Memory and pointers · Lesson 12 of 21

Stack, heap, new and delete

Memory that frees itself, and memory you're responsible for.

10 minutes3 games

A program uses two areas of memory:

The stack: ordinary local variables. Very fast, freed automatically when the scope ends (}), but small (often a few megabytes).
The heap: a large area you allocate from at runtime with new, which stays until you free it with delete.

	Stack	Heap
How	int x = 5;	new int(5)
Who frees	automatic at `}`	you, with `delete` (or a smart pointer)
Speed	very fast	slower
Size	small	large
Lifetime	until the scope ends	until you free it

C++

#include <iostream>

int main() {
    int onStack = 10;                 // freed automatically

    int* single = new int(42);        // one int on the heap
    int* many = new int[5];           // an array of 5 on the heap
    for (int i = 0; i < 5; i++) many[i] = i * i;

    std::cout << onStack << " " << *single << " " << many[4] << "\n";

    delete single;                    // new   -> delete
    delete[] many;                    // new[] -> delete[]
    single = nullptr;                 // don't leave it dangling
    many = nullptr;
    return 0;
}

Spot the bug

This function runs every second on a server, and after days memory runs out. Why?

Quick question

A program needs an array of 50 million numbers. Where should it live?

Exercise

Allocate a heap array of n = 6 numbers, fill it with 1, 2, 4, 8, 16, 32 (each double the last), print their sum, and don't forget to free the memory.

Expected output: 63

C++

#include <iostream>

int main() {
    int n = 6;
    
    return 0;
}

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