右值的成员访问运算符应该是一个 xvalue 吗？

Question

在 cpprefernce section: Value categories, it states that "the member of object expression, where a is an rvalue and m is a non-static data member of non-reference type" is an xvalue. In the standard (I found in: N4140, the draft C++14 standard) 中指出（第 87 页）"An expression is an xvalue if it is... a class member access expression designating a non-static data member of non-reference type in which the object expression is an xvalue."

我想用下面的代码检查我对此的理解：

struct B { // B for Boring...
  int i{0};
};

template <typename T> struct V {
  V() { cout << "V" << endl; }
};

template <typename T> struct V<T &> { // Partial Specialization
  V() { cout << "V&" << endl; }
};

template <typename T> struct V<T &&> { // Partial Specialization
  V() { cout << "V&&" << endl; }
};

int main() {
  int i{1};
  V<decltype((1))> v1;       // V
  V<decltype((i))> v2;       // V&
  V<decltype((move(i)))> v3; // V&&

  V<decltype((B().i))> v4;       // V, why not V&&?
  V<decltype((move(B()).i))> v5; // V&& as expected
}

如果我的计算是正确的，那么 B().i 是一个 "member of object expression, where [B()] is an rvalue," 并且根据引用，表达式应该是一个 xvalue，而 decltype 返回的类型应该是 int&&。请注意，我使用的是 gcc version 7.3.0 (Ubuntu 7.3.0-27ubuntu1~18.04)，我在没有标志和 -std=c++14.

的情况下进行了尝试

（明确地说，我也在此处检查我对 "decltype" 的理解，但我之前链接到的标准和参考资料在 decltype 的行为上逐字一致且非常清楚）。

抱歉，这不是问题...我说得对吗？是否应更新参考以澄清此行为？

Answer 1

是的，你是对的，这是一个错误，这看起来类似于 but this case dealt speifically with arrays which had a carve out in [expr.sub]p1 which said the result was an lvalue but deemed a defect。否则同样的逻辑适用。

我们可以看到它是fixed in clang 4.0 and if we check clang < 4.0 live, we obtain the same non-conforming results you see from gcc less than the most HEAD but clang 4.0 and > live这也是固定的。

另请注意 is also only fixed in gcc HEAD 中的问题。

另请注意 [expr.ref]p4.2

... If E1 is an lvalue, then E1.E2 is an lvalue; if E1 is an xvalue, then E1.E2 is an xvalue; otherwise, it is a prvalue...

在 C++11 之后更改为 this:

... If E1 is an lvalue, then E1.E2 is an lvalue; otherwise E1.E2 is an xvalue. ...

看起来更改最初是 DR 616 although what it has to do with indeterminate values 的一部分，我不确定。

右值的成员访问运算符应该是一个 xvalue 吗？

Should the member access operator of an rvalue be an xvalue?

c++

rvalue

decltype

lvalue

xvalue