交换参考的临时元组 [英] swap temporary tuples of references

问题描述

我编写一个自定义迭代器，当解引用返回一个引用的元组。由于元组本身是短暂的，我不认为我可以从操作符*（）返回一个引用。我认为我的迭代器语义上有意义，因为它有引用语义，即使operator *返回一个值。

I'm writing a custom iterator that, when dereferenced returns a tuple of references. Since the tuple itself is ephemeral, I don't think I can return a reference from operator*(). I think my iterator makes sense semantically, since it has reference semantics, even though operator* returns a value.

问题是，当我尝试调用std :: swap （或者，当std ::排序时），如下所示，我得到错误，因为交换期望l值。这个问题有很容易解决吗？

The issue is, when I try to call std::swap (or rather, when std::sort does), like below, I get errors because the swap expects l-values. Is there an easy fix to this problem?

#include <vector>

class test {
  public:
  test()
    :v1(10), v2(10)
  {}

  class iterator {
    public:
    iterator(std::vector<int>& _v1,
             std::vector<int>& _v2)
      :v1(_v1), v2(_v2){}

    std::tuple<int&, int&> operator*(){
      return std::tuple<int&, int&>{v1[5], v2[5]};
    }
    std::vector<int>& v1;
    std::vector<int>& v2;
  };

  std::vector<int> v1, v2;
};



int main(){
  test t;
  //error, Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/type_traits:3003:1: note: candidate function [with _Tp = std::__1::tuple<int &, int &>] not viable: expects an l-value for 1st argument

  //deep within the bowels of std::sort ...
  std::swap(*(test::iterator(t.v1, t.v2)),
            *(test::iterator(t.v1, t.v2)));
}

推荐答案

http://stackoverflow.com/a/8116843/596781\">垃圾场合，可能需要获得对临时值的引用。这很容易实现与 std :: move ：

On rare occasions it may be desirable to get an lvalue reference to temporary. This easily achieved with a cast opposite to std::move:

template <typename T>
T & stay(T && t) { return t; }

用法：

std::swap(stay(foo()), stay(bar()));

---

正如你已经说过的，如果你不能更改调用网站，最好的选择是编写自己的引用包装并使用ADL：

---

As you already said, if you can't change the call site, your best option may be to write your own reference wrapper and use ADL for that:

namespace detail
{
    struct IntRefPair
    {
        int & a, & b;
        IntRefPair(int & x, int & y) : a(x), b(y) {}
    };

    void swap(IntRefPair && lhs, IntRefPair && rhs)
    { 
        std::swap(lhs.a, rhs.a);
        std::swap(lhs.b, rhs.b);
    }
}

// ...

IntRefPair operator*() { return IntRefPair(v1[5], v2[5]); } }

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