(C++) State (Design Pattern)

 

The State is a Design Pattern to 'allow an object to alter its behavior when its internal state changes. The object will appear to change its class' [1]

 

Below is an example from [2].

 

• Download the Qt Creator project 'CppDesignPatternState' (zip)

 

 

 

 

 

Technical facts

 

Application type(s)

• Desktop application

Operating system(s) or programming environment(s)

• Ubuntu 10.10 (maverick)

IDE(s):

• Qt Creator 2.0.1

Project type:

• Console application

C++ standard:

• C++98

Compiler(s):

• G++ 4.4.5

Libraries used:

• Boost: version 1.42
• STL: GNU ISO C++ Library, version 4.4.5

 

 

 

 

 

Qt project file: CppDesignPatternState.pro

 

---

#------------------------------------------------- # # Project created by QtCreator 2011-04-14T15:44:05 # #------------------------------------------------- QT       += core QT       -= gui TARGET = CppDesignPatternState CONFIG   += console CONFIG   -= app_bundle TEMPLATE = app SOURCES += main.cpp \     gumballmachine.cpp \     hasquarterstate.cpp \     noquarterstate.cpp \     soldoutstate.cpp \     soldstate.cpp HEADERS += \     gumballmachine.h \     hasquarterstate.h \     noquarterstate.h \     state.h \     soldoutstate.h \     soldstate.h

 

 

 

 

 

gumballmachine.cpp

 

---

//--------------------------------------------------------------------------- /* CppDesignPatternState, demonstrates the State design pattern Copyright 2011 Richel Bilderbeek This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see <http://www.gnu.org/licenses/>. */ //--------------------------------------------------------------------------- //From http://www.richelbilderbeek.nl/CppDesignPatternState.htm //--------------------------------------------------------------------------- #include <iostream> //--------------------------------------------------------------------------- #include "gumballmachine.h" #include "hasquarterstate.h" #include "noquarterstate.h" #include "soldoutstate.h" #include "soldstate.h" #include "state.h" //--------------------------------------------------------------------------- GumballMachine::GumballMachine(const int n_gumballs)   : m_has_quarter_state(new HasQuarterState(this)),     m_no_quarter_state(new NoQuarterState(this)),     m_sold_out_state(new SoldOutState(this)),     m_sold_state(new SoldState(this)),     m_count(n_gumballs) {   //delete m_state; //Should not be possible   if (m_count)   {     SetState(GetNoQuarterState());   }   else   {     SetState(GetSoldOutState());   } } //--------------------------------------------------------------------------- void GumballMachine::EjectQuarter() {   m_state->EjectQuarter(); } //--------------------------------------------------------------------------- int GumballMachine::GetCount() const {   assert(m_count >= 0);   return m_count; } //--------------------------------------------------------------------------- State * GumballMachine::GetHasQuarterState() {   return m_has_quarter_state.get(); } //--------------------------------------------------------------------------- State * GumballMachine::GetNoQuarterState() {   return m_no_quarter_state.get(); } //--------------------------------------------------------------------------- State * GumballMachine::GetSoldOutState() {   return m_sold_out_state.get(); } //--------------------------------------------------------------------------- State * GumballMachine::GetSoldState() {   return m_sold_state.get(); } //--------------------------------------------------------------------------- const State * GumballMachine::GetState() const {   return m_state; } //--------------------------------------------------------------------------- void GumballMachine::InsertQuarter() {   m_state->InsertQuarter(); } //--------------------------------------------------------------------------- void GumballMachine::ReleaseBall() {   std::cout << "A gumball comes rolling out of the solt...\n";   --m_count;   assert(m_count >= 0); } //--------------------------------------------------------------------------- void GumballMachine::SetState(State * const state) {   assert(state);   //No need to delete the old m_state, this is done   //by the boost::scoped_ptr's   m_state = state; } //--------------------------------------------------------------------------- void GumballMachine::TurnCrank() {   m_state->TurnCrank(); } //--------------------------------------------------------------------------- std::ostream& operator<<(std::ostream& os, const GumballMachine& gumballMachine) {   os     << "Mighty Gumball, Inc.\n"     << "C++-enabled Standing Gumball Model #2004\n"     << "Inventory: " << gumballMachine.GetCount()     << " gumballs\n"     << gumballMachine.GetState()->GetDescription()     << '\n';   return os; } //--------------------------------------------------------------------------- std::ostream& operator<<(std::ostream& os, const GumballMachine * const gumballMachine) {   assert(gumballMachine);   return operator<<(os,*gumballMachine); } //--------------------------------------------------------------------------- std::ostream& operator<<(std::ostream& os, const boost::scoped_ptr<GumballMachine>& gumballMachine) {   assert(gumballMachine);   return operator<<(os,gumballMachine.get()); } //---------------------------------------------------------------------------

