Policy-based design

       One problem which often arises during programming is how to build a base set of functionality which can be extended by the user, while still being modular enough to make it easy to replace only certain parts of an implementation without having to resort to copy & paste techniques. I guess everybody of us has faced this problem at least once, and came up with different solutions. There is a powerful and elegant technique called policy-based design for solving this kind of problem.

      Simple example

#include<iostream>#include<string>using namespace std;template<typename output_policy,typename language_policy>class HelloWorld:public output_policy,public language_policy{    using output_policy::Print;    using language_policy::Message;public:    void Run(){Print(Message());    }};class HelloWorld_OutputPolicy_WriteToCout{protected:template<typename message_type>    void Print(message_type message)    {std::cout << message << std::endl;    }};class HelloWorld_LangguagePolicy_English{protected:std::string Message()    {        return "Hello World!";    }};class HelloWorld_LanguagePolicy_German{protected:std::string Message()    {        return "Hello Policy!";    }};int main(){    typedef HelloWorld<HelloWorld_OutputPolicy_WriteToCout,HelloWorld_LangguagePolicy_English> myPolicyClass;    myPolicyClass hello1;    hello1.Run();    typedef HelloWorld<HelloWorld_OutputPolicy_WriteToCout,HelloWorld_LanguagePolicy_German> myOtherPolicyClass;    myOtherPolicyClass hello2;    hello2.Run();}

      Makefile

all:policy# which compilerCC=g++ # Where are include file keptINCLUDE = .# Where to installINSTDIR = /usr/local/bin# Options for developmentCFLAGS = -g -Wall -ansipolicy:policy.o$(CC) -o policy policy.opolicy.o:policy.cpp#$(CC) -I$(INCLUDE) $(CFLAGS) -c msgqueue.c#$(CC) -D_REENTRANT -c msgqueue.c -lpthread$(CC) -c policy.cpp -o policy.oclean:-rm  policy.o policyinstall:policy@if [-d $(INSTDIR) ];\        then \        cp policy $(INSTDIR);\        chmod a+x $(INSTDIR)/policy;\        chmod og-w $(INSTDIR)/policy;\        echo "Install in $(INSTDIR)";\    else \       echo "Sorry,$(INSTDIR) does not exist";\    fi 

      Runing result:

       Hello World

       Hello Policy