C++中标准输入流cin与Ctrl+Z使用时的问题

今天使用C++编写了一段小程序，练习使用标准库的算法，代码如下：

 #include <iostream>
 #include <algorithm>
 #include <vector>
 #include <string>

 using std::vector;
 using std::cin;
 using std::cout;
 using std::endl;
 using std::string;

 int main() {
     vector<string> vec;
     string str;
     cout << "Please enter some string..." << endl;
     while (cin >> str) {
         vec.push_back(str);
     }
     cout << "Please enter the key word: ";
     cin >> str;
     auto result = find(vec.cbegin(), vec.cend(), str);
     if (result == vec.cend()) {
         cout << "Failed to find the key word..." << endl;
     } else {
         cout << "Succeed to find the key word, it is the " << result - vec.cbegin() +  << "th word..." << endl;
     }
     system("pause");
     return ;
 }

然而运行时却出现了问题，运行截图如下：

从运行结果来看，第20行的cin >> str;根本没有执行,于是输出cin的状态位看看出了什么问题:

 cout << "cin.fail() = " << cin.fail() << endl;
 cout << "cin.bad() = " << cin.bad() << endl;
 cout << "cin.eof() = " << cin.eof() << endl;

可以看出，输入流cin的failbit以及eofbit已经被置位，但badbit没有置位，这说明cin流并没有崩溃，但是流已经到达了文件结束，IO操作失败，因此cin >> str;自然没有成功执行。

而问题出现的原因在于while(cin>>str)语句在结束输入时使用了Ctrl+Z，告诉cin用户已经结束了输入，所以eofbit与failbit被置位。

为了让程序正常运行，只需调用cin.clear()让cin的所有条件状态位复位，将状态设置为有效即可。

 #include <iostream>
 #include <algorithm>
 #include <vector>
 #include <string>

 using std::vector;
 using std::cin;
 using std::cout;
 using std::endl;
 using std::string;

 int main() {
     vector<string> vec;
     string str;
     cout << "Please enter some string..." << endl;
     while (cin >> str) {
         vec.push_back(str);
         cout << "Push str = " << str << endl;
     }
     cout << "Please enter the key word: ";
     cin.clear();
     cin >> str;
     cout << "str = " << str << endl;
     cout << "cin.fail() = " << cin.fail() << endl;
     cout << "cin.bad() = " << cin.bad() << endl;
     cout << "cin.eof() = " << cin.eof() << endl;
     auto result = find(vec.cbegin(), vec.cend(), str);
     if (result == vec.cend()) {
         cout << "Failed to find the key word..." << endl;
     }
     else {
         cout << "Succeed to find the key word, it is the " << result - vec.cbegin() +  << "th word..." << endl;
     }
     system("pause");
     return ;
 }

运行结果如下所示：