Re: Clearing the input buffer - then pausing

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Tom,

Thank you very much for the effort you put into explaing things in such
great detail. It is greatly appreciated. Although it should have been
obvious, I didn't consider that ignore also blocked. So is guess my
remaining issue is whether or not it is possible, in C or in C++, to portably
cause a program to pause at some arbitrary point, regardless of what may or
may not already be in the buffer, then continue when the Enter key (preceded
by 0 or more arbitrary characters) is struck.

Thanks,
Ray

"Tom Widmer" wrote:

> Ray Mitchell wrote:
> > VS .NET 2003: I should know this, but I don't! I would like to pause my
> > program at some arbitrary point and resume only when the Enter key is pressed
> > once (preceeded by zero or more other characters). The code below works if I
> > do not comment out the first cin line. If I do, however, it takes two
> > strokes of the Enter key. What appears to be happening is that the ignore
> > method does not operate on a new buffer, but that's just a guess. I've tried
> > several different things but it has become painfully clear that I should
> > consider another line of work! Additionally, it seems there must be less
> > complex way to accomplish this task. Thanks.
>
> You need to understand how console input interacts with std::cin.
> Firstly, you have to remember that std::cin is just a stream interface -
> it processes a stream of characters that might come from a pipe (via OS
> piping), or from a file, or from the console; std::cin can't itself tell
> which it is getting.
>
> When using a console, every character that appears on the screen is
> inserted into the cin stream, but this only happens for a line of text
> when enter is pressed. So, to your example:
>
> >
> > Note: cin.ignore(INT_MAX, '\n');
> > It seemed to me that cin.ignore(INT_MAX); would be more appropriate, but it
> > is worse.
> >
> > #include <iostream>
> > #include <climits>
> > using namespace std;
> >
> > int main()
> > {
> > int value = 0;
> > cout << "Enter an intergal value: ";
> 1:
> > cin >> value; // Requires 2nd Enter if this line commented out
> > cout << "The value was " << value << endl;
> >
> > int ch;
> 2:
> > cin.ignore(INT_MAX, '\n'); // It seemed that cin.ignore(INT_MAX); would be
> 3:
> > while ((ch = cin.peek()) != '\n' && ch != EOF)
> > ch = cin.get();
> >
> > return 0;
> > }
>
> With the line not commented out:
> The input stream is given, for example:
> 1\nsome random characters\n
>
> At 1:, cin blocks until the first \n is entered. The stream buffer looks
> like this:
> 1\n
> Then the cin >> value pulls the '1' off the stream. So the stream now
> looks like this:
> \n
> Now, 2 doesn't block, since it consumes characters up until the \n
> character in the buffer (just consuming 1 character). Now the stream is
> empty.
> Finally, 3: (the first cin.peek call) blocks until another \n is
> pressed, so the buffer might now contain:
> some random text\n
> The get calls now pull of the characters up to the \n (which isn't
> consumed since you only peek at it) without blocking, so the final
> buffer state is just:
> \n
>
> Now lets see what happens with the line commented out. At 2:, the cin
> buffer is empty, so cin.ignore blocks. You enter some text and press
> enter, transferring the text into the cin stream:
> some random text\n
> Now, the ignore call pulls those characters off the stream until it hits
> a \n character, so the stream is now "empty".
> Next, 3: blocks until a second enter is pressed, because the peek call
> finds an empty buffer. You enter some more characters and press enter:
> some more random text\n
> Finally, the peek calls unblockingly consume those characters up to the
> \n, leaving the final unconsumed characters as:
> \n
>
> Is that clearer? If you comment out the cin >> value, you should also
> comment out the ignore line.
>
> Tom
>
.

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