How to resize array in C++?

Question

I need to do the equivalent of the following C# code in C++

Array.Resize(ref A, A.Length - 1);

How to achieve this in C++?

Answer 1

1. Use std::vector or
2. Write your own method. Allocate chunk of memory using new. with that memory you can expand till the limit of memory chunk.

Answer 2

Raw arrays aren't resizable in C++.

You should be using something like a Vector class which does allow resizing..

std::vector allows you to resize it as well as allowing dynamic resizing when you add elements (often making the manual resizing unnecessary for adding).

Answer 3

You cannot resize array, you can only allocate new one (with a bigger size) and copy old array's contents. If you don't want to use std::vector (for some reason) here is the code to it:

int size = 10;
int* arr = new int[size];

void resize() {
    size_t newSize = size * 2;
    int* newArr = new int[newSize];

    memcpy( newArr, arr, size * sizeof(int) );

    size = newSize;
    delete [] arr;
    arr = newArr;
}

code is from here http://www.cplusplus.com/forum/general/11111/.

Answer 4

You can do smth like this for 1D arrays. Here we use int*& because we want our pointer to be changeable.

#include<algorithm> // for copy

void resize(int*& a, size_t& n)
{
   size_t new_n = 2 * n;
   int* new_a = new int[new_n];
   copy(a, a + n, new_a);
   delete[] a;
   a = new_a;
   n = new_n;
}

For 2D arrays:

#include<algorithm> // for copy

void resize(int**& a, size_t& n)
{
   size_t new_n = 2 * n, i = 0;
   int** new_a = new int* [new_n];
   for (i = 0; i != new_n; ++i)
       new_a[i] = new int[100];
   for (i = 0; i != n; ++i)
   {
       copy(a[i], a[i] + 100, new_a[i]);
       delete[] a[i];
   }
   delete[] a;
   a = new_a;
   n = new_n;
}

Invoking of 1D array:

void myfn(int*& a, size_t& n)
{
   // do smth
   resize(a, n);
}

Invoking of 2D array:

void myfn(int**& a, size_t& n)
{
   // do smth
   resize(a, n);
}

The declaration of this function should be earlier than one of myfn. And its definition too. These functions were tested and they work correctly.

Answer 5

The size of an array is static in C++. You cannot dynamically resize it. That's what std::vector is for:

std::vector<int> v; // size of the vector starts at 0

v.push_back(10); // v now has 1 element
v.push_back(20); // v now has 2 elements
v.push_back(30); // v now has 3 elements

v.pop_back(); // removes the 30 and resizes v to 2

v.resize(v.size() - 1); // resizes v to 1

Answer 6

You cannot do that, see this question's answers. You may use std:vector instead.