 

 

 

 

 

gumballmachine.h

 

---

//--------------------------------------------------------------------------- /* CppDesignPatternState, demonstrates the State design pattern Copyright 2011 Richel Bilderbeek This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see <http://www.gnu.org/licenses/>. */ //--------------------------------------------------------------------------- //From http://www.richelbilderbeek.nl/CppDesignPatternState.htm //--------------------------------------------------------------------------- #ifndef GUMBALLMACHINE_H #define GUMBALLMACHINE_H //--------------------------------------------------------------------------- #include <iosfwd> //--------------------------------------------------------------------------- #include <boost/checked_delete.hpp> #include <boost/scoped_ptr.hpp> //--------------------------------------------------------------------------- struct State; //--------------------------------------------------------------------------- struct GumballMachine {   GumballMachine(const int n_gumballs = 0);   void EjectQuarter();   void InsertQuarter();   void TurnCrank();   void ReleaseBall();   int GetCount() const;   //State machinery   State * GetNoQuarterState();   State * GetHasQuarterState();   State * GetSoldOutState();   State * GetSoldState();   const State * GetState() const;   void SetState(State * const state);   private:   virtual ~GumballMachine() {}   //Do not forget the template brackets, as stated in   //Herb Sutter. Exceptional C++ style. 2005. ISBN: 0-201-76042-8. Item 8: 'Befriending templates'.   friend void boost::checked_delete<>(GumballMachine* x);   boost::scoped_ptr<State> m_has_quarter_state;   boost::scoped_ptr<State> m_no_quarter_state;   boost::scoped_ptr<State> m_sold_out_state;   boost::scoped_ptr<State> m_sold_state;   State * m_state;   int m_count; }; //--------------------------------------------------------------------------- std::ostream& operator<<(std::ostream& os, const GumballMachine& gumballMachine); std::ostream& operator<<(std::ostream& os, const GumballMachine * const gumballMachine); std::ostream& operator<<(std::ostream& os, const boost::scoped_ptr<GumballMachine>& gumballMachine); //--------------------------------------------------------------------------- #endif // GUMBALLMACHINE_H

 

 

 

 

 

hasquarterstate.cpp

 

---

//--------------------------------------------------------------------------- /* CppDesignPatternState, demonstrates the State design pattern Copyright 2011 Richel Bilderbeek This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see <http://www.gnu.org/licenses/>. */ //--------------------------------------------------------------------------- //From http://www.richelbilderbeek.nl/CppDesignPatternState.htm //--------------------------------------------------------------------------- #include <cassert> #include <iostream> //--------------------------------------------------------------------------- #include "gumballmachine.h" #include "hasquarterstate.h" //--------------------------------------------------------------------------- HasQuarterState::HasQuarterState(GumballMachine * gummballMachine)   : m_gumballMachine(gummballMachine) {   assert(m_gumballMachine);   //delete m_gumballMachine; //Should not be possible } //--------------------------------------------------------------------------- void HasQuarterState::Dispense() {   std::cout << "No gummball dispensed\n"; } //--------------------------------------------------------------------------- void HasQuarterState::EjectQuarter() {   std::cout << "Quarter returned\n";   //Change the state   m_gumballMachine->SetState(     m_gumballMachine->GetNoQuarterState()); } //--------------------------------------------------------------------------- const std::string HasQuarterState::GetDescription() const {   return "The machine has had a quarter inserted"; } //--------------------------------------------------------------------------- void HasQuarterState::InsertQuarter() {   std::cout << "You can insert another quarter\n"; } //--------------------------------------------------------------------------- void HasQuarterState::TurnCrank() {   std::cout << "You turned...\n";   //Change the state   m_gumballMachine->SetState(     m_gumballMachine->GetSoldState()); } //---------------------------------------------------------------------------

