参考

https://en.cppreference.com/w/cpp/language/value_category

左值

历史习惯问题，叫左值，可以放在等号左边的对象和函数。例如，如果E是一个指针类型的表达式，*E就是一个左值表达式，可以指向E指向的对象或函数。另一个例子就是如果函数调用的返回值是一个左值引用，也是一个左值。
— 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. —end example ]

右值

prvalue和xvalue，都属于右值。有些名词的定义，没有办法说出来，只能通过举例来说明概念，这个右值就属于这一类。
右值的属性：右值不能使用取址符来取址。举例：
An rvalue can’t be used as the left-hand operand of the built-in assignment or compound assignment operators.
An rvalue may be used to initialize a const lvalue reference, in which case the lifetime of the object identified by the rvalue is extended until the scope of the reference ends.
An rvalue may be used to initialize an rvalue reference, in which case the lifetime of the object identified by the rvalue is extended until the scope of the reference ends.
When used as a function argument and when two overloads of the function are available, one taking rvalue reference parameter and the other taking lvalue reference to const parameter, an rvalue binds to the rvalue reference overload (thus, if both copy and move constructors are available, an rvalue argument invokes the move constructor, and likewise with copy and move assignment operators).

临时值

xvalue，也是是指向一个对象，但是通常快要终结的对象。xvalue是特定表达式牵扯rvalue引用的结果。
— 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 (8.3.2). [ Example: The result of calling a function whose return type is an rvalue reference is an xvalue. —end example ]

glvalue

glvalue是一般化的lvalue，是一个lvalue或者xvalue。
— A glvalue (“generalized” lvalue) is an lvalue or an xvalue.
rvalue，也是历史习惯的称号，rvalue可以出现在赋值符号的右边。rvalue是一个xvalue，临时对象或者是一个只，不关联任何对象。
— An rvalue (so called, historically, because rvalues could appear on the right-hand side of an assignment expression) is an xvalue, a temporary object (12.2) or subobject thereof, or a value that is not associated with an object.

纯右值

prvalue，纯右值；就是不是xvalue的rvalue。例子：就是如果一个函数调用返回了一个非引用的对象，就是纯右值。
— 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. —end example ]

总结

每一个表达式只能属于其中的一种：lvalue，xvalue或者rvalue。

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c++: rvalue, prvalue, lvalue, glvalue

文章目录

参考

左值

右值

临时值

glvalue

纯右值

总结

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