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I encountered a strange problem earlier. I was working at insertion in B-trees and I wrote a simple display function. I ran it and it didn't show anything in the console even though I inserted some values.

I entered in the debug mode, I followed the flow of the program and the debugger showed me that the tree had values in it. Then I wanted to display only the first node's elements and I had the same problem: an empty console.

I asked my teacher what would be the mistake and he told me to put an endl after cout, like this:

cout << node->keys[i] << endl;

It worked! He told me then that probably I addressed a NULL pointer in my program and Eclipse doesn't say anything about that but nothing about how endl could help.

Does anybody know what could be the problem and how that endl solves it? I'm very very confused. I didn't understand what flushing the buffer has to do with my display function.

Sorin
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5 Answers5

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I'm a bit late to answer and how to flush has been explained by the other answers already so I wanna answer your comment on Johnsy's answer at least.

First the reason that output is buffered is that writing the data to some output stream and not in the memory is usually a really slow operation (this ofc depends if you wanna write to a file on a ssd or hdd or just to the display but they all are way slower than the ram).

So c++ writes it to an internal buffer first and only actually writes to the output when the buffer is full or you flush the stream. It does so to avoid the slow operation of writing on most output streams.

So now why do you need to flush the buffer before it gets displayed... Like already said it only actually writes it out for you to see when the buffer is full or it gets explicitly flushed. Now when the program ends normally all streams get flushed automatically so what happens prolly is a crash of the Program (a crash won't flush the buffer) so your program quits w/o it ever displaying.

Since your program displays everything correctly when you add the endl I guess you're trying to output a node with a nullptrat the very end and crash just before returning from main. You could easily test that by adding std::cout << "end of the program" << std::endl; just before your returnin main and testing if it gets displayed.

AliciaBytes
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    Thank you! I got it now, I didn't know that the buffer is flushed at the end of the program and if the program crashes the buffer is not displayed. I tried to write something just before return in main and it was not showing on the screen. – Sorin Mar 12 '13 at 15:15
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std::endl flushes output stream....

cout is buffered, output will not be displayed immediately, they will be buffered, until the buffer overflows, then all buffered data will be displayed. If you want display the ouput sooner, flush it.

Inserts a endline character into the output sequence os and flushes it as if by calling os.put(os.widen('\n')) followed by os.flush().[1]

You can use std::cout.flush(); too.

masoud
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std::endl inserts a endline character into the output sequence (os, in your case cout) and flushes it as if by calling os.put(os.widen('\n')) followed by os.flush().

std::cout controls output to a stream buffer of implementation-defined type (derived from std::streambuf), associated with the standard C output stream stdout. This output is buffered (unlike std::cerr)

Before you put endl into your source code, your buffer was not being flushed and the output was not appearing on your screen.

johnsyweb
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  • Thanks for the answer, but it is still confusing for me. Why do I have to flush the buffer before I want to display something? – Sorin Mar 12 '13 at 13:21
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    Adding characters to a buffer is one of the cheapest things code can do. Communicating with the outside world -- a network interface, a disk drive, a display server -- is one of the most expensive. Even today, you don't want to ask the operating system to mediate any more transactions than are really necessary. The difference is comparable to putting things on your grocery list and going to the store. – jthill Mar 12 '13 at 16:12
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std::endl not only appends a newline character to the output stream, but also causes it to be flushed. I'm not quite sure how large the buffer on std::cout is, but it is possible that you're not seeing anything because the buffer is not full and thus does not get flushed. You could try to call cout.flush(); after appending your data (without appending std::endl) and see if that solves the problem.

Kristian Duske
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As M M has mentioned in his answer as have others above, the std::endl flushes output stream. What it means is to display or put the results/output to the standard output source immediately/real time.

Using '\n' instead of std::endl will also result in similar output but may not be displayed immediately if some operation is still going on.

The common down side of std::endl is considered to be degradation of performance, though it may not be of significance if the output is streamed to a display instead of a file - where the '\n' is the preferred method.

Hope this helps.

LiaqatG
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