hjk41的日志

设置cout的输出格式有两种办法，一种是用cout的成员变量，一种是用<iomanip>里面提供的函数
比如要设置输出的行宽，就有两种方法：

#include<iostram>
#include<iomanip>
using namepsace std;
void main(){
    // using str::width()
    cout.width(20);
    cout<<12<<endl;

    // using iomanip
    cout<<setw(20)<<12<<endl;
}

两种方法是等价的，事实上setw(20)这个函数会返回一个manipulator对象，而cout所属的ostream类又重载了操作符<<，当它接收到一个manipulator对象时就会执行与该对象相应的操作。比如cout<<setw(20)所对应的处理代码大概就会是这样的：

[i]
if(manipulatorIn.type==is_seting_width){
    this->width( manipulator.width() );
}[/i]

因为第二种方法书写比较简单，所以这里只介绍第二种方法。

<iomanip>共有6个成员函数，分别如下：
resetiosflags // Clears the specified flags.
setbase // Set base for integers.
setfill // Sets the character that will be used to fill spaces in a right-justified display.
setiosflags // Sets the specified flags.
setprecision // Sets the precision for floating-point values.
setw // Specifies the width of the display field.
其中最常用的是setprecision，setw和setbase。
setprecision是设置输出的精度，即显示几位数字。它对后面的cout都有影响。
setw是设置输出的行宽，行宽如果小于precision的值，那么就按precision显示，行宽大于precision值就在数字前进行填充；setw()只对紧跟其后的数字有效。
setbase是设置输出数字的基数，如输出8进制数则用setbase(8)；setbase后面的参数只能是8,10和16,其它参数无效。setbase对其后的cout都有影响。
setfill设置填充所用的字符。如果设定的行宽大于输出的精度，那么就会在数字前进行填充，而填充所用的字符则用setfill来设置。setfill对其后的cout都有影响。
setioflags与resetioflags是一对函数，它们用于改变一些flag值，这些flag值有：

boolalpha, to insert or extract objects of type bool as names (such as true and false) rather than as numeric values.
dec, to insert or extract integer values in decimal format.
fixed, to insert floating-point values in fixed-point format (with no exponent field).
hex, to insert or extract integer values in hexadecimal format.
internal, to pad to a field width as needed by inserting fill characters at a point internal to a generated numeric field.
left, to pad to a field width as needed by inserting fill characters at the end of a generated field (left justification).
oct, to insert or extract integer values in octal format.
right, to pad to a field width as needed by inserting fill characters at the beginning of a generated field (right justification).
scientific, to insert floating-point values in scientific format (with an exponent field).
showbase, to insert a prefix that reveals the base of a generated integer field.
showpoint, to insert a decimal point unconditionally in a generated floating-point field.
showpos, to insert a plus sign in a nonnegative generated numeric field.
skipws, to skip leading white space before certain extractions.
unitbuf, to flush output after each insertion.
uppercase, to insert uppercase equivalents of lowercase letters in certain insertions.
In addition, several useful values are:

adjustfield, where internal | left | right
basefield, where dec | hex | oct
floatfield, where fixed | scientific

所以事实上用这两个函数可以做很多事，只不过要记这些flag实在太麻烦了。

示例：

const double a=0.123456789;
const int b=16;
cout<< "a="<<a<<endl;
cout<< "setprecision(5)"<<endl;
cout<< setprecision(5)<<a<<endl;
cout<< "setw(10)"<<endl;
cout<< setw(10)<<a<<endl;
cout << "setiosflags(ios_base::scientific)"<<endl;
cout<< setiosflags(ios_base::scientific)<<a<<endl;
cout<< "resetiosflags(ios_base::scientific)"<<endl;
cout<< resetiosflags(ios_base::scientific)<<a<<endl;
cout<< "b="<<b<<endl;
cout<< "setbase(8)"<<endl;
cout<<setbase(8)<<b<<endl;

结果如下：

a=0.123457
setprecision(5)
0.12346
setw(10)
   0.12346
setiosflags(ios_base::scientific)
1.23457e-001
resetiosflags(ios_base::scientific)
0.12346
b=16
setbase(8)
20