c ++/boost融合处理父类 [英] c++/boost fusion handle parent class
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问题描述
假设我有这样的类层次结构:
Lets suppose I have such classes hierarchy:
enum class Type { DUMMY };
struct Base {
int a;
explicit Base(int a) : a(a) {}
virtual ~Base() {}
virtual Type type() = 0;
};
struct Foo1 : public Base {
double b;
Foo1(int a, double b) : Base{a}, b(b) {}
Type type() override { return Type::DUMMY; }
};
全部使用单继承性从Base派生而未定义
任何virtual方法，但覆盖type()方法除外.
all derived from Base using single inheritance and not defined
any virtual methods, except overriding type() method.
我想为每个从Base派生到序列化和调试输出的元信息.正如我所见，增强融合是我想要的:
And I want to have meta info for each derived from Base to serialization and debug output. And as I see boost fusion is what I want:
#include <iostream>
#include <string>
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/fusion/include/at_c.hpp>
#include <boost/fusion/include/for_each.hpp>
#include <boost/fusion/include/mpl.hpp>
#include <boost/fusion/include/zip.hpp>
#include <boost/mpl/range_c.hpp>
namespace fusion = boost::fusion;
namespace mpl = boost::mpl;
enum class Type { DUMMY };
struct Base {
int a;
explicit Base(int a) : a(a) {}
virtual ~Base() {}
virtual Type type() = 0;
};
struct Foo1 : public Base {
double b;
Foo1(int a, double b) : Base{a}, b(b) {}
Type type() override { return Type::DUMMY; }
};
BOOST_FUSION_ADAPT_STRUCT(Foo1, (double, b))
template <typename Sequence> struct XmlFieldNamePrinter {
XmlFieldNamePrinter(const Sequence &seq) : seq_(seq) {}
const Sequence &seq_;
template <typename Index> void operator()(Index idx) const {
std::string field_name =
fusion::extension::struct_member_name<Sequence, idx>::call();
std::cout << '<' << field_name << '>' << fusion::at<Index>(seq_) << "</"
<< field_name << ">\n";
;
}
};
template <typename Sequence> void printXml(Sequence const &v) {
typedef mpl::range_c<unsigned, 0, fusion::result_of::size<Sequence>::value>
Indices;
fusion::for_each(Indices(), XmlFieldNamePrinter<Sequence>(v));
}
int main() {
Foo1 saveMe = {3, 3.4};
printXml(saveMe);
}
但是如何处理Base数据成员?
我不想将它们的描述包含在BOOST_FUSION_ADAPT_STRUCT(Foo1中，
像这样:
But how handle Base data memebers?
I do not want include their description into BOOST_FUSION_ADAPT_STRUCT(Foo1,
like this:
BOOST_FUSION_ADAPT_STRUCT(Foo1,
(int, a)
(double, b))
因为我必须对从Base继承的每个结构都执行此操作，
因此，我希望使用与此类似的语法(当然不进行编译):
because of I have to do it for every struct that inherit from Base,
so I would prefer syntax similar to this(not compiled of course):
BOOST_FUSION_ADAPT_STRUCT(Base, (int, a))
BOOST_FUSION_ADAPT_STRUCT(Foo1,
(Base, __parent__)
(double, b))
如何实现类似的语法?
How can I achieve similar syntax?
推荐答案
您需要分别处理Fusion序列，例如使用SFINAE:
You need to treat Fusion sequences separately, e.g. using SFINAE:
template <typename Index, typename IsSeq = IsSeq<Index> >
typename boost::disable_if<IsSeq, void>::type operator()(Index idx) const {
std::string field_name = fusion::extension::struct_member_name<Sequence, idx>::call();
std::cout << '<' << field_name << '>' << fusion::at<Index>(seq_) << "</" << field_name << ">\n";
}
template <typename Index, typename IsSeq = IsSeq<Index> >
typename boost::enable_if<IsSeq, void>::type operator()(Index idx) const {
std::string field_name = fusion::extension::struct_member_name<Sequence, idx>::call();
std::cout << "<" << field_name << ">\n\t";
typedef typename IsSeq::sequence_type SubSeq;
typedef mpl::range_c<unsigned, 0, fusion::result_of::size<SubSeq>::value> SubIndices;
fusion::for_each(SubIndices(), XmlFieldNamePrinter<SubSeq>(fusion::at<Index>(seq_)));
std::cout << "</" << field_name << ">\n";
}
IsSeq<Index>辅助特征的实现如下.现在您可以使用:
The implementation of the IsSeq<Index> helper trait is below. Now you can use:
BOOST_FUSION_ADAPT_STRUCT(Base, a)
BOOST_FUSION_ADAPT_STRUCT(Foo1, base, b)
并获得
<base>
<a>3</a>
</base>
<b>3.4</b>
实时演示
#include <iostream>
#include <string>
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/fusion/include/at_c.hpp>
#include <boost/fusion/include/for_each.hpp>
#include <boost/fusion/include/mpl.hpp>
#include <boost/fusion/include/zip.hpp>
#include <boost/mpl/range_c.hpp>
namespace fusion = boost::fusion;
namespace mpl = boost::mpl;
enum class Type { DUMMY };
struct Base {
int a;
explicit Base(int a) : a(a) {}
virtual ~Base() {}
virtual Type type() = 0;
Base &base = *this;
};
struct Foo1 : public Base {
double b;
Foo1(int a, double b) : Base{ a }, b(b) {}
Type type() override { return Type::DUMMY; }
};
BOOST_FUSION_ADAPT_STRUCT(Base, a)
BOOST_FUSION_ADAPT_STRUCT(Foo1, base, b)
template <typename Sequence> struct XmlFieldNamePrinter {
XmlFieldNamePrinter(const Sequence &seq) : seq_(seq) {}
const Sequence &seq_;
template <typename Index,
typename T = typename fusion::result_of::at_c<Sequence, Index::value>::type,
typename BareT = typename boost::remove_reference<T>::type
>
struct IsSeq : mpl::bool_<fusion::traits::is_sequence<BareT>::value> {
typedef BareT sequence_type; // if true_
};
template <typename Index, typename IsSeq = IsSeq<Index> >
typename boost::disable_if<IsSeq, void>::type operator()(Index idx) const {
std::string field_name = fusion::extension::struct_member_name<Sequence, idx>::call();
std::cout << '<' << field_name << '>' << fusion::at<Index>(seq_) << "</" << field_name << ">\n";
}
template <typename Index, typename IsSeq = IsSeq<Index> >
typename boost::enable_if<IsSeq, void>::type operator()(Index idx) const {
std::string field_name = fusion::extension::struct_member_name<Sequence, idx>::call();
std::cout << "<" << field_name << ">\n\t";
typedef typename IsSeq::sequence_type SubSeq;
typedef mpl::range_c<unsigned, 0, fusion::result_of::size<SubSeq>::value> SubIndices;
fusion::for_each(SubIndices(), XmlFieldNamePrinter<SubSeq>(fusion::at<Index>(seq_)));
std::cout << "</" << field_name << ">\n";
}
};
template <typename Sequence> void printXml(Sequence const &v) {
typedef mpl::range_c<unsigned, 0, fusion::result_of::size<Sequence>::value> Indices;
fusion::for_each(Indices(), XmlFieldNamePrinter<Sequence>(v));
}
int main() {
Foo1 saveMe = { 3, 3.4 };
printXml(saveMe);
}
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