Is a pointer variable created for stack local array?

Question

Consider the following code:

#include <iostream>

int main()
{
    int index;
    std::cin >> index;

    int* dynArray = new int[5];
    dynArray[index] = 1;

    int stackArray[5];
    stackArray[index] = 1;
}

I know for sure that dynArray is a simple int* pointer and it takes additional sizeof(int*) bytes on stack.

Question 1: Is an additional pointer variable also created on stack for stackArray? If so, does that always happen? If not, then how does stackArray[index] = 1; work without knowing array base, i.e. how does the compiler decide what value to add to index for calculating address?

Question 2: Is there any difference between C/C++?

I want an answer for these 2 environments:

GCC/Linux, x86
Visual Stuidio 2013, x86

For first, int* dynArray = new int[5] is not the same type as int stackArray[5]. — 101010
using gcc you can compile the code using the -S flag to generate the assembly code and verify by yourself how the two arrays are allocated. I would expect the dynArray to be in the heap though. Don't know if there is a similar option for VS2013 — sergico

Kerrek SB Kerrek SB · Accepted Answer · 2014-05-24T15:17:34

dynarray is a variable. Its type is int *, and it has automatic storage duration ("on the stack"). Its value is a pointer to some element somewhere else. The fact that it happens to point to an element of some array somewhere else is to some extent incidental.
stackArray is a variable. Its type is int[5] and it has automatic storage duration. Its value is an array of five integers. When you use the name of the variable in an expression like stackArray[index], that expression denotes the corresponding array element (which is a subobject of the array object).

Note that in your example, the array of which dynarray[index] is an element is not itself a variable, nor is the element itself. You can only access the element objects because you have a pointer to them. (And in fact you cannot express the array object itself at all, since its type is know knowable at compile time. Dynamic arrays are the one part of C++ where you have truly dynamic typing.)

Is a pointer variable created for stack local array?

2 Answers