cin 在一个 while 循环中

Question

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";
  }
}

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

Also how is while loop running when I am entering a number and stops when I enter something non-digit ? How is it evaluating true and false?

Answer 1

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 .

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

• 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 .

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

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).

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";
  }
}

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& .

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,

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.

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