Understanding C++ Pimpl Idiom

In C++, whenever a header file changes, any files that include that header file will need to be recompiled. To avoid such an issue, we can use pimpl (pointer to implementation) idiom.

Hello.h

#ifndef HELLO_H_
#define HELLO_H_

#include <boost/shared_ptr.hpp>

class HelloImpl; // forward declare this

class Hello {
public:
    Hello();
    virtual ~Hello();
    void sayHello() const;
private:
    boost::shared_ptr<HelloImpl> impl;
};

#endif /* HELLO_H_ */

Hello.cpp

#include "Hello.h"
#include <iostream>
using namespace std;

class HelloImpl {
public:
    void sayHello() const {
        cout << "Hello World" << endl;
    }
};

Hello::Hello() : impl(new HelloImpl) {
}

Hello::~Hello() {
}

void Hello::sayHello() const {
    impl->sayHello();
}

In this example, the use of smart pointer, such as Boost shared_ptr is recommended to avoid the hassle of writing the copy constructor, assignment operator, and destructor. With this approach, whenever the implementation changes, the client is insulated from the changes.