 

 

 

 

 

hasquarterstate.h

 

---

//--------------------------------------------------------------------------- /* CppDesignPatternState, demonstrates the State design pattern Copyright 2011 Richel Bilderbeek This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see <http://www.gnu.org/licenses/>. */ //--------------------------------------------------------------------------- //From http://www.richelbilderbeek.nl/CppDesignPatternState.htm //--------------------------------------------------------------------------- #ifndef HASQUARTERSTATE_H #define HASQUARTERSTATE_H //--------------------------------------------------------------------------- #include "state.h" //--------------------------------------------------------------------------- struct GumballMachine; //--------------------------------------------------------------------------- struct HasQuarterState : public State {   HasQuarterState(GumballMachine * gummballMachine);   void Dispense();   void EjectQuarter();   const std::string GetDescription() const;   void InsertQuarter();   void TurnCrank();   GumballMachine * const m_gumballMachine; }; //--------------------------------------------------------------------------- #endif // HASQUARTERSTATE_H

 

 

 

 

 

main.cpp

 

---

//--------------------------------------------------------------------------- /* CppDesignPatternState, demonstrates the State design pattern Copyright 2011 Richel Bilderbeek This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see <http://www.gnu.org/licenses/>. */ //--------------------------------------------------------------------------- //From http://www.richelbilderbeek.nl/CppDesignPatternState.htm //--------------------------------------------------------------------------- #include <cassert> #include <iostream> //--------------------------------------------------------------------------- #include <boost/scoped_ptr.hpp> //--------------------------------------------------------------------------- #include "state.h" #include "gumballmachine.h" //--------------------------------------------------------------------------- int main() {   boost::scoped_ptr<GumballMachine> gumballMachine(     new GumballMachine(5));   //delete gumballMachine.get(); //Should not be possible   assert(gumballMachine);   std::cout << '\n' << gumballMachine << '\n';   gumballMachine->InsertQuarter();   gumballMachine->TurnCrank();   std::cout << '\n' << gumballMachine << '\n';   gumballMachine->InsertQuarter();   gumballMachine->TurnCrank();   gumballMachine->InsertQuarter();   gumballMachine->TurnCrank();   std::cout << '\n' << gumballMachine << '\n'; } //---------------------------------------------------------------------------

 

 

 

 

 

noquarterstate.cpp

 

---

//--------------------------------------------------------------------------- /* CppDesignPatternState, demonstrates the State design pattern Copyright 2011 Richel Bilderbeek This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see <http://www.gnu.org/licenses/>. */ //--------------------------------------------------------------------------- //From http://www.richelbilderbeek.nl/CppDesignPatternState.htm //--------------------------------------------------------------------------- #include <cassert> #include <iostream> //--------------------------------------------------------------------------- #include "gumballmachine.h" #include "noquarterstate.h" //--------------------------------------------------------------------------- NoQuarterState::NoQuarterState(GumballMachine * gummballMachine)   : m_gumballMachine(gummballMachine) {   assert(m_gumballMachine);   //delete m_gumballMachine; //Should not be possible } //--------------------------------------------------------------------------- void NoQuarterState::Dispense() {   std::cout << "You need to pay first\n"; } //--------------------------------------------------------------------------- void NoQuarterState::EjectQuarter() {   std::cout << "You haven\'t inserted a quarter\n"; } //--------------------------------------------------------------------------- const std::string NoQuarterState::GetDescription() const {   return "The gumballmachine has gumballs in it, but no quarter inserted"; } //--------------------------------------------------------------------------- void NoQuarterState::InsertQuarter() {   std::cout << "You inserted a quarter\n";   //Change the state   m_gumballMachine->SetState(     m_gumballMachine->GetHasQuarterState()); } //--------------------------------------------------------------------------- void NoQuarterState::TurnCrank() {   std::cout << "You turned, but there\'s no quarter\n"; } //---------------------------------------------------------------------------

