xvalue

Xvalue is a term from C++ describing one of the language’s value categories, introduced with move semantics in C++11. An xvalue, short for exPiring value, denotes a glvalue (a value that designates an object with identity) whose resources can be moved from. In practice, xvalues indicate objects that can be transferred (moved) rather than copied, enabling efficient resource ownership transfer.

In the C++ value category system, expressions are classified as glvalue, prvalue, or xvalue. Glvalues include both lvalues (regular named objects) and xvalues. Prvalues are pure rvalues representing temporary values. Xvalues specifically represent objects that can be moved-from, typically to implement move constructors and move assignment operators. Xvalues are often produced by rvalue references or by explicit casts.

Common ways to produce xvalues include:

- std::move(x), which yields an xvalue referring to x

- expressions of type T&& that designate an object, such as a function returning T&&

These forms enable the program to reuse resources from the original object rather than copying them.

Key implications: moving from an xvalue is typically cheaper than copying, but it leaves the moved-from object in a valid but unspecified state. Correct use relies on understanding value categories to ensure resources are transferred safely and efficiently.

Example: std::string s = "hello"; std::string t = std::move(s); The expression std::move(s) is an xvalue, allowing t to take ownership of s’s contents.

See also: move semantics, rvalue references, value categories.