Resource Acquisition Is Initialization

Resource acquisition is initialization (RAII) is a programming idiom used in several object-oriented languages, most prominently C++, where it originated, but also D, Ada, Vala, and Rust. The technique was developed for exception-safe resource management in C++ during 1984–89, primarily by Bjarne Stroustrup and Andrew Koenig, and the term itself was coined by Stroustrup. RAII is generally pronounced as an initialism, sometimes pronounced as "R, A, double I".

In RAII, holding a resource is a class invariant, and is tied to object lifetime: resource allocation (or acquisition) is done during object creation (specifically initialization), by the constructor, while resource deallocation (release) is done during object destruction (specifically finalization), by the destructor. Thus the resource is guaranteed to be held between when initialization finishes and finalization starts (holding the resources is a class invariant), and to be held only when the object is alive. Thus if there are no object leaks, there are no resource leaks.

Other names for this idiom include Constructor Acquires, Destructor Releases (CADRe) and one particular style of use is called Scope-based Resource Management (SBRM). This latter term is for the special case of automatic variables. RAII ties resources to object lifetime, which may not coincide with entry and exit of a scope. (Notably variables allocated on the free store have lifetimes unrelated to any given scope.) However, using RAII for automatic variables (SBRM) is the most common use case.

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