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《C++设计新思维》勘误,附C++14新解法

勘误:

原书(中文版)3.13节,65-69页中GenScatterHierarchy以及FieldHelper均存在问题,当TypeList中类型有重复时,无法通过编译(原因在于“二义性基类”)。

书中出现的二义性问题,可以用一小段代码演示一下:

class A{};
class B:public A{};
class C:public A,public B{};

void test()
{
    C c;
    A& cf =c;//wrong,don't try this at home.  
    B& cbf =c;//right
    A& caf = cbf;//right
}

由于C继承了两个A,一个直接继承,一个间接继承,所以将C转换成A,存在两条路径,无法转换。甚至我们永远无法访问C直接继承的A!

继承B的路径是唯一的,所以可以通过B,再次转换成为A。

《C++设计新思维》书中给出的代码和类图,向右侧转换是唯一的,但是向左侧转换时路径不为一,所以TypeList一旦包含重复类型后,无法通过编译。

这个问题已经解决了,具体解决方案请参看Loki源码 Loki源码

这里我简要说一下,解决方法就是确保左端的直接父类是唯一的,这样即可有一条唯一的路径可以转换到某一基类。

 

C++14 新法:

首先,给出typelist和操作typelist的两个函数

template <typename... T> struct TypeList;

template <int I, typename Arg>
struct at;
template <int I, typename Head, typename... Tail>
struct at<I, TypeList<Head, Tail...>>
{
    typedef typename at<I - 1, TypeList<Tail...>>::type type;
};
template <class Head, typename... Tail>
struct at<0, TypeList<Head, Tail...>>
{
    typedef Head type;
};
template <int N, class Seq>
struct drop;
template <int N, class Head, class... Tail>
struct drop<N, TypeList<Head, Tail...>>
{
    typedef
    typename drop<N-1, TypeList<Tail...>>::type
    type;
};
template <class Head, class... Tail>
struct drop<0, TypeList<Head, Tail...>>
{
    typedef TypeList<Head, Tail...> type;
};

at获取typelist中某个索引值的类型元素,而drop是去除首部的某几个元素。

 

下面给出GenScatterHierarchy代码,此处为了方便使用Genorator代之。

 

namespace Private
{
// The following type helps to overcome subtle flaw in the original
// implementation of GenScatterHierarchy.
// The flaw is revealed when the input type list of GenScatterHierarchy
// contains more then one element of the same type (e.g. LOKI_TYPELIST_2(int, int)).
// In this case GenScatterHierarchy will contain multiple bases of the same
// type and some of them will not be reachable (per 10.3).
// For example before the fix the first element of Tuple<LOKI_TYPELIST_2(int, int)>
// is not reachable in any way!
template<class, class>
struct UniqueTag;
}



template <typename Head,typename... Tails, template <typename> class Unit> class Genorater<TypeList<Head,Tails...>,Unit>
        :public Genorater<Private::UniqueTag<Head,TypeList<Head,Tails...>>,Unit>,public Genorater<TypeList<Tails...>,Unit> {};
template <typename Head,typename... Tails,template <typename> class Unit> class Genorater<Private::UniqueTag<Head,TypeList<Head,Tails...>>,Unit>:public Unit<Head> {};

UniqueTag,用于辅助构建唯一的类型转换路径。

下面给出FieldHelper源码:

template <int I,typename... TList,template <typename> class Unit> Unit<typename at<I,TypeList<TList...>>::type>& FieldHelper(Genorater<TypeList<TList...>,Unit>& obj)
{
 
    Genorater<Private::UniqueTag<typename at<I,TypeList<TList...>>::type,typename drop<I,TypeList<TList...>>::type>,Unit>& leftBase = obj;
    return leftBase;
 
}
 
template <int I,typename... TList, template <typename> class Unit> Unit<typename at<I,TypeList<TList...>>::type>& Field(Genorater<TypeList<TList...>,Unit>& obj)
{
 
    //return FieldHelper(obj,Int2Type<I>());
    return FieldHelper<I>(obj);
}
 

 

最后是测试代码:

typedef TypeList<double,int,double,string> myList;

typedef Genorater<myList,Holder> Info;

int main()
{
    cout << "Hello world!" << endl;
  
    Info obj;
    cout<<typeid(obj).name()<<endl;
    Field<1>(obj).value_=1;
    Field<2>(obj).value_=2;
    Field<3>(obj).value_="hao123";
    cout<< Field<1>(obj).value_<<endl;
    cout<< Field<2>(obj).value_<<endl;
    cout<< Field<3>(obj).value_<<endl;
 
 
    return 0;
}

-------------------------------------------------------------------------华丽的分割线---------------------------------------------------------------------------------------

此处给出一个仿写Loki源码的FieldHelper

namespace Private
{
template<class, class...>
struct UniqueTag;
}

template <typename Head,typename... Tails, template <typename> class Unit> class Genorater<TypeList<Head,Tails...>,Unit>
        :public Genorater<Private::UniqueTag<Head,Tails...>,Unit>,public Genorater<TypeList<Tails...>,Unit> {};
template <typename Head,typename... Tails,template <typename> class Unit> class Genorater<Private::UniqueTag<Head,Tails...>,Unit>:public Unit<Head> {};


template <typename Head,typename... TList,template <typename> class Unit> Unit<Head>& FieldHelper(Genorater<TypeList<Head,TList...>,Unit>& obj,Int2Type<0>){

              Genorater<Private::UniqueTag<Head,TList...>,Unit>& leftBase =  obj;
              return leftBase;
}
template <int I,typename Head,typename... TList, template <typename> class Unit> Unit<typename at<I,TypeList<Head,TList...>>::type>& FieldHelper(Genorater<TypeList<Head,TList...>,Unit>& obj,Int2Type<I>){

Genorater <TypeList<TList...>,Unit>& rightBase = obj;
return FieldHelper(rightBase,Int2Type<I-1>());
}

 

补充一些内容,其实C++11可利用变模板参数而剔除typelist。比如C++11标准的tuple的实现并没有利用typelist技术。

tuple利用了多重继承,实现方法和Genorator方式大同小异。C++11采用左端采用公有继承,右端使用私有继承。并采用int类型模板参数,作为转换路径唯一的标示。

具体见C++11 tuple.h中 相关源码。

接下来我不使用typelist技术来实现单根继承

class empty{};

template <template <typename,typename> class Unit,typename Head,typename... Tail> class GenorateLiner
:public Unit<Head,GenorateLiner<Unit,Tail...>>{
    
};

template <template <typename,typename> class Unit,typename Head> class GenorateLiner<Unit,Head>:Unit<Head,empty>{

};

 

谢谢,转载请表明出处!本文仅对读过《C++设计新思维》一书朋友有用,其他博友慎读(不要对C++产生抵触情绪)

 

posted on 2015-02-03 17:40  RonTang  阅读(2173)  评论(0编辑  收藏