cin.ignore(3) after failure to get a double with cin >> var

I'm learning c++ with c++ Primer Plus, and I'm on chapter 7 and trying to create the following function:

double getDouble(const char * message){
    double temp;
    std::cout << (message);
    if (!(std::cin >> temp)){
        std::cin.clear();
        std::cin.ignore(3); // Why does this work specifically?
        std::cout << "Input not double!" << std::endl;
        return 0.0;
    }
    return temp;

}

istream::ignore well, ignores the next N characters or until the delimiter is hit. The 3 seems like a random magic number to me. Or what did you want to know?

The C++ standard guarantees (§27.6.1.3/23 in the '03 standard) that passing std::numeric_limits<std::streamsize>::max() will ignore everything in the stream until the specified delimiter or end of input is reached. So change

std::cin.ignore(3);

to

std::cin.ignore(std::numeric_limits<std::streamsize>::max());

Using magic numbers such as 3 when calling ignore is very rarely appropriate.