Question

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how do I validate user input as a double in C++?

I am new to C++, and I have a function in which I am wanting the user to input a double value. 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:

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.

Answer 1

If ostream& operator>>(ostream& , T&) fails the extraction of formatted data (such as integer, double, float, …), stream.fail() will be true and thus !stream will evaluate to true too.

So you can use

cin >> radius;
if(!cin){
    cout << "Bad value!";
    cin.clear();
    cin.ignore(numeric_limits<streamsize>::max(), '\n');
    cin >> radius;
}

or simply

while(!(cin >> radius)){
    cout << "Bad value!";
    cin.clear();
    cin.ignore(numeric_limits<streamsize>::max(), '\n');
}

It is important to ignore the 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

cin.ignore(numeric_limits<streamsize>::max(), '\n');

your loop will never end, as the input isn’t cleared from the standard input.

See also:

• 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>).