为什么我们会在读取输入后调用 cin.clear() 和 cin.ignore() ? [英] Why would we call cin.clear() and cin.ignore() after reading input?

问题描述

Google 代码大学的 C++ 教程 曾经有这个代码:

// Description: Illustrate the use of cin to get input
// and how to recover from errors.

#include <iostream>
using namespace std;

int main()
{
  int input_var = 0;
  // Enter the do while loop and stay there until either
  // a non-numeric is entered, or -1 is entered.  Note that
  // cin will accept any integer, 4, 40, 400, etc.
  do {
    cout << "Enter a number (-1 = quit): ";
    // The following line accepts input from the keyboard into
    // variable input_var.
    // cin returns false if an input operation fails, that is, if
    // something other than an int (the type of input_var) is entered.
    if (!(cin >> input_var)) {
      cout << "Please enter numbers only." << endl;
      cin.clear();
      cin.ignore(10000,'
');
    }
    if (input_var != -1) {
      cout << "You entered " << input_var << endl;
    }
  }
  while (input_var != -1);
  cout << "All done." << endl;

  return 0;
}

cin.clear() 和 cin.ignore() 的意义是什么?为什么需要 10000 和 参数?

What is the significance of cin.clear() and cin.ignore()? Why are the 10000 and parameters necessary?

推荐答案

cin.clear() 清除 cin 上的错误标志(以便将来的 I/O操作将正常工作)，然后 cin.ignore(10000, ' ') 跳到下一个换行符(忽略与非数字同一行上的任何其他内容，以便它不会导致另一个解析失败).它最多只能跳过 10000 个字符，因此代码假设用户不会输入很长的无效行.

The cin.clear() clears the error flag on cin (so that future I/O operations will work correctly), and then cin.ignore(10000, ' ') skips to the next newline (to ignore anything else on the same line as the non-number so that it does not cause another parse failure). It will only skip up to 10000 characters, so the code is assuming the user will not put in a very long, invalid line.

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