Re: [Mingw-users] Question about bool data type

[Greg C. <gch...@sb...>]

On 2011-09-23 00:54Z, Mani wrote:
> 
> I see that for double input stored as ints, it works fine..
> 
>     {
>            int x;
> 
>            cin >> x;
>            if(cin.fail()) {
>                          cout << "Input failed" << endl;
>            }
>            cout << "x = " << x << endl;
>     }
> 
> if the user enters 10.3 -- cin succeeds, and it works as expected...

It should display "10".

The reason is not that it
  reads 10.3 as a floating-point number (it doesn't);
  converts that floating-point value to an integer (again, no); and
  prints the integer.
That's not what's happening. Instead, it
  reads "1" and "0", and stores them in a buffer;
  then reads "." and stops reading there because a decimal point
    cannot be part of an integer literal; then
  converts buffer contents "10" to an integer, and prints it; and
  leaves ".3" on the stream.

> can you explain why the difference??

For std::cin >> some_int, input is read and converted as if by
scanf(): i.e., characters are read one at a time, stopping at the
first character that can't be part of an int value; any remaining
characters are left on the input stream; and the characters that
have been read (before the first invalid one) are converted to an
int if possible; otherwise (e.g. if it's too large) failbit is set.

For std::cin >> some_bool, a long int is extracted as above; if
its value is either 0 or 1, then that value is assigned to the bool;
otherwise, failbit is set.

In both cases, int and bool, the operation fails if the destination
cannot represent the result of converting the extracted characters
to an integer. In the case of bool, that excludes everything but one
and zero.

If the stream contains "127.0.0.1", then "127" is extracted from
the stream, and ".0.0.1" is not. This is a valid int, but not a
valid bool. OTOH, if the stream contains "1x" or "0x", then
std::cin >> some_bool assigns values of 1 or 0 respectively to the
bool; but if it contains "10x", then nothing is assigned to the bool.
It is prudent to check the stream state after such operations.