C++ Templates 第十六章 模板与继承 例子
#include <iostream>using namespace std;// 中间模板template <typename Base, int D>class Discrim : public Base{public: Discrim() { cout<<"Discrim()"<<endl; } };// 类Discrim 使的 PolicySelector 可以多次继承自相同的基类template <typename set1, typename set2, typename set3, typename set4>class PolicySelector : public Discrim<set1, 1>, public Discrim<set2, 2>, public Discrim<set3, 3>, public Discrim<set4, 4>{};class DefaultPol1 {public: void print() { cout<<"p1 print()"<<endl; }};class DefaultPol2 {public: void print() { cout<<"p2 print()"<<endl; }};class DefaultPol3 {public: void print() { cout<<"p3 print()"<<endl; }};class DefaultPol4 {public: void print() { cout<<"p4 print()"<<endl; }};// 建立一个缺省值的集合class DefaultPols{public: typedef DefaultPol1 dp1; typedef DefaultPol2 dp2; typedef DefaultPol3 dp3; typedef DefaultPol4 dp4;};class DefaultPolArgs : virtual public DefaultPols {};// 写几个模板覆盖缺省的policy参数// 1template <typename Policy>class Policy1_is : virtual public DefaultPols{public: typedef Policy dp1; virtual void print() { dp1 dd; dd.print(); }};// 2template <typename Policy>class Policy2_is : virtual public DefaultPols{public: typedef Policy dp2; virtual void print() { dp1 dd; dd.print(); }};// 3template <typename Policy>class Policy3_is : virtual public DefaultPols{public: typedef Policy dp3; Policy3_is() { cout<<"Policy3_is()"<<endl; } virtual void print() { dp1 dd; dd.print(); }};// 4template <typename Policy>class Policy4_is : virtual public DefaultPols{public: typedef Policy dp4; virtual void print() { dp1 dd; dd.print(); }};// 这里所有的默认值相同template <typename PolicySet1 = DefaultPolArgs, typename PolicySet2 = DefaultPolArgs, typename PolicySet3 = DefaultPolArgs, typename PolicySet4 = DefaultPolArgs>class BreadSlicer{public: // 创建选择器，PolicySelector 在这里用模板参数分别初始化为对应的类型 // 这样就指定了 PolicySelector 的父类， 从别从自个模板参数中继承而来。。。 // 这一块的理解还不是很清楚？？ typedef PolicySelector<PolicySet1, PolicySet2, PolicySet3, PolicySet4> policies; void print() { typename policies::dp1 p1; p1.print(); typename policies::dp2 p2; p2.print(); typename policies::dp3 p3; p3.print(); typename policies::dp4 p4; p4.print(); } };class CustomPoliA{public: void print() { cout<<"CustomPoliA print()"<<endl; } };class CustomPoliB{public: void print() { cout<<"CustomPoliB print()"<<endl; } };int main(int argc, char **argv) { BreadSlicer<Policy3_is<CustomPoliA>, Policy4_is<CustomPoliB> > bc; bc.print(); return 0;}Starting: /home/robertkun/projects/tevent/build/teventp1 print()p2 print()CustomPoliA print()CustomPoliB print()*** Exited normally ***