 

 

 

 

 

noquarterstate.h

 

---

//--------------------------------------------------------------------------- /* CppDesignPatternState, demonstrates the State design pattern Copyright 2011 Richel Bilderbeek This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see <http://www.gnu.org/licenses/>. */ //--------------------------------------------------------------------------- //From http://www.richelbilderbeek.nl/CppDesignPatternState.htm //--------------------------------------------------------------------------- #ifndef NOQUARTERSTATE_H #define NOQUARTERSTATE_H //--------------------------------------------------------------------------- #include "state.h" //--------------------------------------------------------------------------- struct GumballMachine; //--------------------------------------------------------------------------- struct NoQuarterState : public State {   NoQuarterState(GumballMachine * gummballMachine);   void Dispense();   void EjectQuarter();   const std::string GetDescription() const;   void InsertQuarter();   void TurnCrank();   GumballMachine * const m_gumballMachine; }; //--------------------------------------------------------------------------- #endif // NOQUARTERSTATE_H

 

 

 

 

 

soldoutstate.cpp

 

---

//--------------------------------------------------------------------------- /* CppDesignPatternState, demonstrates the State design pattern Copyright 2011 Richel Bilderbeek This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see <http://www.gnu.org/licenses/>. */ //--------------------------------------------------------------------------- //From http://www.richelbilderbeek.nl/CppDesignPatternState.htm //--------------------------------------------------------------------------- #include <cassert> #include <iostream> //--------------------------------------------------------------------------- #include "gumballmachine.h" #include "soldoutstate.h" //--------------------------------------------------------------------------- SoldOutState::SoldOutState(GumballMachine * gummballMachine)   : m_gumballMachine(gummballMachine) {   assert(m_gumballMachine);   //delete m_gumballMachine; //Should not be possible } //--------------------------------------------------------------------------- void SoldOutState::Dispense() {   std::cout << "You need to pay first\n"; } //--------------------------------------------------------------------------- void SoldOutState::EjectQuarter() {   std::cout << "You haven\'t inserted a quarter\n"; } //--------------------------------------------------------------------------- const std::string SoldOutState::GetDescription() const {   return "The gumballmachine is empty and has no quarter inserted"; } //--------------------------------------------------------------------------- void SoldOutState::InsertQuarter() {   std::cout << "You inserted a quarter\n";   //Change the state   m_gumballMachine->SetState(     m_gumballMachine->GetHasQuarterState()); } //--------------------------------------------------------------------------- void SoldOutState::TurnCrank() {   std::cout << "You turned, but there\'s no quarter\n"; } //---------------------------------------------------------------------------

 

 

 

 

 

soldoutstate.h

 

---

//--------------------------------------------------------------------------- /* CppDesignPatternState, demonstrates the State design pattern Copyright 2011 Richel Bilderbeek This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see <http://www.gnu.org/licenses/>. */ //--------------------------------------------------------------------------- //From http://www.richelbilderbeek.nl/CppDesignPatternState.htm //--------------------------------------------------------------------------- #ifndef SOLDOUTSTATE_H #define SOLDOUTSTATE_H //--------------------------------------------------------------------------- #include "state.h" //--------------------------------------------------------------------------- struct GumballMachine; //--------------------------------------------------------------------------- struct SoldOutState : public State {   SoldOutState(GumballMachine * gummballMachine);   void Dispense();   void EjectQuarter();   const std::string GetDescription() const;   void InsertQuarter();   void TurnCrank();   GumballMachine * const m_gumballMachine; }; //--------------------------------------------------------------------------- #endif // SOLDOUTSTATE_H

 

 

 

 

 

soldstate.cpp

 

