The Command Design Pattern 'encapsulates a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations' [1]

Personally, I view a Command as a class containing a single push button (labeled 'Execute'), where the class itself does not know what this push button does.

In [1] a Command base class is described. But instead I show two of my personal flavors of the Command Design Pattern.

Example of two general template commands


//---------------------------------------------------------------------------

// From http://www.richelbilderbeek.nl/CppDesignPatternCommand.htm

//---------------------------------------------------------------------------

#include <iostream>

#include <boost/shared_ptr.hpp>

//---------------------------------------------------------------------------

template <class T>

struct Command

{

  typedef void (*FunctionPointer)(T& target);



  Command(boost::shared_ptr<T> target, const FunctionPointer functionPointer)

    : mTarget(target), mFunction(functionPointer) { }

  void Execute() { mFunction( *( mTarget.get() ) ); }

  private:

  boost::shared_ptr<T> mTarget;

  FunctionPointer mFunction;

};

//---------------------------------------------------------------------------

template <class T>

struct MemFunCommand

{

  typedef void (T::* MemFunPtr)();



  MemFunCommand(boost::shared_ptr<T> target, const MemFunPtr memFun)

    : mTarget(target), mFunction(memFun) { }

  void Execute() { (mTarget.get()->*mFunction)(); }

  private:

  boost::shared_ptr<T> mTarget;

  MemFunPtr mFunction;

};

//---------------------------------------------------------------------------

struct MdPlayer

{

  MdPlayer(const int id) : mId(id) {}

  void Play()   { std::cout << "Play   #" << mId << std::endl; }

  void Record() { std::cout << "Record #" << mId << std::endl; }

  void IncreaseId() { ++mId; std::cout << "Increased ID to " << mId << std::endl; }

  void DecreaseId() { --mId; std::cout << "Decreased ID to " << mId << std::endl; }

  void DecreaseId() { --mId; std::cout << "Decreased ID to " << mId << std::endl; }

  private:

  int mId;

};

//---------------------------------------------------------------------------

void IncreaseId(MdPlayer& player)

{

  player.IncreaseId();

}

//---------------------------------------------------------------------------

void DecreaseId(MdPlayer& player)

{

  player.DecreaseId();

}

//---------------------------------------------------------------------------

int main()

{

  boost::shared_ptr<MdPlayer> p0(new MdPlayer(0));

  boost::shared_ptr<MdPlayer> p1(new MdPlayer(1));

  MemFunCommand<MdPlayer> b0(p0,&MdPlayer::Play);

  MemFunCommand<MdPlayer> b1(p1,&MdPlayer::Record);



  Command<MdPlayer> c0(p0,&IncreaseId);

  Command<MdPlayer> c1(p1,&DecreaseId);

  b0.Execute();

  b1.Execute();

  c0.Execute();

  c1.Execute();

}

Screen output:


Play   #0


Record #1

Increased ID to 1

Decreased ID to 0

References

Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides. Design Patterns. 1995. ISBN: 0201633612.

Go back to Richel Bilderbeek's C++ page.

Go back to Richel Bilderbeek's homepage.

(C++) Command (Design Pattern)

Example of two general template commands

References