What is a memory leak in C++ and how to avoid it?

A memory leak happens when you allocate memory with new and lose the pointer without delete. The program uses more and more memory.

#include <iostream>
#include <memory>

int main() {
    int* leak = new int(5);
    leak = nullptr;                       // the address is lost: a leak (the memory is never freed)

    auto safe = std::make_unique<int>(5); // freed automatically
    std::cout << *safe << '\n';
}

How to avoid it: use std::unique_ptr and std::shared_ptr, use containers (vector, string) and follow RAII. Tools like AddressSanitizer (-fsanitize=address) and Valgrind find leaks. See Smart Pointers.

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