Boost组件之lexical

字符串→数值
数值→字符串
异常

在C++中Boost里面的lexical_cast实现了数值与字符串之间的转换，使其更加方便与简洁

字符串→数值

如何将字符串”123”转换为int类型整数123？答案是，用标准C的库函数atoi
如果要转换为long类型呢？标准C的库函数atol
如何将”123.12”转换为double类型呢？标准C的库函数atod

如果要转换为long double类型呢？标准C的库函数atold

C++:

    #include <boost/lexical_cast.hpp>
    #include <iostream>
    int main()
    {
            using boost::lexical_cast;
            int a = lexical_cast<int>("123");
            double b = lexical_cast<double>("123.12");
            std::cout<<a<<std::endl
            std::cout<<b<<std::endl;
            return 0;
    }

数值→字符串

sprintf()函数

  char item[100];
  sprintf(item,"%s",12345.678);

C++标准字符串string:

    #include <boost/lexical_cast.hpp>
    #include <string>
    #include <iostream>
    int main()
    {
            using std::string;
            const double d = 123.12;
            string s = boost::lexical_cast<string>(d);
            std::cout<<s<<std::endl;
            return 0;
    }

异常

如果转换失败，则会有异常bad_lexical_cast抛出。该异常类是标准异常类bad_cast的子类

  #include <boost/lexical_cast.hpp>
  #include <iostream>
  using namespace td;
  int main()
  {
          int i;
          try{
                  i = boost::lexical_cast<int>("abcd");
          }
          catch(boost::bad_lexical_cast& e)
          {
                  cout<<e.what()<<endl; 
                  return 1;
          }
          cout<<i<<endl;
          return 0;
  }

异常消息：bad lexical cast: source type value could not be interpreted as target

注意事项

lexical_cast依赖于字符流std::stringstream
- 原理
  
  把源类型读入到字符流中，再写到目标类型中
```
int m = boost::lexcal_cast<int>("123456");
```
  等价：
```
    int item;
    std::stringstream ss;
    ss<<"123";
    s>>item;
```
- 输入数据必须“完整”地转换，否则抛出bad_lexical_cast异常
```
int a = boost::lexical_cast<int>("123.456");
```
  会将123.456强制转成123，导致部分转换
- 浮点数精度问题
```
string item = boost::lexical_cast<string>("123.456789")
```
  理论值等于“123.456789”,实际值只有“1233.456”，默认情况下std::stringstream精度6位，打开头文件<boost/lexical_cast.hpp>修改
```
 interpreter.precision(std::numeric_limits<Source>::digits10);
```

Boost组件之lexical_cast

字符串→数值

数值→字符串

异常

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