flushing ....

This is a discussion on flushing .... within the C++ Programming forums, part of the General Programming Boards category; Why is it that flushall(); works for me in this code and cout.flush(); doesn't?? Code: while ( i<drivers ) { ...

  1. #1
    The superhaterodyne twomers's Avatar
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    flushing ....

    Why is it that flushall(); works for me in this code and cout.flush(); doesn't??

    Code:
    while ( i<drivers )
    {
    	cout.flush();	//not working like flushall();
    
    //	flushall();		//was used, but it don't really want
    			//to need to have it there if possible
    
    	cout<< endl << "What is your name: ";
    	cin.getline( TempName, NAME_SIZE );	
    
    //	to be entered into a struct, but I didn't put it in this
    //	cause it would just clutter up the post 
    
    }
    basically, when I have flushall(); it works fine in that the program doesn't skip the getting name thing. but if I put cout.flush(); it doesn't work and if I don't have any flushing thing there it acts as it would with cout.flush() or rather cout.flush acts as if there wasn't any flusher.

    I bet that if someone compiles this they actually won't need a buffer for it which leads me to further confusion ... argh

  2. #2
    Registered User major_small's Avatar
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    did you try:
    Code:
    std::cout<<std::flush();
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  3. #3
    The superhaterodyne twomers's Avatar
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    Quote Originally Posted by major_small
    did you try:
    Code:
    std::cout<<std::flush();
    thanks major_small for your reply!!

    I just tried it, it may just be that I don't have a header, but I got this error:

    [ERROR]

    U:\CAss\One\Rally.cpp(65) : error C2780: 'class std::basic_ostream<_E,_Tr> &__cdecl std::flush(class std::basic_ostream<_E,_Tr> &)' : expects 1 arguments - 0 provided
    c:\program files\microsoft visual studio\vc98\include\ostream(398) : see declaration of 'flush'

    [/ERROR]

    EDIT:
    I've just deduced there that the problem is the .getline thing in the cin .... is there another way of reading in the whole space thing without using .getline (short of making your own function to do it of course)?
    Last edited by twomers; 02-09-2006 at 02:29 AM.

  4. #4
    Registered User major_small's Avatar
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    I just realized there are no parenthesis on the end of flush when used the way I showed... so try giving those the axe and see if it helps...

    you may also want to add:
    Code:
    std::cin.getline(TempName,NAME_SIZE,'\n');
    just so that it stops when you hit enter (the way you have it, it'll keep taking input until the array is full)
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  5. #5
    ZuK
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    In your example code I don't see any need to flush cout

    Code:
    while ( i<drivers )
    {
    	cout<< endl << "What is your name: ";  // endl will flush cout
    	cin.getline( TempName, NAME_SIZE );	// cin / cout are synchronized so getline flushes cout
    
    }
    Kurt

  6. #6
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    you may also want to add:

    Code:
    std::cin.getline(TempName,NAME_SIZE,'\n');

    just so that it stops when you hit enter (the way you have it, it'll keep taking input until the array is full)
    cin.getline(TempName, NAME_SIZE)

    is eqivalent to:

    cin.getline(TempName, NAME_SIZE, '\n')

  7. #7
    The superhaterodyne twomers's Avatar
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    thanks for your reply Zuk/Kurt and major_small !!

    Quote Originally Posted by ZuK
    In your example code I don't see any need to flush cout
    I didn't see any reason to have to flush either, but when I compiled the code it skipped the "Enter your name" thing and went straight to the second thing without inputting anything ...

    Quote Originally Posted by major_small
    you may also want to add:
    Code:
    std::cin.getline(TempName,NAME_SIZE,'\n');
    that sir, is highly aloof! I had it in the code, but it didn't copy ... must have been a backup file that I copied it from, one which didn't have the '\n' terminator or something similar.

    Quote Originally Posted by major_small
    did you try:
    Code:
    std::cout<<std::flush();
    I just tried that, with the correction that you posted, and it still didn't do anything to help with the flushing ... does anyone know what the cause of that may be?? Could it have anything to do with the code preceeding it?? if so, all that's being done before is creating a dynamically allocated array and testing to see if it was created correctly .... i'm big time confused!

  8. #8
    ZuK
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    >> Could it have anything to do with the code preceeding it??
    Normally it has. But it has nothing to do with cout.
    if you use some statement like
    Code:
    cin >> number;
    before using getline() it will retun an empty string because there would still be a '\n' left in the buffer.
    It's never a good idea to mix different types of input statements.
    But that is all in the FAQ's
    Kurt

  9. #9
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    One easy solution to the issues from mixing cin >> and getline is to add cin.ignore() after any call to cin >>, and specifically after any call to cin>> that comes before a call to getline.

  10. #10
    The superhaterodyne twomers's Avatar
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    Quote Originally Posted by Daved
    One easy solution to the issues from mixing cin >> and getline is to add cin.ignore() after any call to cin >>, and specifically after any call to cin>> that comes before a call to getline.
    good call Daved!! works like a treat now! thanks a bunch! I'm still kinda confused about why I had to do it, cause i don't really think I needed to do it before in similar things ...

  11. #11
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    As ZuK said, operator>> leaves a newline in the buffer. For example, if you prompt the user for a number and they type 3 and then hit enter, then cin >> number will read the 3 into the variable and stop. However, there will be a newline from when the user hit enter that is still in the stream. Later, you call getline, which just reads until it reaches it's delimiter (which is a newline by default). So if there is a newline in the stream from the previous cin >> call, then that newline is picked up by the getline call and nothing else is read in. The cin.ignore() works by ignoring the newline left in the input stream.

    The reason the ignore() isn't necessary when you have a second call to cin >> after the first one is that operator>> ignores leading whitespace (including newlines). So the newline that is in the input buffer from the first call to cin >> is ignored automatically and the second call to cin >> just waits for the user to type in more stuff.

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