Memory: the stack and the heap

A running program uses two main areas of memory. The call stack holds a frame for each function call in progress, with that call's local variables. It grows when you call a function and shrinks when the function returns.

That makes it fast and automatic, but small, so very deep recursion overflows it. The heap holds data whose size or lifetime is not tied to one call, such as lists, objects and long strings. You reach it through references or pointers (stored addresses).

In C++ you control how long heap data lives, ideally through std::vector and smart pointers. Python, Java, JavaScript and C# use garbage collection instead: the runtime frees unused heap objects for you. Many "why did this change?" bugs come from two references to one heap object.