c++11实现一个简单的lexical_cast

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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;}

　　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;}

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