Question

I have a problem about cin.

int main(void)
{
    int a;
    float b;
    cin >> a >> b;
}

When I give a floating number (such as 3.14) as input, neither a nor b get the complete value (3.14): the output is a=3, b=0.14.

I know that cin will split the input by space, tab or Return, but ‘dot’ will not, right?

And why will the following code work?

int main(void)
{
    int i=0;
    int k=0;
    float j=0;

    cin >> i >> k >> j;    // i =3, j=k=0
}

And one more problem, what benefit will compiler do this for us?

Thanks!

Answer 1

The formatted input functions work quite simple:

1. They skip leading whitespace if any.
2. They try to read a format matching the type given.
3. If reading the value fails because the data doesn’t match the required format, they set std::ios_base::failbit. If reading fails the input shouldn’t change the variable attempted to be read (the standard input operators follow this rule but user defined input operator might not).

The first value you try to read is an int. Reading an int means that a optional leading sign is read followed by a sequence of digits (where, depending on your settings and the value given, the stream may read octal or hexadecimal numbers rather than decimal ones). That is, the int receives the value 3 and reading stops right in front of the ..

Depending on what you read next, the next read fails or doesn’t:

• In the first code you try to read a floating point value which starts with an optional sign, followed by an optional integral parts, followed by an optional thousands separator, followed by an optional fractional part, followed by an optional exponent. At least one digit is required in either the integral or the fractional part. In your example, there is only a thousands separate followed by a fractional part.
• When trying to read an integer, a . is found which isn’t a valid part of an int and reading fails.

After attempting to read a value, you should always try if the read operation was successful and report potential errors:

if (in >> value) {
    std::cout << "successfully read '" << value << "'\n";
}
else {
    std::cerr << "failed to read a value from input\n";
}

Note, that after a failed read you may need to clean up as well, e.g., using

in.clear();
in.ignore();

This first clears the error flags (without this, the stream would ignore any further attempts to read the data) and then it ignores the next character.