c – 是否可以使用Boost.Hana进行反序列化？

Hana是一个元编程库.它提供了可用于构建的工具
更复杂的功能,如序列化,但它没有提供这样的功能
功能本身.它根本不是该库的范围.另外,关于
你的特定用例;解析不是一个容易的问题和其他库
像Boost.Spirit已经尝试解决它.

话虽这么说,我勾勒出一个使用Hana反序列化JSON的例子.
结果既不高效也不健壮,但它应该足以给予
你可以一瞥Hana如何用来实现更好的目标.解决
正确的这个问题需要实现一个解析器组合库
à-la Boost.Spirit,我不会在这里做.干得好：

template <typename T>
  std::enable_if_t<std::is_same<T,int>::value,T> from_json(std::istream& in) {
    T result;
    in >> result;
    return result;
}

template <typename T>
  std::enable_if_t<std::is_same<T,std::string>::value,T> from_json(std::istream& in) {
    char quote;
    in >> quote;

    T result;
    char c;
    while (in.get(c) && c != '"') {
        result += c;
    }
    return result;
}


template <typename T>
  std::enable_if_t<hana::Struct<T>::value,T> from_json(std::istream& in) {
    T result;
    char brace;
    in >> brace;

    hana::for_each(hana::keys(result),[&](auto key) {
        in.ignore(std::numeric_limits<std::streamsize>::max(),':');
        auto& member = hana::at_key(result,key);
        using Member = std::remove_reference_t<decltype(member)>;
        member = from_json<Member>(in);
    });
    in >> brace;
    return result;
}

template <typename Xs>
  std::enable_if_t<hana::Sequence<Xs>::value,Xs> from_json(std::istream& in) {
    Xs result;
    char bracket;
    in >> bracket;
    hana::length(result).times.with_index([&](auto i) {
        if (i != 0u) {
            char comma;
            in >> comma;
        }

        auto& element = hana::at(result,i);
        using Element = std::remove_reference_t<decltype(element)>;
        element = from_json<Element>(in);
    });
    in >> bracket;
    return result;
}

然后你可以像使用它一样

struct Car {
    BOOST_HANA_DEFINE_STRUCT(Car,(std::string,brand),model)
    );
};

struct Person {
    BOOST_HANA_DEFINE_STRUCT(Person,name),last_name),(int,age)
    );
};

int main() {
    std::istringstream json(R"EOS(
        [
            {
                "name": "John","last_name": "Doe","age": 30
            },{
                "brand": "BMW","model": "Z3"
            },{
                "brand": "Audi","model": "A4"
            }
        ]
    )EOS");

    auto actual = from_json<hana::tuple<Person,Car,Car>>(json);

    auto expected = hana::make_tuple(Person{"John","Doe",30},Car{"BMW","Z3"},Car{"Audi","A4"});

    assert(actual == expected);
}

完整示例可用于here.