Accessing a Dynamic Array causing a runtime error - c++

Question

I've been messing around with dynamic memory and I've hit a huge wall.
I'm trying to create a program where the user enters as many strings as they want, then can quit whenever, however after a second string is entered, the program crashes with out giving me any specific error message.
#include "stdafx.h"
#include "string.h"
#include "stdio.h"
#include "stdlib.h"
#include "new"
int _tmain(int argc, _TCHAR* argv[])
{
//Variables
int i=0,end=0,requiresSize=1;
char ** temp;
char item[256]="a";
char ** requires;
//Initialize each element in requiers
requires = new char * [requiresSize];
for(int j=0;j<requiresSize*2;j++){
requires[j]= new char[256];
}
while(strcmp(item,"q-")){
end=0;
printf("Enter h- for help.\nEnter q- to quit.\n");
printf("Please enter a string\n");
gets_s(item);
if(!strcmp(item,"h-")){
printf("Enter a string to add to the list.\nEnter p- to print the list.\n");
end=1;
}
if(!strcmp(item,"q-")){
break;
}
if(!strcmp(item,"p-")){
if(requires[0]!=NULL){
for(int j=0;j<requiresSize;j++){
printf("%d. %s\n",j,requires[j]);
}
}
end=1;
}
while(end==0){
printf("check1:i=%d\n",i);
//if search index is larger than size of the array,reallocate the array
if(i>= requiresSize){
temp = new char * [requiresSize*2];
//Initialize each element in temp
printf("check2:temp initalized\n");
for(int j=0;j<requiresSize*2;j++){
temp[j]= new char[256];
}
printf("check3:temp itmes initialized\n");
for(int j =0;j<requiresSize;j++){
//for each element in requires, copy that element to temp
temp[j]=requires[j];
}
printf("check4:copied requires into temp\n");
delete * requires;
requires = temp;
printf("check5:deleted requires and set requires equal to temp\n");
delete temp;
requiresSize = requiresSize *2;
}
printf("check6:\n");
//if the index at requires is not empty, check to see if it is the same as given item
if(requires[i]!= NULL){
printf("check8:index at requires is not empty\n");
//I know the error occurs here, something to do with accessing requires[i]
if(!strcmp( item, requires[i])){
printf("check9:index at requires is the same as item\n");
//if they are the same, break out of the loop, item is already included
break;
}else{
printf("check10:index at requires is different\n");
//otherwise, increase the index and check again (continue loop)
i++;
break;
}
}else{
printf("check11:index at requires is null, item added\n");
//if the index is empty, add the item to the list and break out of loop
requires[i]= item;
break;
}
printf("check7\n");
}
}
delete requires;
return 0;
}
Thank you in advance.

Answer 1

You need to realize that an assignment statement such as temp = requires (in this case) is only copying over the pointer, so that temp is now pointing to the same location in memory as requires; it does NOT copy over that memory.
This is leading to two problems:
You are allocating new 256-char arrays to each element of temp, and then reassigning each char* in temp to point to a different location, leaking all of that memory; there is now no way to reference the newly-allocated memory, and thus no way for you to free it.
You are assigning the pointer temp to requires, which, again, just means the two are now pointed to the same location in memory, and then deleting temp, which frees that memory (that, again, requires is also now pointing to).
Also, if you use new[] to allocate an array, you must use delete[] to free it. So requires = new char * [requiresSize]; requires you to use delete [] requires; at the end of the program, instead of just delete requires;. Same for each 256-char element of requires.
So, replace temp[j]=requires[j]; with an appropriate call to strcpy (or strncpy). And do not delete temp; the delete [] requires; at the end will handle that, since it is now pointing to that bit of memory.