C++ memory allocation for an array of pointers

Question

I have a question involving memory allocation. Let's say I create an array of pointers like this.

int **numbers = new int *[1024*1024];

I had assumed that this would need 8MB of memory (8-byte pointer on Mac 64-bit) but that's not the case. Memory is only allocated when each pointer is assigned a value. So if I NULL all the pointers then I see 8MB being allocated.

for(int i=0; i<1024*1024; i++)
{
    numbers[i] = NULL;
}

How does my app know which pointers have an assigned a value without allocating memory for it?

Answer 1

From C++ perspective your memory is allocated and there. You better not confuse yourself by what OS is reporting.

Your "app" definitely does not know whether it assigned a pointer or not -- just you as the programmer are responsible to not use whatever pointers before assignment.

If curious on possible backgroung, look around for memory overcommit like here or in other articles.

Answer 2

You are observing a feature of the OS, called overcommit-accounting.

Unassigned memory only gets reserved address-space (process specific virtual memory) by default. Only when assigning values to it, the pages are actually mapped to physical addresses in the page-table.