flush

[Roby]

I just don't understand, what flush() means in C++? I mean flush with cout object, like this:

Code: Select all

...
cout.flush();
...

Is there also for cin object? Thanx...

[Krzysztof Duleba]

Google for it. It takes less than writing a post.

[Roby]

I still don't get it... from http://www.cs.hmc.edu/~geoff/classes/hm ... es/io.html and http://www.gillius.org/ctut/app_b.htm it explain that flush usage is to output the buffer... but, when I try with these code:

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#include <iostream.h>

int main()
{
 char mystrin[10] = "HELLO";

 cout << "My string is " << mystring << " plus a null character\n" << flush;

 return 0;
}

I didn't catch something different if I removed that flush manipulator. Can't someone explain to me please? Thanx in advance...

[Krzysztof Duleba]

Code: Select all

#include <iostream>

using namespace std;

int main(){
 cout << "a" << flush;
 while(1){}
}

But now it does make a difference, doesn't it?

You should use flush only when you know what you're doing, in very few and specific situations (for instance when you're dealing with pipes).

[Roby]

Code: Select all

#include <iostream>

using namespace std;

int main(){
 cout << "a" << flush;
 while(1){}
}

But now it does make a difference, doesn't it?

You should use flush only when you know what you're doing, in very few and specific situations (for instance when you're dealing with pipes).

Confused... it's just looping forever, isn't it? or anything else? I really don't understand...

[sumankar]

[...]Confused... it's just looping forever, isn't it? or anything else? I really don't understand...

Don't miss the wood for the trees. The loop is not important in itself. Take out the `flush' from the cout statement and then try to run the program. Can you see any difference?

Here's the relevant section from one of the websites you've posted:

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Forcing all buffered output to actually be printed is known as "flushing" the stream. A flush can be forced by calling the flush function associated with each output stream, inserting the magic variable flush into the stream, or inserting endl.

Note: you can flush only output streams. Doing that on an input stream is undefined behaviour.

[jtmh]

Maybe, with files, it's easier to understand:

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#include <iostream>
#include <fstream>

using namespace std;

int main()
{
	ofstream outfile("__test__.txt");

	outfile << "You will not see this string till outfile.flush() is run.";
	cin.get(); // open __test__.txt to check if it contains the string above
	           // and then press <Enter> in the console window to continue
	outfile.flush();
	cin.get(); // RE-open __test__.txt to check it out again
	           // and then press <Enter> in the console window to exit

	return 0;
}

Run the compiled program and follow the instructions in the comments, and you should be able to see the effect of flush(). Hope this could help.

[Moha]

May be the best effect of flush can be demonestrated by using cerr and cout together. cerr is unbuffer stream but cout is a buffer one.

[Roby]

Maybe, with files, it's easier to understand:

Code: Select all

#include <iostream>
#include <fstream>

using namespace std;

int main()
{
	ofstream outfile("__test__.txt");

	outfile << "You will not see this string till outfile.flush() is run.";
	cin.get(); // open __test__.txt to check if it contains the string above
	           // and then press <Enter> in the console window to continue
	outfile.flush();
	cin.get(); // RE-open __test__.txt to check it out again
	           // and then press <Enter> in the console window to exit

	return 0;
}

Run the compiled program and follow the instructions in the comments, and you should be able to see the effect of flush(). Hope this could help.

Thanx moha... now I understand... thanx a lot...