std::cin 失败时的行为

Question

C++ 新手，我正在检查 cin 在意外输入时的行为并编写了以下代码

#include <iostream>

using std::cout;
using std::endl;
using std::cin;

int main() {
    
    int num = -1;

    cin >> num;
    cout << num << "\n";

    char alph = 'z';

    cin >> alph;
    cout << alph << "\n";

    return 0;
}

所以，它应该是一个数字，然后是一个字符。以下是不同类型输入的结果

5 a
5
a

5a
5
a

aa
0
z

预计第一个。

在第二篇中，我读到 here 因为 std::cin 能够提取 5 操作将成功，将 a 留在缓冲区中。因此，a 被我们的字符输入。

第三种情况我看不懂。这里第一个 cin 提取失败。

然后呢？因为第二个 cin 没有做任何事情。 a 中的 none 是否留在缓冲区中。

Answer 1

您引用的 link 解释了正在发生的事情：

https://www.learncpp.com/cpp-tutorial/stdcin-and-handling-invalid-input/

When the user enters input in response to an extraction operation, that data is placed in a buffer inside of std::cin.

When the extraction operator is used, the following procedure happens:

If there is data already in the input buffer, that data is used for extraction.

If the input buffer contains no data, the user is asked to input data for extraction (this is the case most of the time). When the user hits enter, a ‘\n’ character will be placed in the input buffer.

operator>> extracts as much data from the input buffer as it can into the variable (ignoring any leading whitespace characters, such as spaces, tabs, or ‘\n’).

Any data that can not be extracted is left in the input buffer for the next extraction.

到目前为止，还不错。文章继续：

[Upon an input error] std::cin goes immediately into “failure mode”, but also assigns the closest in-range value to the variable. Consequently, x is left with the assigned value of 32767.

Additional inputs are skipped, leaving y with the initialized value of 0.

这解释了您看到的“0”。它还解释了为什么“z”没有替换为“a”。

std::cin 失败时的行为

Behavior of std::cin on failure

c++

cin