prvalue

Prvalue, short for pure rvalue, is a category of expressions in C++ that denotes a value but does not designate a persistent object. Introduced with the language’s value categories, prvalues contrast with lvalues, which designate objects with identity, and with xvalues, which denote an expiring value that can be moved from.

Prvalues are not glvalues (generalized lvalues). An expression such as 5 or a temporary constructed object is a prvalue. In contrast, the expression x is an lvalue if referring to a named object, and the expression x+1 is a prvalue.

Use and materialization: A prvalue can initialize an object or be used to initialize a temporary object. When you write int a = 5; the prvalue 5 initializes a. When you write T() in an initialization, T() produces a temporary of type T. Prvalues participate in move semantics, and move constructors can be invoked to move from prvalues.

C++17 changes: The language introduced guaranteed copy elision for many prvalue-to-object initializations, allowing compilers to construct the object directly in the destination without creating a temporary. This tightens the link between prvalues and the objects they initialize, reducing unnecessary temporaries in practice.

Relation and further reading: Prvalues are part of the broader concept of value categories, which also include lvalues, glvalues, and xvalues. See also lvalue, xvalue, glvalue, and value category for precise definitions in the standard.