How to implement insertion sort algorithm in C++ with arrays and pointers?

Question

I am trying to learn C++, arrays and pointers. I decided to implement the insertion sort algorithm. So, here is my code and my wrong output. What should I do to correct it? Can you please tell me what is my mistake and what should I avoid if it is a common error?

My code:

// InsertionSort.cpp

#include "stdafx.h"
#include <iostream>

int DeclareAnInteger();
int* DeclareAndShowTheArray(int n);
int* InsertionSort(int *A, int n);

int main()
{

   int n = DeclareAnInteger();
   int *A;
   A = DeclareAndShowTheArray(n);
   int *B;
   B = InsertionSort(A, n);

   system("PAUSE");
   return 0;
}

int DeclareAnInteger()
{
   int n;
   std::cout << "Please enter a positive integer n: ";
   std::cin >> n;
   return n;
}

int* DeclareAndShowTheArray(int n)
{
   int *A;
   A = (int *)alloca(sizeof(int) * n);
   for (int i = 0; i < n; i++)
   {
      std::cout << "Please enter the value of A[" << i + 1 << "]: ";
      std::cin >> A[i];
   }
   std::cout << "The unsorted array is: ";
   for (int i = 0; i < n; i++)
   {
      std::cout << A[i];
      std::cout << "\t";
   }
   std::cout << "\n";
   return A;
}

int* InsertionSort(int *A, int n)
{
   int k;
   //int *A = new int[n];
   for (k = 1; k < n; k++)
   {
      int key = A[k];
      int m   = k - 1;
      while (m >= 0 & A[m] > key)
      {
         A[m + 1] = A[m];
         m        = m - 1;
      }
      A[m + 1] = key;
   }
   std::cout << "The sorted array is: ";
   for (int i = 0; i < n; i++)
   {
      std::cout << A[i];
      std::cout << "\t";
   }
   std::cout << "\n" << std::endl;
   return A;
}

My output:

Answer 1

This here is a big problem:

A = (int *)alloca(sizeof(int) * n);

The alloca function allocates on the stack, and it will be lost when the function returns which gives you a stray pointer and undefined behavior when you dereference this pointer.

If you're programming C++ then use new, if you program C then use malloc.