setf

ios_base::fmtflags:  使用setf函数设置 

independent flags	boolalpha	read/write bool elements as alphabetic strings (true and false).
	showbase	write integral values preceded by their corresponding numeric base prefix.
	showpoint	write floating-point values including always the decimal point.
	showpos	write non-negative numerical values preceded by a plus sign (+).
	skipws	skip leading whitespaces on certain input operations.
	unitbuf	flush output after each inserting operation.
	uppercase	write uppercase letters replacing lowercase letters in certain insertion operations.
numerical base (basefield)	dec	read/write integral values using decimal base format.
	hex	read/write integral values using hexadecimal base format.
	oct	read/write integral values using octal base format.
float format (floatfield)	fixed	write floating point values in fixed-point notation.   ---------usually used  将浮点数用定点数表示
	scientific	write floating-point values in scientific notation.                                       科学计数法
adjustment (adjustfield)	internal	the output is padded to the field width by inserting fill characters at a specified internal point. 特定处补零
	left	the output is padded to the field width appending fill characters at the end.            结束补零
	right	the output is padded to the field width by inserting fill characters at the beginning. 开头补零

  std::cout.setf ( std::ios::showbase );  带0x 等前缀。