c++11实现一个简单的lexical

本文主要是介绍c++11实现一个简单的lexical_cast，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

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 targetstd::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;
}  

　　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 numerictemplate <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 booltemplate <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 stringtemplate <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;
}