c++ force std::cout flush (print to screen)

Question

I have code such as the following:

std::cout << "Beginning computations..."; // output 1
computations();
std::cout << " done!\n";                  // output 2

The problem, however, is that often output #1 and output #2 appear (virtually) simultaneously. That is, often output #1 does not get printed to the screen until after computations() returns. Since the entire purpose of output #1 is to indicate that something is going on in the background (and thus to encourage patience from the user), this problem is not good.

Is there any way to force the std::cout buffer to get printed before the computations() call? Alternatively, is there some other way (using something other than std::cout) to print to standard out that would fix this problem?

Gantt

I hear std::endl is pretty popular for this kinda thing... – Least

@ebyrob But this will end the line, and if he wants to continue printing on the same line after the computation, he needs to manually flush. – Ruthy

@Ruthy that's very true. In that case he can either use fprintf(), std::flush (as below), or modify his version of cout to automatically flush at the end of each line of code... I had a SO thread for that last one but seem to have lost it. – Least

@Gantt what you facing I am not facing. it's working fine. I just made computation function with one cout line. but it is printing that "Beginning computations..." before calling the function. but I need to find at which particular case buffer will not free. – Magenmagena

Am using gcc compiler. Added "-static" flag and it worked. See this so ans – Suburbanize

Answer 1

Just insert std::flush:

std::cout << "Beginning computations..." << std::flush;

Also note that inserting std::endl will also flush after writing a newline.

Lidda

@user1810087 Sorry, I edited it in, but pretty quickly. I think it was before you answered. – Lidda

I enjoyed the comments :D – Halide

@JosephMansfield but in my case it is not happening means there is no need to use of std::flush. can you give a case where buffer is not full so cout is not printing. – Magenmagena

Answer 2

In addition to Joseph Mansfield answer, std::endl does the flush too (besides a new line).

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().

Firmament