结构绑定和变量类型 [英] Structural binding and type of variables

问题描述

我想问一个关于结构绑定以及变量如何获取其类型的问题。
这是我编译的代码。

I would like to ask a question about structural binding and how the variables get their types. Here is the code I compile.

struct T {
  explicit T(int a = 1) {
    a_ = a;
    std::cout << "Default" << std::endl;
  }
  T(const T& t) {
    a_ = t.a_;
    std::cout << "Copy" << std::endl;
  }
  int a_;
};

int main() {

  std::tuple<bool, T> bi{true, T(1)};
  std::cout << "START" << std::endl;
  auto& [b, i] = bi;
  static_assert(std::is_same<decltype(i), T>::value);
  std::cout << i.a_ << std::endl;
  i.a_++;
  std::cout << i.a_ << std::endl;
  std::cout << (&i == &get<1>(bi)) << std::endl;


  return 0;
}

代码会编译，结果是：

Default
Copy
START
1
2
1

您可以看到变量 i 实际上是指得到< 1>（bi）。
但是，变量 i 的类型是 T ，因为 std :: is_same< decltype（i），T> :: value 。
您能否解释一下为什么结构绑定如此工作？
我已经在 C ++模板：完整指南书中读过
，作者指出变量 i 必须具有类型 std :: tuple_element< 1，std :: tuple< bool，T> > :: type& 。我不明白为什么 decltype（i）只是 T 。

As you can see the variable i actually refers to get<1>(bi). However, the type of the variable i is T since std::is_same<decltype(i), T>::value. Could you please explain why structural binding works this way? I have read about it in the book "C++ Templates: The Complete Guide" and the authors state that the variable i must have type std::tuple_element<1, std::tuple<bool, T> >::type&&. I don't understand why decltype(i) is just T.

推荐答案

decltype 很有趣。其规范包含用于结构化绑定的特殊条款

decltype is funny that way. Its specification contains a special clause for structured bindings

4 对于表达式 e ，用表示的类型decltype（e）是
，定义如下：

4 For an expression e, the type denoted by decltype(e) is defined as follows:

• 如果 e 是未括号的id表达式，命名为结构化绑定（[dcl.struct.bind]）， decltype（e）被引用类型为结构化绑定声明的规范中给出的
  ；

• if e is an unparenthesized id-expression naming a structured binding ([dcl.struct.bind]), decltype(e) is the referenced type as given in the specification of the structured binding declaration;

...

因此，尽管每个结构化绑定确实是对元组的引用，但 decltype 报告引用对象的类型。可能是因为结构化绑定的意思是像常规命名变量一样。

So while each structured binding is indeed a reference into the tuple, decltype reports the type of the referent. Likely since structured bindings are meant to feel like regular named variables.

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