使用 mpl::vector 作为函数表

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结构成员。目前，第一遍生成键值对的 stl::unordered_map。下一步是解释值字符串并将它们作为要存储在结构中的目标类型返回。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 将键映射到结构成员，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 >
> {};

Length 和 Energy 类型是 boost::units::quantities 的 typedef。

这对 boost 元编程可行吗？如果我在正确的轨道上，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），但我不确定这是否值得麻烦...您不能使用更简单的方法，也许使用继承和多态性？

不过，这里有一个使用 Boost.MPL 的可行解决方案：

// 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);
}

使用示例：

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