C++ cin.fail() 的正确使用方法
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Correct way to use cin.fail()
提问by James Kanze
What is the correct way to use cin.fail();?
正确的使用方法是cin.fail();什么？
I am making a program where you need to input something. It is not very clear if you need to input a number or character. When a user inputs a character instead of a number the console goes crazy. How can I use cin.fail()to fix this?
我正在制作一个您需要输入内容的程序。是否需要输入数字或字符不是很清楚。当用户输入字符而不是数字时，控制台会发疯。我怎样才能cin.fail()解决这个问题？
Or is there a better way?
或者，还有更好的方法？
采纳答案by Shumail
cin.fail()returns true if the last cin command failed, and false otherwise.
cin.fail()如果最后一个 cin 命令失败，则返回 true，否则返回 false。
An example:
一个例子：
int main() {
int i, j = 0;
while (1) {
i++;
cin >> j;
if (cin.fail()) return 0;
cout << "Integer " << i << ": " << j << endl;
}
}
Now suppose you have a text file - input.txt and it's contents are:
现在假设你有一个文本文件 - input.txt，它的内容是：
30 40 50 60 70 -100 Fred 99 88 77 66
When you will run above short program on that, it will result like:
当你运行上面的短程序时，结果如下：
Integer 1: 30
Integer 2: 40
Integer 3: 50
Integer 4: 60
Integer 5: 70
Integer 6: -100
it will not continue after 6th value as it quits after reading the seventh word, because that is not an integer: cin.fail()returns true.
它不会在第 6 个值之后继续，因为它在读取第 7 个单词后退出，因为它不是整数：cin.fail()返回true。
回答by James Kanze
std::cin.fail()is used to test whether the preceding input
succeeded. It is, however, more idiomatic to just use the
stream as if it were a boolean:
std::cin.fail()用于测试前面的输入是否成功。然而，更习惯的做法是将流当作布尔值来使用：
if ( std::cin ) {
// last input succeeded, i.e. !std::cin.fail()
}
if ( !std::cin ) {
// last input failed, i.e. std::cin.fail()
}
In contexts where the syntax of the input permit either a number
of a character, the usual solution is to read it by lines (or in
some other string form), and parse it; when you detect that
there is a number, you can use an std::istringstreamto
convert it, or any number of other alternatives (strtol, or
std::stoiif you have C++11).
在输入的语法允许多个字符的上下文中，通常的解决方案是按行（或以其他字符串形式）读取它，然后解析它；当您检测到有一个数字时，您可以使用 anstd::istringstream来转换它，或者使用任意数量的其他替代方法（strtol，或者
std::stoi如果您有 C++11）。
It is, however, possible to extract the data directly from the stream:
但是，可以直接从流中提取数据：
bool isNumeric;
std::string stringValue;
double numericValue;
if ( std::cin >> numericValue ) {
isNumeric = true;
} else {
isNumeric = false;
std::cin.clear();
if ( !(std::cin >> stringValue) ) {
// Shouldn't get here.
}
}
