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)