非const引用类型从右值 [英] non-const reference of type from an rvalue

问题描述

请考虑以下代码：

  class Widget {}; 
 
 template< typename T> 
 T&& foo2（T& t）{
 return std :: forward< T& 
} 
 
 ///返回第一个元素
 template< typename T> 
 typename std :: tuple_element< 0，typename std :: decay< T> :: type> :: type&& foo（T& t）{
 return std ::向前typename std :: tuple_element< 0，typename std :: decay< T> :: type> :: type> 
（std :: get< 0>（t））; 
} 
 
 Widget w; 
 auto list = std :: make_tuple（
 w，
 Widget（）
）; 
 
 
 int main（）
 {
 auto& l = foo（list）; // this is not work 
 // auto& l2 = foo2（std :: get< 0>（list））; //这一个工程。 
} 
  

http://coliru.stacked-crooked.com/a/4d3b74ca6f043e45

当我尝试编译我收到以下错误：

 错误：无效的初始化类型'Widget& type'std :: tuple_element< 0ul，std :: tuple< Widget，Widget> > :: type {aka Widget}'
  

好吧， / p>

• 首先，

b
$ b

PS正如你所看到的，我尝试写函数，其操作与左值和右值。如果第一个元素是临时的，我想返回rvalue，如果它不是-lvalue。

解决方案

tuple_element 返回元素类型，而不是引用类型（除非元素类型本身是引用类型）。

您需要返回

这可以用条件表达：

  typename std :: conditional< std :: is_lvalue_reference< T> :: value，
 typename std :: add_lvalue_reference< 
 typename std :: tuple_element< 0，typename std :: decay< T> :: type> :: type> :: type，
 typename std :: tuple_element< 0，typename std :: decay< ; T> :: type> :: type> :: type 
  

decltype ，因为 std :: get 已为您执行此计算：

  decltype（std :: get< 0>（std :: declval< T&&>（）））&& 
  

Consider the following code:

class Widget{};

template<typename T>
T &&foo2(T &&t){
    return std::forward<T>( t );
}

/// Return 1st element
template<typename T>
typename std::tuple_element<0, typename std::decay<T>::type >::type  &&foo(T &&t){
    return std::forward< typename std::tuple_element<0, typename std::decay<T>::type >::type >
            ( std::get<0>(t) );
}

Widget w;
auto list = std::make_tuple(
    w,
    Widget()
);


int main()
{
  auto &l  = foo(list );                      // This is NOT work
  //auto &l2 = foo2( std::get<0>(list) );     // This one works.
}

http://coliru.stacked-crooked.com/a/4d3b74ca6f043e45

When I tried to compile this I got the following error:

error: invalid initialization of non-const reference of type 'Widget&' from an rvalue of type 'std::tuple_element<0ul, std::tuple<Widget, Widget> >::type {aka Widget}'

Well, and that would be ok, but:

• at first, that Widget w is not temporary. Why it treat it like temporary?

• at second, why foo2 works than?

P.S. As you see, I try to write function which operates both with lvalue and rvalue. If first element is temporary I want to return rvalue, if it is not - lvalue.

解决方案

tuple_element returns the element type, not a reference type (unless the element type is itself a reference type).

You need to have it return a reference type if the type T is a reference type.

This can be expressed with a conditional:

typename std::conditional<std::is_lvalue_reference<T>::value,
    typename std::add_lvalue_reference<
        typename std::tuple_element<0, typename std::decay<T>::type >::type>::type,
    typename std::tuple_element<0, typename std::decay<T>::type >::type>::type

Or, more easily, using decltype, since std::get already performs this calculation for you:

decltype(std::get<0>(std::declval<T &&>())) &&

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