glvalues

Glvalue, short for generalized lvalue, is a category of expression in C++ that denotes an object or a function with a distinct identity and a memory location. A glvalue designates an object or function; it has an identifiable location in memory and, in the case of objects, an address that can be retrieved with the address-of operator. Glvalues include traditional lvalues, such as named variables and dereferenced pointers, and xvalues (expiring values), which designate objects that may be moved from without necessarily being reassigned.

In C++11 and later, the language distinguishes three value categories: glvalues, prvalues, and the newer xvalues within glvalues. Prvalues are pure rvalues that represent temporary values not associated with a persistent memory location. Glvalues can be used where an object is required; lvalues are those glvalues that designate modifiable objects and can appear on the left-hand side of an assignment, while xvalues designate resources that can be stolen via move operations.

The concept supports move semantics, overload resolution, and template deduction by making explicit whether an expression refers to an existing object or to a temporary. For instance, a variable a is a glvalue; std::move(a) yields an xvalue, also a glvalue, used to indicate that a may be moved from. Array subscripts, dereferenced pointers, and function names also yield glvalues under appropriate types.

Understanding glvalues helps explain how expressions interact with operators like &, and how language features such as auto type deduction and perfect forwarding treat different value categories. Historically, the term generalized lvalue was introduced to unify lvalues and xvalues under a single category for clarity in the language standard.