Question

我正在编写一些C ++，它读取XML字符串并使用与XML文件中的属性键匹配的值填充一些struct成员。目前，第一次传递会产生一个关键值对的stl :: unordered_map。下一步是解释值字符串并将它们作为目标类型返回以存储在结构中。有些类型有点复杂，但我有可以转换它们的方法。

我想要做的是使用mpl :: vector将键映射到struct成员，使用get_value_ *方法转换值。我认为它看起来有点像：

typedef boost::mpl mpl;
using namespace std;

template<string K, typename T, T& dest, boost::function<void(MapTypePtr, string, T& dest)> >
struct MapperRow {};

struct Mapper : mpl::vector<
    MapperRow < "death_star_radius", Length, death_star.radius, get_value_units >,
    MapperRow < "death_star_energy", Energy, death_star.energy, get_value_units >,
    MapperRow < "operational",       bool,   death_star.operational, get_value_simple >,
    MapperRow < "num_tie_fighters",  int,    death_star.tie_fighers, get_value_simple >
> {};

长度和能量类型是boost :: units :: quantity的typedef。

升压元编程是否可行？如果我走在正确的轨道上，我该如何让它运行？我需要迭代mpl :: vector吗？

Answer 1

这绝对是可能的（见下文），但我不确定这是值得的麻烦......难道你不能使用更简单的方法，也许是继承和多态？

尽管如此，这是一个使用Boost.MPL的工作解决方案：

// MapperRow holds the necessary parsing informations: type of the member,
// type of the object, pointer to the appropriate member, parsing function
template<typename Type, typename Clazz, Type Clazz::*Member, 
         void (*Parser)(const std::string &, Type &)>
struct MapperRow
{
    typedef Type type;
    typedef Clazz clazz;
    typedef Type Clazz::*memberType;
    static const memberType member;
    typedef void (*parserType)(const std::string &, Type &);
    static const parserType parser;
};

template <typename Type, typename Clazz, Type Clazz::*Member,
          void (*Parser)(const std::string &, Type &)>
const typename MapperRow<Type, Clazz, Member, Parser>::memberType
    MapperRow<Type, Clazz, Member, Parser>::member = Member;

template <typename Type, typename Clazz, Type Clazz::*Member,
          void (*Parser)(const std::string &, Type &)>
const typename MapperRow<Type, Clazz, Member, Parser>::parserType
    MapperRow<Type, Clazz, Member, Parser>::parser = Parser;


// fill iterates over a map key->MapperRow, trying to find the given key.
// if found, it calls the parsing function, otherwise it asserts false (should
// probably throw an exception instead)
template <typename Clazz, typename First, typename Last>
struct fill_impl
{
    static void apply(Clazz &obj, const std::string &key,
                      const std::string &value)
    {
        typedef typename mpl::deref<First>::type entry;
        static const char *curKey = 
            mpl::c_str< typename 
                mpl::first<entry>::type
            >::value;

        if (key == curKey)
        {
            typedef typename mpl::second<entry>::type Row;

            Row::parser(value, obj.*Row::member);
        }
        else
        {
            fill_impl<
                Clazz, typename
                mpl::next<First>::type,
                Last
            >::apply(obj, key, value);
        }
    }
};

template <typename Clazz, typename Last>
struct fill_impl<Clazz, Last, Last>
{
    static void apply(Clazz &obj, const std::string &key,
                      const std::string &value)
    {
        assert(false && "key not found");
    }
};

template <typename Map, typename Clazz>
void fill(Clazz &obj, const std::string &key, const std::string &value)
{
    fill_impl< 
        Clazz, typename 
        mpl::begin<Map>::type, typename 
        mpl::end<Map>::type
    >::apply(obj, key, value);
}

样品使用：

template <typename T>
void get_value_units(const std::string &str, T &value)
{
    value = T::from_value(boost::lexical_cast<typename T::value_type>(str));
}

template <typename T>
void get_value_simple(const std::string &str, T &value)
{
    value = boost::lexical_cast<T>(str);
}

typedef boost::units::quantity<boost::units::si::energy> Energy;
typedef boost::units::quantity<boost::units::si::length> Length;

struct DeathStar
{
    Length radius;
    Energy energy;
    bool operational;
    int tie_fighters;
};

// Could be clearer with MPLLIBS_STRING*
typedef mpl::map<
    mpl::pair<
        mpl::string<'deat','h_st','ar_r','adiu','s'>, 
        MapperRow< Length , DeathStar, &DeathStar::radius, 
                   &get_value_units<Length> > >,
    mpl::pair<
        mpl::string<'deat','h_st','ar_e','nerg','y'>,
        MapperRow< Energy, DeathStar, &DeathStar::energy, 
                   &get_value_units<Energy> > >,
    mpl::pair< 
        mpl::string<'oper','atio','nal'>, 
        MapperRow< bool, DeathStar, &DeathStar::operational, 
                   &get_value_simple<bool> > >,
    mpl::pair< 
        mpl::string<'num_','tie_','figh','ters'>, 
        MapperRow< int, DeathStar, &DeathStar::tie_fighters, 
                   &get_value_simple<int> > >
> death_star_map;

int main()
{
    DeathStar ds;
    fill<death_star_map>(ds, "death_star_radius", "12");
    fill<death_star_map>(ds, "death_star_energy", "34");
    fill<death_star_map>(ds, "operational", "1");
    fill<death_star_map>(ds, "num_tie_fighters", "56");

    std::cout << "Radius: " << ds.radius << '\n';
    std::cout << "Energy: " << ds.energy << '\n';
    std::cout << "Operational: " << std::boolalpha << ds.operational << '\n';
    std::cout << "Tie fighters: " << ds.tie_fighters << '\n';
}

* MPLLIBS_STRING

使用mpl :: vector作为函数表

1 个答案: