c++ primer 1.44 if 语句 例题 不解

[maosengshulei]

#include <iostream>

int main() { // currVal is the number we're counting; we'll read new values into val

int currVal = 0, val = 0;

// read first number and ensure that we have data to process
if (std::cin >> currVal) {        
	int cnt = 1;  // store the count for the current value we're processing
	while (std::cin >> val) { // read the remaining numbers 
		if (val == currVal)   // if the values are the same
			++cnt;            // add 1 to cnt 
		else { // otherwise, print the count for the previous value
			std::cout << currVal << " occurs " 
			          << cnt << " times" << std::endl;
			currVal = val;    // remember the new value
			cnt = 1;          // reset the counter
		}
	}  // while loop ends here
	// remember to print the count for the last value in the file
	std::cout << currVal << " occurs " 
	          << cnt << " times" << std::endl;
} // outermost if statement ends here
return 0;

}

源码如上 为何自己运行时最后一个数无法输出啊？

[zpole]

while 没有退出

[maosengshulei]

意思是运行时输入完一串数后要输入 ctrl+z 才能结束是吗

[aprikyblue]

> while(std::cin>>val)

operator>>返回结果为 std::cin 本身，然后其通过类型转换来进行 if 判断
正常输入是一直为 true
当 ctrl+z 输入 EOF ，使得 cin 流状态变为无效，然后求值变为 false ，此时 while 退出
cin.clear()可以重新将 cin 置为有效