C++ example for Builder Design Pattern

The Builder Design Pattern separates the construction of a complex object from its representation so that the same construction process can create different representations. This design pattern is not used very often in practice.

The following is an example of Builder Design Pattern:


//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
//Builder is part of Creational Patterns
//Creational Patterns deal with initializing and configuring classes and objects
//Builder Separates object construction from its representation

//We will take an example of creating Vehicles using Vehicle class.
//The VehicleBuilder is the abstract interface for creating the parts of Vehicle
//Different ConcreteBuilder classes are used to construct the final Product
//The Shop class is the Director that defines the sequence of construction
//The final product, Vehicle class shows the different type of Vehicle
//constructed and consists of different parts that can be assembles in final result

#include <iostream>
#include <string>
#include <map>

using namespace
std;

//The 'Product' class
class Vehicle
{

public
:
Vehicle(const string& vehicleType) : vehicleType_(vehicleType)
{
}


void
SetPart(const string& partName, const string& partValue)
{

parts_[partName] = partValue;
}


const
string& GetpartValue(const string& partName)
{

map<string, string>::const_iterator it;
it = parts_.find(partName);
if
(it != parts_.end())
return
it->second;
else

{

parts_[partName] = "Not found";
return
parts_[partName];
}
}


void
const Show(void)
{

cout << "\n---------------------------" << endl;
cout << "Vehicle Type : " << vehicleType_ << endl;
cout << "Frame : " << parts_["frame"] << endl;
cout << "Engine : " << parts_["engine"] << endl;
cout << "#Wheels : " << parts_["wheels"] << endl;
cout << "#Doors : " << parts_["doors"] << endl;
}


private
:
Vehicle(); //Default constructor is private so not allowed to be used
string vehicleType_;
map<string, string> parts_;
};


//Create an abstract 'Builder' class
class VehicleBuilder
{

public
:
//Default constructor wont work as pointer needs init
VehicleBuilder()
{

vehicle = NULL;
}

//Destructor made virtual
virtual ~VehicleBuilder()
{

if
(vehicle)
{

delete
vehicle;
vehicle = NULL;
}
}

const
Vehicle& getVehicle(void)
{

return
*vehicle;
}

virtual
void BuildFrame() = 0;
virtual
void BuildEngine() = 0;
virtual
void BuildWheels() = 0;
virtual
void BuildDoors() = 0;
protected
:
Vehicle* vehicle;
};


//The Concrete 'Builder' #1 class
class MotorCycleBuilder : public VehicleBuilder
{

public
:
MotorCycleBuilder()
{

vehicle = new Vehicle("MotorCycle");
}

void
BuildFrame()
{

vehicle->SetPart("frame", "MotorCycle Frame");
}

virtual
void BuildEngine()
{

vehicle->SetPart("engine", "500 cc");
}

virtual
void BuildWheels()
{

vehicle->SetPart("wheels", "2");
}

virtual
void BuildDoors()
{

vehicle->SetPart("doors", "0");
}
};


//The Concrete 'Builder' #2 class
class CarBuilder : public VehicleBuilder
{

public
:
CarBuilder()
{

vehicle = new Vehicle("Car");
}

void
BuildFrame()
{

vehicle->SetPart("frame", "Car Frame");
}

virtual
void BuildEngine()
{

vehicle->SetPart("engine", "2500 cc");
}

virtual
void BuildWheels()
{

vehicle->SetPart("wheels", "4");
}

virtual
void BuildDoors()
{

vehicle->SetPart("doors", "4");
}
};


//The Concrete 'Builder' #3 class
class ScooterBuilder : public VehicleBuilder
{

public
:
ScooterBuilder()
{

vehicle = new Vehicle("Scooter");
}

void
BuildFrame()
{

vehicle->SetPart("frame", "Scooter Frame");
}

virtual
void BuildEngine()
{

vehicle->SetPart("engine", "50 cc");
}

virtual
void BuildWheels()
{

vehicle->SetPart("wheels", "2");
}

virtual
void BuildDoors()
{

vehicle->SetPart("doors", "0");
}
};


//The 'Director' class
class Shop
{

public
:
void
Construct(VehicleBuilder* vehicleBuilder)
{

vehicleBuilder->BuildFrame();
vehicleBuilder->BuildEngine();
vehicleBuilder->BuildWheels();
vehicleBuilder->BuildDoors();
}
};


int
main()
{

VehicleBuilder *builder = NULL;

Shop shop; //New Instance of class created locally.

//Construct vehicle 1 and destroy instance when done
builder = new ScooterBuilder();
shop.Construct(builder);
(
const_cast<Vehicle&>(builder->getVehicle())).Show();
delete
builder;

//Construct vehicle 2 and destroy instance when done
builder = new CarBuilder();
shop.Construct(builder);
(
const_cast<Vehicle&>(builder->getVehicle())).Show();
delete
builder;

//Construct vehicle 3 and destroy instance when done
builder = new MotorCycleBuilder();
shop.Construct(builder);
(
const_cast<Vehicle&>(builder->getVehicle())).Show();
delete
builder;

return
0;
}


The output is as follows:


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