字符串转换为数字

long double stod(const char *ptr , bool *bOk = 0 , int *d = 0)
{
    assert(ptr != 0);
    const char *cursor = ptr;
    switch(*cursor)
    {
    case '0':
    {
        ++cursor;
        switch(*cursor)
        {
        //! 此时被认为是8进制数
        case '0':
        case '1':
        case '2':
        case '3':
        case '4':
        case '5':
        case '6':
        case '7':
        {
            long long llRet = *cursor - '0';
            ++cursor;
            while(*cursor)
            {
                if ('0' <= *cursor && *cursor <= '7')
                {
                    llRet *= 8LL;
                    llRet += *cursor - '0';
                    ++cursor;
                }
                else
                {
                    break;
                }
            }
            if (bOk)
                *bOk = true;

            if (d)
                *d = 8;

            return static_cast<long double>(llRet);
        }

        //! 此时被认为是16进制数
        case 'x':
        case 'X':
        {
            ++cursor;
            long long llRet = 0LL;
            while(*cursor)
            {
                if (('a' <= *cursor && *cursor <= 'f'))
                {
                    llRet *= 16LL;
                    llRet += *cursor - 'a' + 10LL;
                    ++cursor;
                }
                else if ('A' <= *cursor && *cursor <= 'Z')
                {
                    llRet *= 16LL;
                    llRet += *cursor - 'A' + 10LL;
                    ++cursor;
                }
                else if ('0' <= *cursor && *cursor <= '9')
                {
                    llRet *= 16LL;
                    llRet += *cursor - '0';
                    ++cursor;
                }
                else
                {
                    break;
                }
            }

            if (bOk)
                *bOk = true;

            if (d)
                *d = 16;

            return static_cast<long double>(llRet);
        }

        //! 此时被认为是小数
        case '.':
        {
            long double ret = 0.0;
            long double div = 10.0L;
            ++cursor;
            while(*cursor)
            {
                if ('0' <= *cursor && *cursor <= '9')
                {
                    ret += static_cast<long double>(*cursor - '0') / div;
                    div *= 10.0L;
                    ++cursor;
                }
                else
                    break;
            }
            if (bOk)
                *bOk = true;

            if (d)
                *d = 10;

            return ret;
        }

        //! 此时被认为是十进制数
        case '8':
        case '9':
        {
            long long llRet = *cursor - '0';
            ++cursor;
            while(*cursor)
            {
                if ('0' <= *cursor && *cursor <= '9')
                {
                    llRet *= 10LL;
                    llRet += *cursor - '0';
                    ++cursor;
                }
                else if (*cursor == '.')
                {
                    ++cursor;
                    long double ret = 0.0L;
                    long double div = 10.0L;
                    while(*cursor)
                    {
                        if ('0' <= *cursor && *cursor <= '9')
                        {
                            ret += static_cast<long double>(*cursor - '0') / div;
                            div *= 10.0L;
                            ++cursor;
                        }
                        else
                        {
                            if (bOk)
                                *bOk = true;

                            if (d)
                                *d = 10;

                            return static_cast<long double>(ret + llRet);
                        }
                    }
                }
                else
                    break;
            }
            if (bOk)
                *bOk = true;

            if (d)
                *d = 10;

            return static_cast<long double>(llRet);
        }

        //! 转换失败
        default:
        {
            if (bOk)
                *bOk = false;
            return 0.0;
        }
        }
    }
        break;
    case '-':
    {
        ++cursor;
        long long llRet = 0LL;
        while(*cursor)
        {
            if ('0' <= *cursor && *cursor <= '9')
            {
                llRet *= 10LL;
                llRet += *cursor - '0';
                ++cursor;
            }
            else if (*cursor == '.')
            {
                long double ret = 0.0;
                long double div = 10.0L;
                ++cursor;
                while(*cursor)
                {
                    if (isdigit(*cursor))
                    {
                        ret += static_cast<long double>(*cursor - '0') / div;
                        div *= 10.0L;
                        ++cursor;
                    }
                    else
                    {
                        if (bOk)
                            *bOk = true;

                        if (d)
                            *d = 10;

                        return static_cast<long double>(-llRet - ret);
                    }
                }
            }
            else
                break;
        }
        if (bOk)
            *bOk = true;

        if (d)
            *d = 10;

        return static_cast<long double>(-llRet);
    }
    case '.':
    {
        long double ret = 0.0L;
        long double div = 10.0L;
        ++cursor;
        while(*cursor)
        {
            if (isdigit(*cursor))
            {
                ret += static_cast<long double>(*cursor - '0') / div;
                div *= 10.0L;
                ++cursor;
            }
            else
                break;
        }
        if (bOk)
            *bOk = true;

        if (d)
            *d = 10;

        return ret;
    }
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':
    {
        long long llRet = *cursor - '0';
        ++cursor;
        while(*cursor)
        {
            if (isdigit(*cursor))
            {
                llRet *= 10LL;
                llRet += *cursor - '0';
                ++cursor;
            }
            else if (*cursor == '.')
            {
                ++cursor;
                long double ret = 0.0L;
                long double div = 10.0L;
                while(*cursor)
                {
                    if(isdigit(*cursor))
                    {
                        ret += static_cast<long double>(*cursor - '0') / div;
                        div *= 10.0L;
                        ++cursor;
                    }
                    else
                    {
                        if (bOk)
                            *bOk = true;

                        if (d)
                            *d = 10;

                        return static_cast<long double>(llRet + ret);
                    }
                }
            }
            else
                break;
        }

        if (bOk)
            *bOk = true;

        if (d)
            *d = 10;

        return static_cast<long double>(llRet);
    }
    default:
    {
        if (bOk)
            *bOk = false;

        return 0.0;
    }
    }
}