Policy-based Design - Simple Example

Simple Example

Presented below is a simple (contrived) example of a C++ hello world program, where the text to be printed and the method of printing it are decomposed using policies.

#include #include template class HelloWorld : private OutputPolicy, private LanguagePolicy { using OutputPolicy::print; using LanguagePolicy::message; public: // Behaviour method void run const { // Two policy methods print(message); } }; class OutputPolicyWriteToCout { protected: template void print(MessageType const &message) const { std::cout << message << std::endl; } }; class LanguagePolicyEnglish { protected: std::string message const { return "Hello, World!"; } }; class LanguagePolicyGerman { protected: std::string message const { return "Hallo Welt!"; } }; int main { /* Example 1 */ typedef HelloWorld HelloWorldEnglish; HelloWorldEnglish hello_world; hello_world.run; // prints "Hello, World!" /* Example 2 * Does the same, but uses another language policy */ typedef HelloWorld HelloWorldGerman; HelloWorldGerman hello_world2; hello_world2.run; // prints "Hallo Welt!" }

You could easily write another OutputPolicy by adding a new class with the member function print and take that as the new OutputPolicy.

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