C++ Builder Design Pattern

Builder design pattern in useful when you want to build a complex object. Intent of this pattern is to separate the construction of object from its representation. In this article, we will explore builder design pattern with a simple implementation in C++.

Builder Design Pattern

• Builder pattern in useful when you want to build a complex object. Intent of this pattern is to separate the construction of object from its representation.
• Abstract the construction of object and then derived concrete implementations will give respective construction parts.
• The director makes sure that the product (complex object) is properly created using the builder interface and the parts are built with correct order.

Builder design pattern implementation

Below example shows an implementation with a multi-cuisine cook cooking different meals. Treat Meal as a complete product, in that sense well known examples of pizza or maze make better products (look more tightly coupled product than the meal used here).

#include <iostream>
#include <string>
#include <queue>

using namespace std;

// Product
class Meal {
public:
    Meal() {}
    ~Meal() {}

    void setMealItem(string mealItem) { mMeal.push(mealItem);}
    void serveMeal() {
        int i = 1;
        while(!mMeal.empty()) {
            cout << " Serve item " << i++ << ":" << mMeal.front() << endl ;
            mMeal.pop();
          }
    }

private:
    queue <string> mMeal;
};

// Builder
class MealBuilder {
public:
    MealBuilder() {}
    ~MealBuilder() {}

    const Meal& getMeal() { return mMeal; }
    virtual void buildStarter() = 0;
    virtual void buildMainCourse() = 0;
    virtual void buildDessert() = 0;

protected:
    Meal mMeal;
};

// Director: Makes sure the right sequence of food is prepared and served.
class MultiCuisineCook {
public:
    MultiCuisineCook():mMealBuilder(NULL) {}
    ~MultiCuisineCook() { if (mMealBuilder) delete mMealBuilder;}

    void setMealBuilder(MealBuilder *mealBuilder) {
        if (mMealBuilder) delete mMealBuilder;
        mMealBuilder = mealBuilder;
    }

    const Meal& getMeal() { return mMealBuilder->getMeal(); }

    void  createMeal() {
        mMealBuilder->buildStarter();
        mMealBuilder->buildMainCourse();
        mMealBuilder->buildDessert();
    }

private:
    MealBuilder *mMealBuilder;

};

// Concrete Meal Builder 1
class IndianMealBuilder : public MealBuilder {
public:
    IndianMealBuilder() {}
    ~IndianMealBuilder() {}

    void buildStarter() { mMeal.setMealItem("FriedOnion");}
    void buildMainCourse() { mMeal.setMealItem("CheeseCurry");}
    void buildDessert() { mMeal.setMealItem("SweetBalls");}

};

// Concrete Meal Builder 2
class ChineeseMealBuilder : public MealBuilder {
public:
    ChineeseMealBuilder() {}
    ~ChineeseMealBuilder() {}

    void buildStarter() { mMeal.setMealItem("Manchurian");}
    void buildMainCourse() { mMeal.setMealItem("FriedNoodles");}
    void buildDessert() { mMeal.setMealItem("MangoPudding");}

};

// Concrete Meal Builder 3
class MexicanMealBuilder : public MealBuilder {
public:
    MexicanMealBuilder() {}
    ~MexicanMealBuilder() {}

    void buildStarter() { mMeal.setMealItem("ChipsNSalsa");}
    void buildMainCourse() { mMeal.setMealItem("RiceTacoBeans");}
    void buildDessert() { mMeal.setMealItem("FriedIcecream");}

};

int main()
{
    MultiCuisineCook cook;

    cout << "Build a Chineese Meal!" << endl;
    cook.setMealBuilder(new ChineeseMealBuilder());
    cook.createMeal();

    Meal chineeseMeal = cook.getMeal();
    chineeseMeal.serveMeal();

    cout << "Build a Mexican Meal!" << endl;
    cook.setMealBuilder(new MexicanMealBuilder());
    cook.createMeal();

    Meal mexicanMeal = cook.getMeal();
    mexicanMeal.serveMeal();

    return 0;
}

Output:-

Build a Chineese Meal! 
 Serve item 1:Manchurian 
 Serve item 2:FriedNoodles 
 Serve item 3:MangoPudding 
Build a Mexican Meal! 
 Serve item 1:ChipsNSalsa 
 Serve item 2:RiceTacoBeans 
 Serve item 3:FriedIcecream