为什么不能从元组分配对，但是可以从对分配元组? [英] Why can't you assign a pair from a tuple, but tuple can be assigned from a pair?

问题描述

我不清楚为什么分配tuple<X,Y>=pair<X,Y>

但是分配pair<X,Y>=tuple<X,Y>

    std::pair<int, double> x { 1 , 5.5};
    std::tuple<int, double> y { 1 , 5.5};
    int a;
    double b;
    std::tie(a,b) = x;
    std::tie(a,b) = y;
    x = y;  // THIS LINE (line 12)
    y = x;  // but this is fine ???

这不是对称的吗?

使用g ++ 4.8.1会出现以下错误:

Using g++ 4.8.1 gives the following errors:

tp.cpp:12:4: error: no match for operator= (operand types are std::pair<int, double> and std::tuple<int, double>)
  x = y;
    ^
tp.cpp:12:4: note: candidates are:
In file included from /opt/gcc-4.8.1/include/c++/4.8.1/utility:70:0,
                 from /opt/gcc-4.8.1/include/c++/4.8.1/tuple:38,
                 from tp.cpp:1:
/opt/gcc-4.8.1/include/c++/4.8.1/bits/stl_pair.h:158:7: note: std::pair<_T1, _T2>& std::pair<_T1, _T2>::operator=(const std::pair<_T1, _T2>&) [with _T1 = int; _T2 = double]
       operator=(const pair& __p)
       ^
/opt/gcc-4.8.1/include/c++/4.8.1/bits/stl_pair.h:158:7: note:   no known conversion for argument 1 from std::tuple<int, double> to const std::pair<int, double>&
/opt/gcc-4.8.1/include/c++/4.8.1/bits/stl_pair.h:166:7: note: std::pair<_T1, _T2>& std::pair<_T1, _T2>::operator=(std::pair<_T1, _T2>&&) [with _T1 = int; _T2 = double]
       operator=(pair&& __p)
       ^
/opt/gcc-4.8.1/include/c++/4.8.1/bits/stl_pair.h:166:7: note:   no known conversion for argument 1 from std::tuple<int, double> to std::pair<int, double>&&
/opt/gcc-4.8.1/include/c++/4.8.1/bits/stl_pair.h:177:2: note: template<class _U1, class _U2> std::pair<_T1, _T2>& std::pair<_T1, _T2>::operator=(const std::pair<_U1, _U2>&) [with _U1 = _U1; _U2 = _U2; _T1 = int; _T2 = double]
  operator=(const pair<_U1, _U2>& __p)
  ^
/opt/gcc-4.8.1/include/c++/4.8.1/bits/stl_pair.h:177:2: note:   template argument deduction/substitution failed:
tp.cpp:12:4: note:   std::tuple<int, double> is not derived from const std::pair<_T1, _T2>
  x = y;
    ^
In file included from /opt/gcc-4.8.1/include/c++/4.8.1/utility:70:0,
                 from /opt/gcc-4.8.1/include/c++/4.8.1/tuple:38,
                 from tp.cpp:1:
/opt/gcc-4.8.1/include/c++/4.8.1/bits/stl_pair.h:186:2: note: template<class _U1, class _U2> std::pair<_T1, _T2>& std::pair<_T1, _T2>::operator=(std::pair<_U1, _U2>&&) [with _U1 = _U1; _U2 = _U2; _T1 = int; _T2 = double]
  operator=(pair<_U1, _U2>&& __p)
  ^
/opt/gcc-4.8.1/include/c++/4.8.1/bits/stl_pair.h:186:2: note:   template argument deduction/substitution failed:
tp.cpp:12:4: note:   std::tuple<int, double> is not derived from std::pair<_T1, _T2>
  x = y;
    ^

推荐答案

我认为这是另一种情况:

I think this is another case of:

没有人提出来.

No one proposed it.

最后，您的代码可与 libc ++ (作为扩展名)一起使用. libc ++实现了类似元组"的概念，其中包括tuple，pair和array，然后具有在类似元组"类型上运行的成员模板(在tuple和pair上).这种方法虽然并非完全没有问题，但确实有希望.

Fwiw, your code works with libc++ (as an extension). libc++ implemented a "tuple-like" concept which includes tuple, pair and array, and then has member templates (on tuple and pair) that operate on "tuple-like" types. The approach hasn't been completely without problems, but it does seem promising.

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