Cout format output common functions and flags summary

surface 1 Stream member function and controller for controlling output format

Stream member functions	Control characters with the same function	effect
precision(n)	setprecision(n)	Set the precision of the real number to n position
width(n)	setw(n)	Set the field width to n position
fill(c)	setfill(c)	Set fill symbol c
setf(ios::state)	setiosflags(ios::state)	Set output format status , The format status... Should be given in parentheses , Content and controller setiosflags The contents in brackets are the same , As shown in the table 2 Shown
unsetf(ios::state)	resetioflags(ios::state)	Terminate the set output format state , What should be specified in brackets

surface 2 Set the format flag of the format state

Format mark	effect
ios::left	The output data is aligned to the left in a wide range of the local area
ios::right	The output data is aligned to the right within a wide range of the domain
ios::internal	The sign bit of the value is left aligned within the field width , Align values right , The middle is filled with fill characters
ios::dec	Set the base of the integer to 10
ios::oct	Set the base of the integer to 8
ios::hex	Set the base of the integer to 16
ios::showbase	Force the cardinality of the output integer ( Octal number 0 Lead , Hexadecimal number 0x Lead )
ios::showpoint	Forces the output of the floating-point number of small points and mantissa 0
ios::uppercase	In the form of scientific notation E And in hexadecimal output letters in uppercase
ios::showpos	A positive number shows “+” Number
ios::scientific	Floating point number scientific notation format output
ios::fixed	Fixed point format for floating point numbers ( Decimal form ) Output
ios::boolalpha	With “true”/“false” Output Boolean values in the form of
ios::unitbuf	Refresh all streams after each output
ios::stdio	Clear after each output stdout, stderr

#include <iostream>
using namespace std;
int main(){
    int a = 31;
    cout.width(8);
    cout.fill('*');
    cout.unsetf(ios::dec);
    cout.setf(ios::hex);
    cout << a << endl;
}
Output is ：
******1f

Use controllers to control the output format
We can also use the controller to control the output format , In this way, there is no need to call the member function of the flow , You can call the relevant control characters directly in the output input stream , We can use the following code to implement the same output format as above ：

#include <iostream>
#include <iomanip>  // Don't forget to include this header file
using namespace std;
int main(){
    int a = 31;
    cout << setw(8) << setfill('*') << resetiosflags(ios::dec) << setiosflags(ios::hex) << a << endl;
}

Be careful ：
When using control characters, you need to include header files <iomanip>.
surface 2 Some of the format flags in are mutually exclusive （ such as dec、hex and oct You can only choose one ）, If you want to change the setting to another mutually exclusive state , Member functions should be called unsetf（ Corresponding to member function self） or resetiosflags（ Corresponding to the control character setiosflags）, Terminate the original set state , Then set other States . For example, the default decimal system was terminated in the previous example （ios::dec）, Then set hexadecimal （ios::hex）, If you don't terminate the original decimal mark , The output will still be in decimal .
Member functions in various functions width(n) And control characters setw(n) Only valid for the first subsequent output item , Other functions and flags are always valid for the output and input stream .
surface 2 The format flag in ios Class is defined as an enumeration value , Each format is marked with a bit Of 0 or 1 representative （ as follows enum _Ios_Fmtflags Shown ）, So you can use bits or operators “|” Combine multiple format flags , In the stream member function setf()/unsetf() And the controller setioflags()/resetioflags() Simultaneous setting . such as ：cout.setf(ios::dec | ios::showpos);
enum _Ios_Fmtflags 
    { 
      _S_boolalpha  = 1L << 0,
      _S_dec        = 1L << 1,
      _S_fixed      = 1L << 2,
      _S_hex        = 1L << 3,
      _S_internal   = 1L << 4,
      _S_left       = 1L << 5,
      _S_oct        = 1L << 6,
      _S_right      = 1L << 7,
      _S_scientific     = 1L << 8,
      _S_showbase   = 1L << 9,
      _S_showpoint  = 1L << 10,
      _S_showpos    = 1L << 11,
      _S_skipws     = 1L << 12,
      _S_unitbuf    = 1L << 13,
      _S_uppercase  = 1L << 14,
      _S_adjustfield    = _S_left | _S_right | _S_internal,
      _S_basefield  = _S_dec | _S_oct | _S_hex,
      _S_floatfield     = _S_scientific | _S_fixed,
      _S_ios_fmtflags_end = 1L << 16,
      _S_ios_fmtflags_max = __INT_MAX__,
      _S_ios_fmtflags_min = ~__INT_MAX__
    };

Format flags simplify control output formats

In the above two methods , To set the format flag bit, you need to call the function （setf and setiosflags）, More trouble , Fortunately C++ There is also a set of functions about format flags , It can be used directly in input and output streams , For example, to implement the previous output format, we can write ：
#include <iostream>
#include <iomanip> 
using namespace std;
int main(){
    int a = 31;
    cout << setw(8) << setfill('*') << hex << a << endl;
}

above hex It's a function like this , except hex, surface 2 All format flags in are set in this way , If you want to terminate, just add “no” Can , such as showpos and noshowpos（ surface 2 None of the first six items no edition ）. 
In addition, it can be noted that , Here we use hex When setting hexadecimal, you don't have to terminate the decimal system as before , Why is that ？ First look at it. hex Function definition of ：

inline ios_base&
hex(ios_base& __base)
{
    __base.setf(ios_base::hex, ios_base::basefield);
    return __base;
}

You can see hex It's actually called setf function , But here it is. setf As we used above setf Functions are different , This is overloaded with two formal parameters setf function , Its definition is as follows ：

fmtflags
setf(fmtflags __fmtfl, fmtflags __mask)
{
      fmtflags __old = _M_flags;
      _M_flags &= ~__mask;
      _M_flags |= (__fmtfl & __mask);
      return __old;
}

You can see here setf First the mask The corresponding bits are set 0, Then the position to be set 1. Specific to the hex function , It is to _S_basefield(_S_dec | _S_oct | _S_hex) All first 0, then _S_hex Location 1. With this setf function , We are using hex There is no need to terminate first dec 了 , You can use it directly hex.