当我们使用 std::cin 将“char 类型”值存储在“整数类型”变量中时会发生什么？

Question

int i;
std::cin >> i; // i = 'a'

当我们尝试这样做时，

std::cin

有什么反应？
我们知道，当

std::cin

获取一个值时，它会将其转换为 ASCII 或其他内容，然后将其存储在变量中，那么

std::cin

对此有何反应？

Answer 1

不，它不存储您输入的字符的 ASCII 值

i

。相反，该流将为输入流放置一个失败标志，这意味着整数的读取导致失败。这是演示这一点的代码。

int i;
cout << cin.fail();     // 0 as cin doesn't failed yet
cin >> i;               // A char is entered instead of integer
cout << cin.fail();     // 1 as cin failed to read a character  

Answer 2

如果您正在读取的值符合您的预期，则应该对

std::cin

进行错误检查，否则

std::cin

将失败并且

i

的值将设置为

0

：

#include <iostream>

int main()
{
    int i;
    if(std::cin >> i)
    {
        std::cout << i << std::endl;
    }
    else 
    {
        std::cin.clear();
        std::cout << "No integer" << std::endl;
    }

    return 0;
}

您还可以使用

while

循环来处理错误检测。一个例子如下：

#include <iostream>
#include <limits>


int main()
{
    int age { };

    // executes the loop if the input fails or is out of range (e.g., no
    // characters were read)
    while ( ( std::cout << "How old are you? " ) &&
            ( !( std::cin >> age ) || age < 1 || age > 200 ) )
    {
        std::cout << "That's not a number between 1 and 200; ";
        // clear bad input flag
        std::cin.clear();
        // discard bad input
        std::cin.ignore( std::numeric_limits<std::streamsize>::max(), '\n' );
    }

    std::cout << "You are " << age << " years old.\n";
}

输入/输出示例：

How old are you? a
That's not a number between 1 and 200; How old are you? #12gj
That's not a number between 1 and 200; How old are you? foo
That's not a number between 1 and 200; How old are you? 84
You are 84 years old.