不能创建一个带有空元组的一元元组（c ++ 0x） [英] cannot create a unary tuple with an empty tuple in it (c++0x)

问题描述

我在尝试元组，遇到创建元组的问题。
代码示例如下。

  // a.cpp 
 #include< tuple> 
 using namespace std; 
 
 int main（）{
 auto te = make_tuple（）; // this line is ok 
 auto tte = make_tuple（te）; //此行给出错误。 
 return 0; 
} 
  

我使用g ++ 4.5（g ++ -std = c ++ 0x a.cpp）和MS VC ++ 2010。
两个编译器都在main（）的第二行给我一个错误。

我的问题是：
因为'te'良好定义的变量，为什么不能创建te是内容的另一个元组。这个语义是否正确？

我猜这是一种边界情况，但如果算术正确，应该允许零。
$ b

FYI，来自gcc的错误消息是：

  $ gcc -std = c ++ 0x a.cpp 
 
在a.cpp中包含的文件中：1：0：
c：\mingw\bin\ ../ lib / gcc / mingw32 / 4.5.2 / include / c ++ / tuple：in constructor 
'std :: tuple< _Elements> :: tuple（std :: tuple< _UElements ...>&）[with _UElements = {}，
 _Elements = {std :: tuple<>}]'：
 
c：\mingw\bin\../lib/gcc/mingw32/ 4.5.2 / include / c ++ / tuple：551 ：62：从
实例化std :: tuple< typename std :: __ decay_and_strip< _Elements> :: __ type ...> 
 std :: make_tuple（_Elements&& ...）[with _Elements = {std :: tuple<>&}，typename 
 std :: __ decay_and_strip< _Elements> :: __ type =< ; type error>]'
 a.cpp：6：27：从这里实例化
 
c：\mingw\bin\ ../ lib / gcc / mingw32 / 4.5.2 / tuple<>'到类型'const std :: _ Tuple_impl< 0u>&'
 上的/ include / c ++ / tuple：259：70：error：invalid 
 static_cast code> 




解决方案

这看起来像是编译器匹配你的 std ::对于 std :: tuple< std :: tuple<>> 的以下构造函数使用元组<> （见20.4.2p15-17在N3242中）：

模板< class ... UTypes> tuple（const tuple< UTypes ...>& u）;

需要：
sizeof ...（Types）== sizeof ...（UTypes）。
is_constructible< Ti，const Ui&> :: value 对所有 i / p>

效果：
用
构造 * this 的每个元素u的相应元素。

注：此构造函数不应
参与重载解析
，除非 const Ui& 对于所有 i ，隐式地
可转换为 Ti 。

< blockquote>

我认为这是一个在你的编译器中执行 std :: tuple 的错误; remark意味着这个构造函数不应该被考虑，因为它不会编译。

I was experimenting with tuples and encountered a problem with creating tuples. The code example is as follows.

//a.cpp
#include <tuple>
using namespace std;

int main() {
  auto te = make_tuple();    //this line is ok
  auto tte = make_tuple(te); //this line gives an error.
  return 0;
}

I compiled it with both g++ 4.5 (g++ -std=c++0x a.cpp) and MS VC++2010. Both compilers are giving me an error on the second line in main().

My question is this: Since 'te' is a well-defined variable, why can't another tuple be created with te being the content. Is this semantic correct?

I guess this is kind of a boundary case, but if the arithmetic is correct, zero should be allowed, IMHO.

FYI, the error message from gcc is:

$ gcc -std=c++0x a.cpp

In file included from a.cpp:1:0:
c:\mingw\bin\../lib/gcc/mingw32/4.5.2/include/c++/tuple: In constructor
  'std::tuple<_Elements>::tuple(std::tuple<_UElements ...>&) [with _UElements = {},
  _Elements = {std::tuple<>}]':

c:\mingw\bin\../lib/gcc/mingw32/4.5.2/include/c++/tuple:551:62:   instantiated from
  'std::tuple<typename std::__decay_and_strip<_Elements>::__type ...> 
  std::make_tuple(_Elements&& ...) [with _Elements = {std::tuple<>&}, typename
  std::__decay_and_strip<_Elements>::__type = <type error>]'
a.cpp:6:27:   instantiated from here

c:\mingw\bin\../lib/gcc/mingw32/4.5.2/include/c++/tuple:259:70: error: invalid
  static_cast from type 'std::tuple<>' to type 'const std::_Tuple_impl<0u>&'

解决方案

This looks like the compiler has matched your std::tuple<> against the following constructor of std::tuple<std::tuple<>> (See 20.4.2p15-17 in N3242):

template <class... UTypes> tuple(const tuple<UTypes...>& u);

Requires: sizeof...(Types) == sizeof...(UTypes). is_constructible<Ti , const Ui &>::value is true for all i.

Effects: Constructs each element of *this with the corresponding element of u.

Remark: This constructor shall not participate in overload resolution unless const Ui & is implicitly convertible to Ti for all i.

I think this is a bug in the implementation of std::tuple from your compiler; the "remark" implies that this constructor should not be considered, since it won't compile.

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