Each C++ expression (an operator with its arguments, a literal, a variable name, etc) is characterized by two independent properties: a type and a value category. Each expression has some non-reference type, and each expression belongs to exactly one of the three primary value categories.
1. Value category meanings
Expressions in C++ are assigned a particular value category, based on the result of those expressions. Value categories for expressions can affect C++ function overload resolution.
Value categories determines two important-but-separate properties about an expression. One property is whether the expression has identity. An expression has identity if it refers to an object that has a variable name. The variable name may not be involved in the expression, but the object can still have one.
The other property is whether it is legal to implicitly move from the expression's value. Or more specifically, whether the expression, when used as a function parameter, will bind to r-value parameter types or not.
C++ defines 3 value categories which represent the useful combination of these properties: lvalue (expressions with identity but not movable from), xvalue (expressions with identity that are moveable from), and prvalue (expressions without identity that are moveable from). C++ does not have expressions which have no identity and cannot be moved from.
C++ defines two other value categories, each based solely on one of these properties: glvalue (expressions with identity) and rvalue (expressions that can be moved from). These act as useful groupings of the prior categories.
2. ISO/IEC 14882:2011
（1）basic concepts
This Clause presents the basic concepts of the C ++ language. It explains the difference between an object and a name and how they relate to the value categories for expressions. It introduces the concepts of a declaration and a definition and presents C ++ ’s notion of type, scope, linkage, and storage duration. The mechanisms for starting and terminating a program are discussed. Finally, this Clause presents the fundamental types of the language and lists the ways of constructing compound types from these.
An entity is a value, object, reference, function, enumerator, type, class member, template, template spe- cialization, namespace, parameter pack, or this.
A name is a use of an identifier (2.11), operator-function-id (13.5), literal-operator-id (13.5.8), conversion- function-id (12.3.2), or template-id (14.2) that denotes an entity or label (6.6.4, 6.1).
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（2）types
3.9 and the subclauses thereof impose requirements on implementations regarding the representation of types. There are two kinds of types: fundamental types and compound types. Types describe objects (1.8), references (8.3.2), or functions (8.3.5).
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（3）complete type and incomplete type
A class that has been declared but not defined, or an array of unknown size or of incomplete element type, is an incompletely-defined object type. Incompletely-defined object types and the void types are incomplete types (3.9.1). Objects shall not be defined to have an incomplete type.
A class type (such as “class X”) might be incomplete at one point in a translation unit and complete later on; the type “class X” is the same type at both points. The declared type of an array object might be an array of incomplete class type and therefore incomplete; if the class type is completed later on in the translation unit, the array type becomes complete; the array type at those two points is the same type. The declared type of an array object might be an array of unknown size and therefore be incomplete at one point in a translation unit and complete later on; the array types at those two points (“array of unknown bound of T” and “array of N T”) are different types. The type of a pointer to array of unknown size, or of a type defined by a typedef declaration to be an array of unknown size, cannot be completed.
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（4）scala types...
Arithmetic types (3.9.1), enumeration types, pointer types, pointer to member types (3.9.2), std::nullptr_t, and cv-qualified versions of these types (3.9.3) are collectively called scalar types. Scalar types, POD classes (Clause 9), arrays of such types and cv-qualified versions of these types (3.9.3) are collectively called POD types. Scalar types, trivially copyable class types (Clause 9), arrays of such types, and cv-qualified versions of these types (3.9.3) are collectively called trivially copyable types. Scalar types, trivial class types (Clause 9), arrays of such types and cv-qualified versions of these types (3.9.3) are collectively called trivial types. Scalar types, standard-layout class types (Clause 9), arrays of such types and cv-qualified versions of these types (3.9.3) are collectively called standard-layout types.
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（5）cv-qualified type
A type mentioned in 3.9.1 and 3.9.2 is a cv-unqualified type. Each type which is a cv-unqualified complete or incomplete object type or is void (3.9) has three corresponding cv-qualified versions of its type: a const-qualified version, a volatile-qualified version, and a const-volatile-qualified version. The term object type (1.8) includes the cv-qualifiers specified when the object is created.
The presence of a const specifier in a decl-specifier-seq declares an object of const-qualified object type; such object is called a const object. The presence of a volatile specifier in a decl-specifier-seq declares an object of volatile-qualified object type; such object is called a volatile object. The presence of both cv-qualifiers in a decl-specifier-seq declares an object of const-volatile-qualified object type; such object is called a const volatile object. The cv-qualified or cv-unqualified versions of a type are distinct types; however, they shall have the same representation and alignment requirements (3.9).
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（6）lvalues and rvalues
Expressions are categorized according to the taxonomy in Figure 1.
![[C++] Types and Value categories_第1张图片](http://img.e-com-net.com/image/info10/06ffaa7c515a4d889bc467aa7bda590e.jpg)
（1）lvalue
An lvalue (so called, historically, because lvalues could appear on the left-hand side of an assignment expression) designates a function or an object.
Example:
If E is an expression of pointer type, then *E is an lvalue expression referring to the object or function to which E points. As another example, the result of calling a function whose return type is an lvalue reference is an lvalue.
（2）xvalue
An xvalue (an “eXpiring” value) also refers to an object, usually near the end of its lifetime (so that its resources may be moved, for example). An xvalue is the result of certain kinds of expressions involving rvalue references.
Example:
The result of calling a function whose return type is an rvalue reference is an xvalue.
（3）gvalue
A glvalue (“generalized” lvalue) is an lvalue or an xvalue.
（4）rvalue
An rvalue (so called, historically, because rvalues could appear on the right-hand side of an assignment expression) is an xvalue, a temporary object or subobject thereof, or a value that is not associated with an object.
（5）prvalue
A prvalue (“pure” rvalue) is an rvalue that is not an xvalue.
Example:
The result of calling a function whose return type is not a reference is a prvalue. The value of a literal such as 12, 7.3e5, or true is also a prvalue.
Every expression belongs to exactly one of the fundamental classifications in this taxonomy: lvalue, xvalue, or prvalue. This property of an expression is called its value category.
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（7）lvalue-to-rvalue conversion
A glvalue (3.10) of a non-function, non-array type T can be converted to a prvalue. If T is an incomplete type, a program that necessitates this conversion is ill-formed. If the object to which the glvalue refers is not an object of type T and is not an object of a type derived from T, or if the object is uninitialized, a program that necessitates this conversion has undefined behavior. If T is a non-class type, the type of the prvalue is the cv-unqualified version of T. Otherwise, the type of the prvalue is T.
When an lvalue-to-rvalue conversion occurs in an unevaluated operand or a subexpression thereof (Clause 5) the value contained in the referenced object is not accessed. Otherwise, if the glvalue has a class type, the conversion copy-initializes a temporary of type T from the glvalue and the result of the conversion is a prvalue for the temporary. Otherwise, if the glvalue has (possibly cv-qualified) type std::nullptr_t, the prvalue result is a null pointer constant (4.10). Otherwise, the value contained in the object indicated by the glvalue is the prvalue result.
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（8）lvalue reference and rvalue reference
A reference type that is declared using & is called an lvalue reference, and a reference type that is declared using && is called an rvalue reference. Lvalue references and rvalue references are distinct types. Except where explicitly noted, they are semantically equivalent and commonly referred to as references.
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参考
ISO/IEC 14882:2011
stackoverflow: value-category-meanings
Value categories
C++ Rvalue References Explained