How do stack and heap allocation differ in C++?

The stack is a LIFO region managed automatically: allocation is a pointer bump, extremely fast, and memory is reclaimed when the scope ends. The heap is a large pool managed by new/delete or smart pointers; allocation is slower, can fragment, and you must free it.

void f() {
    int x = 5;                          // stack
    int* p = new int(5);                // heap
    std::unique_ptr<int> q =
        std::make_unique<int>(5);       // heap, RAII-managed
    delete p;                           // must free manually
}                                       // x and q cleaned automatically

Stack size is limited (often 1-8MB), so large arrays or deep recursion overflow it. Heap objects can outlive the scope that created them and support dynamic sizes.

Prefer stack allocation and RAII containers such as std::vector and std::string, and reach for the heap only when you need a lifetime beyond the scope, polymorphic ownership, or a runtime-determined size. This avoids leaks and fragmentation.