标签：
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。
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>()); }
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）
标签：
原文地址：http://www.cnblogs.com/tangzhenqiang/p/4270609.html
