为什么下面的c ++代码保持输出“坏数据，再试一次”？ [英] Why the following c++ code keeps output "bad data, try again"?

问题描述

int main()
{
        int temp;
        while (cin>>temp, !cin.eof())
        {
                if (cin.bad())
                {
                        throw runtime_error("IO stream corrupted");
                }

                if (cin.fail())
                {
                        cerr<<"bad data, try again";
                        cin.clear(istream::failbit);
                        continue;
                }
        }

        return 0;
}

如果输入 x 然后输入，输出将是：

，尝试重新输入数据againbad
data，try aa，try againbad data，try
againbad data，try ta，try againbad
data，try againbad data，try ata，try
againbad data，try againbad数据，tr
数据，尝试againbad数据，尝试againbad
数据，t

, try againbad data, try againbad data, try a a, try againbad data, try againbad data, try ta, try againbad data, try againbad data, try ata, try againbad data, try againbad data, tr data, try againbad data, try againbad data, t

为什么？ >

Why?

推荐答案

ios :: clear（）方法实际上用其参数替换流控制状态位。您每次发出 cin.clear时将 cin 的控制状态设置为 fail （istream :: failbit）; 。

The ios::clear() method actually replaces the stream control state bits with its argument. You're setting cin's control state to fail every time you issue cin.clear(istream::failbit);.

你应该简单地调用 cin.clear（）; 而不是参数。这会将流的控制状态重置为好。

You should simply call cin.clear(); instead, without arguments. That will reset the stream's control state to good.

编辑： ，我忘了。

您还需要调用 istream :: ignore（）丢弃您刚才输入的无效 x 令牌，因为 clear 不会冲刷待处理的输入：

You also need to call istream::ignore() to discard the invalid x token you've just entered, since clear() doesn't flush pending input:

if (cin.fail()) {
    cerr << "bad data, try again\n";
    cin.clear();
    cin.ignore(numeric_limits<streamsize>::max(), '\n');
    continue;
}

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