C++: Enter an article without giving the number of words. Use a cin loop to get each word. How to stop the loop at the end of the article?

Question

Enter an article without giving the number of words. Use a cin loop to get each word. How to stop the loop at the end of the article?
For example:
Input: to be or not to be is a question
Code:

string a;
while( ? ) //无法停止循环
{
    cin >> a;
    // Do some thing...
}

Answer 1

while(cin >> a)
{
    // Do some thing...
}

cin在读取输入流的末尾是会设置标记位failbit为false

Answer 2

if(a == "quit")
    break;