dynamic_cast 和右值引用

Question

class A{
public:
    virtual ~A() {};
};

class B : public A{ };

int main(){

    A&& p = B();

    dynamic_cast<B&&>(std::move(p));

}

抛出错误（g++ 5.2.0）：

error: conversion to non-const reference type 'std::remove_reference<A&>::type& {aka class A&}' from rvalue of type 'A' [-fpermissive]

它试图将 std::move(p) 强制转换为键入 A&，但我不明白为什么。我认为有必要在转换为右值引用之前将 p 转换为右值，但如果我删除 std::move 它编译正常。来自 cppreference:

dynamic_cast < new_type > ( expression )

Similar to other cast expressions, the result is:

an lvalue if new_type is an lvalue reference type (expression must be an lvalue)

an xvalue if new_type is an rvalue reference type (expression may be lvalue or rvalue)

连N3337的5.2.7:

dynamic_cast<T>(v)

If T is a pointer type, v shall be a prvalue of a pointer to complete class type, and the result is a prvalue of type T. If T is an lvalue reference type, v shall be an lvalue of a complete class type, and the result is an lvalue of the type referred to by T. If T is an rvalue reference type, v shall be an expression having a complete class type, and the result is an xvalue of the type referred to by T.

唯一的要求是我使用完整的 class 类型，std::move(p) 是，不是吗？

Answer 1

这似乎有效：

B&& b = std::move(dynamic_cast<B&>(p));

不能告诉你为什么你的不正确。

Answer 2

您的代码当然没问题：

If T is an rvalue reference type, v shall be an expression having a complete class type, and the result is an xvalue of the type referred to by T.

据推测，dynamic_cast 在引入右值引用时未正确更新，并且仍然执行 pre-C++11 规则，即右值只能绑定到 const 左值引用（请注意，将目标类型更改为 B const&& 时它甚至不起作用，尽管错误消息暗示了这一点！）。

归档为#69390。

dynamic_cast 和右值引用

dynamic_cast and rvalue reference

c++

dynamic-cast

rvalue

language-lawyer

c++11