c++ - C++ 中的 xvalue 是什么

Question

我试图了解值(value)类别的差异，我发现 this document由微软开发。

- An xvalue is a glvalue that [...]
- An lvalue is a glvalue that is not an xvalue.
- An rvalue is a prvalue or an xvalue.

为什么xvalue是glvalue？他们为什么不说xvalue是左值或右值？在我看来，这个措辞是有意传达某种信息，但我不明白为什么

Answer 1

Why didn't they say An xvalue is an glvalue or an rvalue?

因为右值尚未定义，并且右值将根据 xvalue 定义。必须在不依赖另一个的情况下定义一个，以避免定义变得循环。

还因为 xvalue 既是左值又是右值。

此外，xvalue 是左值，但不是左值。更进一步，xvalue 是一个右值，而不是纯右值。最后，xvalue 是一个既不是左值也不是纯右值的表达式。

I don't understand why they only mentioned gvalues in that sentence

仅定义了左值和右值。请参阅前面提到的避免循环定义。

What are xvalues in C++

好吧，您已经看到了它的定义。该标准有以下非规范性注释，提供了更多信息:

[basic.lval]

[ Note: An expression is an xvalue if it is:

• the result of calling a function, whether implicitly or explicitly, whose return type is an rvalue reference to object type ([expr.call]),
• a cast to an rvalue reference to object type ([expr.type.conv], [expr.dynamic.cast], [expr.static.cast] [expr.reinterpret.cast], [expr.const.cast], [expr.cast]),
• a subscripting operation with an xvalue array operand ([expr.sub]),
• a class member access expression designating a non-static data member of non-reference type in which the object expression is an xvalue ([expr.ref]), or
• a .* pointer-to-member expression in which the first operand is an xvalue and the second operand is a pointer to data member ([expr.mptr.oper]).