学习使用点滴-1-字符处理

1.字符转换
 1.1 相关类
   lexical_cast
   bad_lexical_cast
 1.2 定义文件：#include <boost\lexical_cast.hpp>
 1.3 功能简介：
   用来进行类型转换的，字符串与数值之间的转换
 1.4 使用举例：
  Eclipse 3.0, GUN G++ , Winxp测试通过。
  #include <iostream>
  #include <boost\lexical_cast.hpp>

  using namespace std;

  main()
  {
   int i=100;
   char* pstr = "10101";
 
   cout<<"Before convert i is:"<<i<<endl;
   i = boost::lexical_cast< int >(pstr);
   cout<<"After convert i is:"<<i<<endl;
 
   return 0;
  }
  //print: Before convert i is:100
  //     After convert i is:10101
 
 1.5 其他：
  转换的异常处理和一般的异常处理相同。
  如：try
        {
            //进行转换
        }
        catch(bad_lexical_cast &)
        {
            //处理异常
        }
   
 2.格式化字符串
 2.1 相关类
   format
 2.2 定义文件：#include <boost\format.hpp>
 2.3 功能简介:
   构建一个格式化的字符串，其中的某些字符是待定的，其可以以参数的形式写入。
 2.4 使用举例：
    Eclipse 3.0, GUN G++ , Winxp测试通过。
    #include <iostream>
    #include <boost\format.hpp>
    #include <string>

    using namespace std;
  
    using boost::format;
    using boost::io::group;
    int main()
    {
         format fmt("%1% %2% %3% %4% %5% %6%\n");
         //使用%1%进行参数替换,1代表第一个参数
         string str;
         int i=100;
         cout<<fmt  % 'g' % 'a' % 'l' % 'p' % 'h' % 'y'<<endl;
         //%操作符压入参数
         string s_tmp =boost::str(fmt);
         //str()自由函数,fomat->string
         //同样也可使用format.str();

         cout << s_tmp;
         return 0;
    }
 2.5 格式说明：
    “$“的使用:后面加printf的格式控制符
    “|“的使用:简化printf的格式控制符
    1.无参数顺序:
        format(“%5d %4s %5f“); //%后面直接加printf的格式控制符,参数顺序传入
        format(“%|5| %|4| %|5|);//效果同上,自动判断d,s,f等控制符号
    2.有参数顺序:
        format(“%1% %2% %1%“); 
        format(“%1$5d  %2$5s %1$5d);
        format(“%|1$5|  %|2$5| %|1$5|);//效果同上,自动判断d,s,f等控制符号
    3.具体格式:

        [ N$ ] [ flags ] [ 宽度] [ . 精度] type-char
                    

Flag Meaning effect on internal stream '-' left alignment N/A (applied later on the string) '=' centered alignment N/A (applied later on the string)
- note : added feature, not in printf - '_' internal alignment sets internal alignment
- note : added feature, not in printf - '+' show sign even for positive numbers sets showpos '#' show numerical base, and decimal point sets showbase and showpoint '0' pad with 0's (inserted after sign or base indicator) if not left-aligned, calls setfill('0') and sets internal
Extra actions are taken after stream conversion to handle user-defined output. ' ' if the string does not begin with + or -, insert a space before the converted string N/A (applied later on the string)
Different to printf's behaviour : it is not affected by internal alignment Type-Char Meaning effect on stream p or x hexadecimal output sets hex o octal output sets oct e scientific float format sets floatfield bits to scientific f fixed float format sets floatfield bits to fixed g general -default- float format unset all floatfield bits X, E or G same effect as their lowercase counterparts, but using uppercase letters for number outputs. (exponents, hex digits, ..) same effects as 'x', 'e', or 'g', plus uppercase d, i or u decimal type output sets basefield bits to dec s or S string output precision specification is unset, and its value goes to an internal field for later 'truncation'. (see precision explanation above) c or C 1-character output only the first character of the conversion string is used. % print the character % N/A

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