在一个while循环内的cin [英] cin inside a while loop

问题描述

我写了一个简单的代码:

I have written a simple code:

#include <iostream>
using namespace std;
int main()
{
  int a, b;
  while (cin >> a >> b)     //Note the cin inside while loop
  { 
    cout << a << b << "\n";
  }
}

我们知道 while 循环仅在表达式计算为 true ( 1 )或 false (<代码> 0 ). cin 是如何评估 true 和 false 的.

We know that while loop functions only when the expression evaluates true (1) or false(0). How come cin is evaluating true and false.

当我输入数字时，while循环如何运行，当我输入非数字的内容时，while循环如何运行?它如何评估是非?

推荐答案

编写 cin>>a ，实际上是根据参考

When you writes cin >> a, you are actually using the std::istream::operator>>, according to the reference here, this operator returns an istream& object reference, and took the right hand variable (reference) as its argument. This is how you can chain it like: cin >> a >> b.

要查看此 cin>>一个>>b 链的另一种方式，当发生故障时，需要执行以下两个步骤:

To see this cin >> a >> b chain another way, when break down, it is this two steps:

• 第一步， cin>>一个返回一些中间值，假设它是 x .(您实际上可以尝试 auto x = cin>> a .
• 第二步，您正在执行(cin>> a)>>.b ，当我们使用中间值 x 时，我们可以将其写为 x>>.b .

• First step, cin >> a returns some intermediate value, let's say it is x. (You can actually try auto x = cin >> a.
• Second step, you are doing (cin >> a) >> b, when we use this intermediate value x, we could write it as x >> b.

那么这 x 到底是什么? x 在这里与 cin 保持相同的位置，它是 istream& 类型的对象.

So what the hell is this x? x here stays a same position as the cin, it is an object of istream& type.

因此，当您谈论 true 或 false 时，实际上是在谈论此返回的 istream& 引用是否指向对象.，无论是 true 还是 false .当标准输出捕获到EOF符号时(例如，当您在类似unix的系统中键入Ctrl-C时，或者当您读取到文件的末尾时)，将为 false .

Therefore, when you talk about true or false, you are actually talking about whether this returned istream& reference, refer to an object, whether it is true or false. It would be false when the standard output catch an EOF sign (like when you type Ctrl-C in unix like system, or when you have read to the end of a file).

因此，您的代码可以扩展为

Your code, therefore, could be expanded as

#include <iostream>
using namespace std;
int main()
{
  int a, b;
  auto x = cin >> a >> b
  while (x)
  { 
    cout << a << b << "\n";
  }
}

如果使用的是类似Visual Studio的IDE，则可以将鼠标指向变量 x ，它会提示您 x 的类型，即 istream& .

If you are using an IDE like Visual Studio, you could point your mouse at the variable x, it would prompt you x's type, and that would be an istream&.

此外，由于Bob__，该 istream& 类可以转换为 ios :: operator bool 类，如

Also, thanks to Bob__, this istream& class could be convert to an ios::operator bool class, as is written here, whether it is true or false represents the state(ios_base::iostate) of this stream, it therfore,

使使用流和返回对流的引用作为循环条件的函数成为可能，从而导致惯用的C ++输入循环，例如 while(stream>> value){...} 或 while(getline(stream，string)){...} .仅当输入操作成功时，此类循环才会执行循环的主体.

makes it possible to use streams and functions that return references to streams as loop conditions, resulting in the idiomatic C++ input loops such as while(stream >> value) {...} or while(getline(stream, string)){...}. Such loops execute the loop's body only if the input operation succeeded.

为进一步理解，您应该阅读教科书中的运算符(重载)"一章.

To further your understanding, you should read the operator (overloading) chapter in your textbook.

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