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c++11实现一个简单的lexical_cast

boost中有一个lexical_cast可以用统一的方式来做基本类型之间的转换，比如字符串到数字，数字到字符串，bool和字符串及数字之间的相互转换。boost::lexical_cast的用法比较简单：

#include <boost/lexical_cast.hpp> #include <iostream> #include <string>  #define ERROR_LEXICAL_CAST  1  int main() {  using boost::lexical_cast;  int   a = 0;  double  b = 0.0;  std::string s = "";   int   e = 0;   try  {    // ----- 字符串 --> 数值    a = lexical_cast<int>("123");   b = lexical_cast<double>("123.12");   // ----- 数值 --> 字符串   s = lexical_cast<std::string>("123456.7");    // ----- 异常处理演示   e = lexical_cast<int>("abc");  }  catch(boost::bad_lexical_cast& e)  {   // bad lexical cast: source type value could not be interpreted as target   std::cout << e.what() << std::endl;   return ERROR_LEXICAL_CAST;  }   std::cout << a << std::endl; // 输出：123   std::cout << b << std::endl; // 输出：123.12   std::cout << s << std::endl;  // 输出：123456.7   return 0; }

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c++11中缺少lexical_cast方法，但是c++11已经提供了一些基本类型转换的方法，比如to_string, atoi, atof等等，但是我们不能通过一种通用的方式来做基本类型转换，因此我希望做一个类似boost的lexical_cast做基本类型的转换，这也是我们的 c++社区的一个开发计划。

由于c++11已经提供了一些便利的方法，我要做的事情就变得很简单了，就是把他们糅合在一起并提供一个统一的lexical_cast的方法即可。

实现思路也很简单，转换主要有这几种：1.数字到字符串的转换；2.字符串到数字的转换；3.bool与字符串的相互转换；4.数字转换为bool；具体的实现代码如下：

#include <type_traits> #include <string> #include <cstdlib> #include <algorithm> #include <stdexcept> #include <cctype> #include <cstring> using namespace std; namespace detail {  const char* strue = "true";  const char* sfalse = "false";  template <typename To, typename From>  struct Converter  {  };  //to numeric  template <typename From>  struct Converter<int, From>  {   static int convert(const From& from)   {    return std::atoi(from);   }  };  template <typename From>  struct Converter<long, From>  {   static long convert(const From& from)   {    return std::atol(from);   }  };  template <typename From>  struct Converter<long long, From>  {   static long long convert(const From& from)   {    return std::atoll(from);   }  };  template <typename From>  struct Converter<double, From>  {   static double convert(const From& from)   {    return std::atof(from);   }  };  template <typename From>  struct Converter<float, From>  {   static float convert(const From& from)   {    return (float)std::atof(from);   }  };  //to bool  template <typename From>  struct Converter<bool, From>  {   static typename std::enable_if<std::is_integral<From>::value, bool>::type convert(From from)   {    return !!from;   }  };  bool checkbool(const char* from, const size_t len, const char* s)  {   for (size_t i = 0; i < len; i++)   {    if (from[i] != s[i])    {     return false;    }   }   return true;  }  static bool convert(const char* from)  {   const unsigned int len = strlen(from);   if (len != 4 && len != 5)    throw std::invalid_argument("argument is invalid");   bool r = true;   if (len == 4)   {    r = checkbool(from, len, strue);    if (r)     return true;   }   else   {    r = checkbool(from, len, sfalse);    if (r)     return false;   }   throw std::invalid_argument("argument is invalid");  }  template <>  struct Converter<bool, string>  {   static bool convert(const string& from)   {    return detail::convert(from.c_str());   }  };  template <>  struct Converter<bool, const char*>  {   static bool convert(const char* from)   {    return detail::convert(from);   }  };  template <>  struct Converter<bool, char*>  {   static bool convert(char* from)   {    return detail::convert(from);   }  };  template <unsigned N>  struct Converter<bool, const char[N]>  {   static bool convert(const char(&from)[N])   {    return detail::convert(from);   }  };  template <unsigned N>  struct Converter<bool, char[N]>  {   static bool convert(const char(&from)[N])   {    return detail::convert(from);   }  };  //to string  template <typename From>  struct Converter<string, From>  {   static string convert(const From& from)   {    return std::to_string(from);   }  }; } template <typename To, typename From> typename std::enable_if<!std::is_same<To, From>::value, To>::type lexical_cast(const From& from) {  return detail::Converter<To, From>::convert(from); } template <typename To, typename From> typename std::enable_if<std::is_same<To, From>::value, To>::type lexical_cast(const From& from) {  return from; }

前后花了一个多小时，一个基本的类型转换类就完成了，再测试一下吧，测试代码：

void test() {  cout<<lexical_cast<int>(1)<<endl;  cout << lexical_cast<int>("1") << endl;  cout << lexical_cast<long>("1") << endl;  cout << lexical_cast<string>(1) << endl;  cout << lexical_cast<bool>(1) << endl;  cout << lexical_cast<double>("1.2") << endl;  cout << lexical_cast<float>("1.2") << endl;  string s = "true";  cout << lexical_cast<bool>(s) << endl;  char* p = "false";  cout << lexical_cast<bool>(p) << endl;  const char* q = "false";  cout << lexical_cast<bool>(q) << endl;  cout << lexical_cast<bool>("false") << endl;  cout << lexical_cast<bool>("test") << endl; } int main() {  try  {   test();  }  catch (const std::exception& e)  {   cout << e.what() << endl;  }  return 0; }