c++，如何验证输入的数据是正确的数据类型
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c++, how to verify is the data input is of the correct datatype
提问by Brook Julias
Possible Duplicate:
how do I validate user input as a double in C++?
可能的重复：
如何在 C++ 中将用户输入验证为双精度值？
I am new to C++, and I have a function in which I am wanting the user to input a doublevalue. How would I go about insuring that the value input was of the correct datatype? Also, how would an error be handled? At the moment this is all I have:
我是 C++ 新手，我有一个函数，我希望用户在其中输入一个double值。我将如何确保输入的值是正确的数据类型？另外，错误将如何处理？目前这就是我所拥有的：
if(cin >> radius){}else{}
I using `try{}catch(){}, but I don't think that would the right solution for this issue. Any help would be appreciated.
我使用 `try{}catch(){}，但我认为这不是解决此问题的正确方法。任何帮助，将不胜感激。
回答by Zeta
If ostream& operator>>(ostream& , T&)fails the extraction of formatted data (such as integer, double, float, ...), stream.fail()will be true and thus !streamwill evaluate to true too.
如果ostream& operator>>(ostream& , T&)格式化数据（如整数、双精度、浮点数、...）的提取失败，则为stream.fail()真，因此!stream也将评估为真。
So you can use
所以你可以使用
cin >> radius;
if(!cin){
cout << "Bad value!";
cin.clear();
cin.ignore(numeric_limits<streamsize>::max(), '\n');
cin >> radius;
}
or simply
或者干脆
while(!(cin >> radius)){
cout << "Bad value!";
cin.clear();
cin.ignore(numeric_limits<streamsize>::max(), '\n');
}
It is important to ignorethe rest of the line, since operator>>won't extract any data from the stream anymore as it is in a wrong format. So if you remove
这ignore对行的其余部分很重要，因为operator>>不会再从流中提取任何数据，因为它的格式错误。所以如果你删除
cin.ignore(numeric_limits<streamsize>::max(), '\n');
your loop will never end, as the input isn't cleared from the standard input.
你的循环永远不会结束，因为输入没有从标准输入中清除。
See also:
也可以看看：
std::basic_istream::ignore(cin.ignore)std::basic_istream::fail(cin.fail())std::numeric_limits(used for the maximum number of ignored characters, defined in<limits>).
std::basic_istream::ignore(cin.ignore)std::basic_istream::fail(cin.fail())std::numeric_limits（用于最大忽略字符数，在 中定义<limits>）。
回答by Jesse Good
You need to read the entire line using std::getlineand std::string. That is the way to fully verify that the entire line is of the correct data type:
您需要使用std::getline和阅读整行std::string。这是完全验证整行是否为正确数据类型的方法：
std::string line;
while(std::getline(std::cin, line))
{
std::stringstream ss(line);
if ((ss >> radius) && ss.eof())
{
// Okay break out of loop
break;
}
else
{
// Error!
std::cout << "Invalid input" << std::endl;
}
}
回答by Vikas
This example is self explanatory, however with this approach you can't distinguish between intand doubledata types.
这个例子是不言自明的，但是使用这种方法您无法区分int和double数据类型。
int main()
{
double number = 0;
if (!(std::cin >> number))
{
std::cout << "That's not a number; ";
}
else
{
std::cout << "That's a number; ";
}
}
