RAII

Resource Acquisition Is Initialization (RAII) is a programming idiom in which resource management is tied to object lifetime. The resource is acquired during object initialization and released when the object is destroyed, ensuring resources are freed deterministically.

The core principle is that resource lifetime is bound to the lifetime of a scope-bound object. Constructors acquire the resource, and destructors release it. This provides automatic cleanup, even in the presence of exceptions or early returns, by ensuring destructors run during stack unwinding.

Common resources managed by RAII include memory allocations, operating-system handles, file descriptors, network sockets, and synchronization primitives. In practice, RAII is implemented via wrappers and smart pointers (for example, unique ownership and reference counting) as well as resource-owning classes around streams or files. For synchronization, guard objects acquire a mutex on construction and release it on destruction; this pattern is a classic RAII use-case.

Key design considerations include clear ownership, correct copying and moving semantics, and exception safety. In languages like C++, the Rule of Three or Rule of Five guides the implementation of copy and move constructors and assignment operators. Destructors should not throw, and transferring ownership typically uses moves rather than copies to avoid double release.

RAII relies on deterministic destruction and is most directly supported in languages with explicit destructors. In garbage-collected languages, similar patterns can be mimicked but destruction timing is not guaranteed. Rust implements a closely related ownership and drop mechanism that achieves RAII-like resource management.