---

//--------------------------------------------------------------------------- /* CppDesignPatternState, demonstrates the State design pattern Copyright 2011 Richel Bilderbeek This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see <http://www.gnu.org/licenses/>. */ //--------------------------------------------------------------------------- //From http://www.richelbilderbeek.nl/CppDesignPatternState.htm //--------------------------------------------------------------------------- #include <cassert> #include <iostream> //--------------------------------------------------------------------------- #include "gumballmachine.h" #include "soldstate.h" //--------------------------------------------------------------------------- SoldState::SoldState(GumballMachine * gummballMachine)   : m_gumballMachine(gummballMachine) {   assert(m_gumballMachine);   //delete m_gumballMachine; //Should not be possible } //--------------------------------------------------------------------------- void SoldState::Dispense() {   m_gumballMachine->ReleaseBall();   if (m_gumballMachine->GetCount() > 0)   {     //Change the state     m_gumballMachine->SetState(       m_gumballMachine->GetNoQuarterState());   }   else   {     std::cout << "Oops, out of gumballs\n";     //Change the state     m_gumballMachine->SetState(       m_gumballMachine->GetSoldOutState());   } } //--------------------------------------------------------------------------- void SoldState::EjectQuarter() {   std::cout << "Sorry, you already turned the crank\n"; } //--------------------------------------------------------------------------- const std::string SoldState::GetDescription() const {   return "The gumballmachine has a gumball lying in its slot"; } //--------------------------------------------------------------------------- void SoldState::InsertQuarter() {   std::cout << "Please wait, we\'re already giving you a gumball\n"; } //--------------------------------------------------------------------------- void SoldState::TurnCrank() {   std::cout << "Turning twice doesn\'t get you another gumball\n"; } //---------------------------------------------------------------------------

 

 

 

 

 

soldstate.h

 

---

//--------------------------------------------------------------------------- /* CppDesignPatternState, demonstrates the State design pattern Copyright 2011 Richel Bilderbeek This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see <http://www.gnu.org/licenses/>. */ //--------------------------------------------------------------------------- //From http://www.richelbilderbeek.nl/CppDesignPatternState.htm //--------------------------------------------------------------------------- #ifndef SOLDSTATE_H #define SOLDSTATE_H //--------------------------------------------------------------------------- #include "state.h" //--------------------------------------------------------------------------- struct GumballMachine; //--------------------------------------------------------------------------- struct SoldState : public State {   SoldState(GumballMachine * gummballMachine);   void Dispense();   void EjectQuarter();   const std::string GetDescription() const;   void InsertQuarter();   void TurnCrank();   GumballMachine * const m_gumballMachine; }; //--------------------------------------------------------------------------- #endif // SOLDSTATE_H

 

 

 

 

 

state.h

 

---

//--------------------------------------------------------------------------- /* CppDesignPatternState, demonstrates the State design pattern Copyright 2011 Richel Bilderbeek This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see <http://www.gnu.org/licenses/>. */ //--------------------------------------------------------------------------- //From http://www.richelbilderbeek.nl/CppDesignPatternState.htm //--------------------------------------------------------------------------- #ifndef STATE_H #define STATE_H //--------------------------------------------------------------------------- #include <string> //--------------------------------------------------------------------------- #include <boost/checked_delete.hpp> //--------------------------------------------------------------------------- struct State {   //All state transitions are abstract methods   virtual void Dispense() = 0;   virtual void EjectQuarter() = 0;   virtual void InsertQuarter() = 0;   virtual void TurnCrank() = 0;   virtual const std::string GetDescription() const = 0;   protected:   virtual ~State() {}   //Do not forget the template brackets, as stated in   //Herb Sutter. Exceptional C++ style. 2005. ISBN: 0-201-76042-8. Item 8: 'Befriending templates'.   friend void boost::checked_delete<>(State* x); }; //--------------------------------------------------------------------------- #endif // STATE_H

 

 

 

 

 

References

 

1. Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides. Design Patterns. 1995. ISBN: 0201633612.
2. Eric Freeman, Elisabeth Freeman. Head First Design Patterns. 2004. ISBN: 978-0-596-00712-6.