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CppStateMultiFile

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(C++) State in multiple files

The files in this example are:

• main.cpp, containing the program itself
• Enum.h, contains the possible States
• Context.h
• Context.cpp
• State.h
• State.cpp

All header files have an #include guard.

In its header file, the State has a forward declarion to its Context. Therefore, in its implementation file (State.cpp), the header file of the Context needs to be #included.

main.cpp

include <memory>

include "Context.h"


int main()
{
  const std::auto_ptr<Context> myContext(new Context);
  myContext->setState(plus);
  myContext->doIt();
  myContext->setState(minus);
  myContext->doIt();
}

Enum.h

ifndef __ENUM_H

define __ENUM_H


enum enumState{ plus, minus};
//End of: #ifndef __ENUM_H
endif

Context.h

ifndef __CONTEXT_H

define __CONTEXT_H


include <iostream>

include <memory>

include <cassert>

include "State.h"

include "Enum.h"


class Context
{
  friend class StateBase;
  friend class StatePlus;
  friend class StateMinus;
  public:
  Context();
  void setState(const enumState&);
  void doIt() const;
  private:
  std::auto_ptr<StateBase> mState;
  const int mValue1;
  const int mValue2;
};
//End of: #ifndef __CONTEXT_H
endif

Context.cpp

include "Context.h"

//---------------------------------------------------------------------------
Context::Context() :
  mState(new StatePlus(this)),
  mValue1(std::rand()%100),
  mValue2(std::rand()%100)
{
  //Nothing
}
//---------------------------------------------------------------------------
void Context::setState(const enumState& state)
{
  switch(state)
  {
    case plus: mState.reset(new StatePlus(this)); break;
    case minus: mState.reset(new StateMinus(this)); break;
    default: assert(!"Unknown state"); std::exit(1);
  }
}
//---------------------------------------------------------------------------
void Context::doIt() const
{
  //The function doIt is delegated to the State
  std::cout
    << "Doing it with "
    << mValue1
    << " and "
    << mValue2
    << ": "
    << mState->doIt()
    << std::endl;
}
//---------------------------------------------------------------------------

State.h

ifndef __STATE_H

define __STATE_H


class Context; //Forward declaration of Context
class StateBase
{
  public:
  StateBase() { mContext = 0; }
  virtual int doIt() const = 0;
  protected:
  Context * mContext;
};
class StatePlus : public StateBase
{
  public:
  StatePlus(Context *);
  int doIt() const;
};
class StateMinus : public StateBase
{
  public:
  StateMinus(Context *);
  int doIt() const;
};
//End of: #ifndef __STATE_H
endif

State.cpp

include "State.h"


include "Context.h"

//---------------------------------------------------------------------------
StatePlus::StatePlus(Context * context)
{
  mContext = context;
}
//---------------------------------------------------------------------------
StateMinus::StateMinus(Context * context)
{
  mContext = context;
}
//---------------------------------------------------------------------------
int StatePlus::doIt() const
{
  return (mContext->mValue1 + mContext->mValue2);
}
//---------------------------------------------------------------------------
int StateMinus::doIt() const
{
  return (mContext->mValue1 - mContext->mValue2);
}
//---------------------------------------------------------------------------

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