Why am I able to allocate more memory to an array than my computer actually has

Question

I am able to successfully run the code:

char* p = new char[34493878088];
cout << "Success at " << 34493878088 << " bytes" << endl;
delete[] p;

which as I understand allocates a 34 gigabyte char array. However, my computer only has 16 gigabytes of ram and my pagefile is only 3 gigabytes so how is this possible?

Answer 1

The reserved address space ("virtual memory") is not mapped immediately into the physical memory. This is delayed till the memory is accessed.

When you first time access the memory location, a page fault interrupt initially happens, and only then the kernel configures the memory controller (that is a piece of hardware, part of CPU) to place a page of physical memory for your program at that address location. The program is then resumed from where it was interrupted and does not see these things happening.

A region in address space that is formally reserved but never accessed does not use the actual physical memory and can be larger than the amount of the physical memory available.