How to Implement Policy Classes in C++

Policy-based class design is a powerful technique described in Modern C++: Generic Programming and Design Patterns Applied book. It's similar to a strategy design pattern, but with all the type-safety thanks to the use of C++ templates.

Below is a trivial example how to design a library with policy classes.

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#include <iostream> #include <string> using namespace std;   template<class T> struct HelloPolicy {     static void saySomething(const T& t) {         cout << "Hello World, " << t.getName() << endl;     } };   template<class T> struct ByePolicy {     static void saySomething(const T& t) {         cout << "Bye World, " << t.getName() << endl;     } };   struct Foo {     string getName() const {         return "Foo";     } };   struct Bar {     string getName() const {         return "Bar";     } };   template<template <class> class Policy> class FooPolicy : public Policy<Foo> { public:     void doSomething() {         Foo foo;         Policy<Foo>().saySomething(foo);     } };   template<template <class> class Policy> class BarPolicy : public Policy<Bar> { public:     void doSomething() {         Bar bar;         Policy<Bar>().saySomething(bar);     } };   int main() {     FooPolicy<HelloPolicy> f1;     f1.doSomething();       FooPolicy<ByePolicy> f2;     f2.doSomething();       BarPolicy<HelloPolicy> b1;     b1.doSomething();       BarPolicy<ByePolicy> b2;     b2.doSomething();           return 0;  }

Output:

Hello World, Foo
Bye World, Foo
Hello World, Bar
Bye World, Bar