书上不是说得很清楚了吗？
5.12.3. Other Implicit Conversions
Conversions Defined by the Library Types
Class types can define conversions that the compiler will apply automatically. Of the library types we've used so far, there is one important conversion that we have used. When we read from an istream as a condition

     string s;
     while (cin >> s)

we are implicitly using a conversion defined by the IO library. In a condition such as this one, the expression cin >> s is evaluated, meaning cin is read. Whether the read succeeds or fails, the result of the expression is cin.

The condition in the while expects a value of type bool, but it is given a value of type istream. That istream value is converted to bool. The effect of converting an istream to bool is to test the state of the stream. If the last attempt to read from cin succeeded, then the state of the stream will cause the conversion to bool to be truethe while test will succeed. If the last attempt failedsay because we hit end-of-filethen the conversion to bool will yield false and the while condition will fail.

#include <iostream>
using namespace std;
void main()
{
    int m;
    if (cin>>m)
    {
        cout<<"Yes"<<endl;
    }
    else 
        cout<<"no"<<endl;
}