Memory: Stack vs. Heap

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Stack vs. heap memory allocation from geeksforgeeks.

std::vector and std::array also use different regions of memory in the computer.

Stack

The stack is where the variables we have been creating like:

double x{};
int a{};

live (this is not required by C++, but it is what most systems do). These are sometimes called automatic variables. When we enter a program unit (like main) these variables are placed on the stack (in the order we encounter them). And when we leave the program unit, they are popped off, automatically freeing up the resources.

Note that the stack is generally small, and if we try to put too much data on the stack, we can encounter a stack overflow and the program will crash.

std::array (typically) puts its data on the stack.

Heap

The heap is the region of memory where dynamically allocated resources typically live—this means that the memory is allocated at runtime. We won’t consider manual memory management in this class, but std::vector will allocate its memory on the heap.

In older C++, we needed to be careful to manually free the memory on the heap, since it is not automatic, as with the stack. This could lead to memory leaks and crashes. In modern C++, for objects like std::vector, when the object goes out of scope (e.g., we leave the function where we created it), a special function called the destructor is automatically called freeing the resources.