=====C++ I/O Flags===== ====Format flags==== C++ defines some format flags for standard input and output, which can be manipulated with the [[flags]], [[setf]], and [[unsetf]] functions. For example,


   cout.setf(ios_base::left, ios_base::adjustfield);


    cout << boolalpha;

This is equivalent to:


    cout.setf(ios_base::boolalpha);

===Manipulators defined in === Manipulators from may be used on both input and output streams. Formatting controlled by a boolean has two manipulators with names matching the flag name. * boolalpha / noboolalpha * showbase / noshowbase * showpoint / noshowpoint * showpos / noshowpos * uppercase / nouppercase * unitbuf / nounitbuf Formatting controlled by a field has one manipulator for each flag, which turns off the others: * ios_base::adjustfield * internal * left * right * ios_base::basefield * dec * hex * oct * ios_base::floatfield * fixed * scientific For example, the following two statements are equivalent:


   cout << left;
   cout.setf(ios_base::left, ios_base::adjustfield);

===Manipulators defined in === There is one input manipulator, ws, which extracts whitespace from the stream, effectively skipping it before the next operation. This is rarely used directly, as streams with the skipws flag set should skip whitespace before any extraction. (All the standard extractors do this, user-defined extractors should use istream::sentry, which will also do this.) ===Manipulators defined in === These manipulators may only be used with output streams. ^Manipulator^Description^ |endl|Output a newline character and flush the stream| |ends|Output a null character (does not flush)| |flush|Flush the stream| ===Manipulators defined in === These manipulators may be used with both input and output streams. ^Manipulator^Description^ |resetiosflags( ios_base::fmtflags f )|Turn off the flags specified by f| |setiosflags( ios_base::fmtflags f )|Turn on the flags specified by f| |setbase( int base )|Sets the number base to base| |setfill( char ch )|Sets the fill character to ch| |setprecision( int p )|Sets the number of digits of precision| |setw( int w )|Sets the field width to w| ==== Field ''width'' automatically reset ==== Unlike the other format flags and fields, the ''width'' field is automatically set whenever the ''<<'' operator is used to output text or numbers. This can lead to surprising results. The loop


	for(int i=1;i<1000;i*=10){
	   cout << setw(6) << i << " |" << i << endl;
	}

does not pad the second column as might be expected, but produces: 1 |1 10 |10 100 |100 To pad both columns, the width field should be set before each number is printed,


	for(int i=1;i<1000;i*=10){
	   cout << setw(6) << i << " |" << setw(6) << i << endl;
	}

producing the more pleasing alignment: 1 | 1 10 | 10 100 | 100 Why does the standard require this extra ''setw(6)''? If the standard did not specify that the ''width'' field automatically be reset, the width of the seperator ''" |"'' would need to be set to 0 so that extra spaces would not be added around it. If the language were defined this way, we would have to write:


	for(int i=1;i<1000;i*=10){
	   cout << setw(6) << i << setw(0) << " |" << setw(6) << i << endl;
	}