C++11)

You can use realloc from c if you have array of chars/ints/doubles... or some other fundamental data type or classes with only those variables (eg. array of strings won't work). http://www.cplusplus.com/reference/cstdlib/realloc/

bag = (T*) realloc(bag, new_size * sizeof(T));

Realloc automatically allocate space for your new array (maybe into the same place in memory) and copy all data from given array. "The content of the memory block is preserved up to the lesser of the new and old sizes, even if the block is moved to a new location."

Example:

#include <stdio.h>      /* printf*/
#include <stdlib.h>     /* realloc, free */

#include <iostream>

int main()
{
    int old_size = 5;
    int new_size = 10;

    int *array = new int[old_size];

    printf("Old array\n");
    for (int i=0; i<old_size; i++) {
        array[i] = i;   
        printf("%d ", array[i]);
    }
    printf("\nArray address: %d\n", array);

    array = (int*) realloc(array, new_size * sizeof(int));

    printf("New array\n");
    for (int i=0; i<new_size; i++)
        printf("%d ", array[i]);
    printf("\nArray address: %d\n", array);

    free(array);
    return 0;
}