使用右值引用成员？ [英] Use of rvalue reference members?

问题描述

我想知道右值引用成员使用了什么

  class A {
 // ... 
 //这是有用的吗？ 
 Foo&& f; 
}; 
  

与左值引用成员相比，它有什么优点或缺点吗？什么是它的主要用例？

解决方案

我已经看到一个非常有用的用例的右值引用数据成员，在C ++ 0x草稿中：

  template< class ... Types> 
 tuple< Types&& ...> 
 forward_as_tuple（Types& ... t）noexcept; 
  

效果：构造
的元组引用参数在t
中适合作为参数
转发给函数。因为结果可能
包含对临时
变量的引用，程序应确保
此函数的返回值
不超过其任何参数。
（例如，程序应该通常
不将结果存储在命名的
变量中）。

返回： tuple< Types&& ...>（std :: forward< Types>（t）...）

当rvalue用作forward_as_tuple的参数时，元组具有右值引用数据成员，否则有左值引用数据成员。

发现forward_as_tuple后来有帮助，当需要捕获可变参数，完美地转发它们打包为一个元组，并重新扩展他们在转发到函子的时间点。我在这种风格中使用forward_as_tuple在实现LWG 1385中提出的tuple_cat的增强版本时：

http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#1385 p>

I was wondering what use an rvalue reference member has

class A {
  // ...
  // Is this one useful?
  Foo &&f;
};

Does it have any benefits or drawbacks compared to an lvalue reference member? What is a prime usecase of it?

解决方案

I've seen one very motivating use case for rvalue reference data members, and it is in the C++0x draft:

template<class... Types>
tuple<Types&&...>
forward_as_tuple(Types&&... t) noexcept;

Effects: Constructs a tuple of references to the arguments in t suitable for forwarding as arguments to a function. Because the result may contain references to temporary variables, a program shall ensure that the return value of this function does not outlive any of its arguments. (e.g., the program should typically not store the result in a named variable).

Returns: tuple<Types&&...>(std::forward<Types>(t)...)

The tuple has rvalue reference data members when rvalues are used as arguments to forward_as_tuple, and otherwise has lvalue reference data members.

I've found forward_as_tuple subsequently helpful when needing to catch variadic arguments, perfectly forward them packed as a tuple, and re-expand them later at the point of forwarding to a functor. I used forward_as_tuple in this style when implementing an enhanced version of tuple_cat proposed in LWG 1385:

http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#1385

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