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Builder

Builder em C++

O Builder é um padrão de projeto criacional, que permite a construção de objetos complexos passo a passo.

Diferente de outros padrões de criação, o Builder não exige que os produtos tenham uma interface comum. Isso torna possível produzir produtos diferentes usando o mesmo processo de construção.

Uso do padrão em C++

Complexidade:

Popularidade:

Exemplos de uso: O padrão Builder é um padrão bem conhecido no mundo C++. É especialmente útil quando você precisa criar um objeto com muitas opções possíveis de configuração.

Identificação: O padrão Builder pode ser reconhecido na classe que possui um único método de criação e vários métodos para configurar o objeto resultante. Os métodos do Builder geralmente suportam encadeamento (por exemplo, algumBuilder->configValorA(1)->configValorB(2)->criar()).

Exemplo conceitual

Este exemplo ilustra a estrutura do padrão de projeto Builder. Ele se concentra em responder a estas perguntas:

  • De quais classes ele consiste?
  • Quais papéis essas classes desempenham?
  • De que maneira os elementos do padrão estão relacionados?

main.cc: Exemplo conceitual

/**
 * It makes sense to use the Builder pattern only when your products are quite
 * complex and require extensive configuration.
 *
 * Unlike in other creational patterns, different concrete builders can produce
 * unrelated products. In other words, results of various builders may not
 * always follow the same interface.
 */

class Product1{
    public:
    std::vector<std::string> parts_;
    void ListParts()const{
        std::cout << "Product parts: ";
        for (size_t i=0;i<parts_.size();i++){
            if(parts_[i]== parts_.back()){
                std::cout << parts_[i];
            }else{
                std::cout << parts_[i] << ", ";
            }
        }
        std::cout << "\n\n"; 
    }
};


/**
 * The Builder interface specifies methods for creating the different parts of
 * the Product objects.
 */
class Builder{
    public:
    virtual ~Builder(){}
    virtual void ProducePartA() const =0;
    virtual void ProducePartB() const =0;
    virtual void ProducePartC() const =0;
};
/**
 * The Concrete Builder classes follow the Builder interface and provide
 * specific implementations of the building steps. Your program may have several
 * variations of Builders, implemented differently.
 */
class ConcreteBuilder1 : public Builder{
    private:

    Product1* product;

    /**
     * A fresh builder instance should contain a blank product object, which is
     * used in further assembly.
     */
    public:

    ConcreteBuilder1(){
        this->Reset();
    }

    ~ConcreteBuilder1(){
        delete product;
    }

    void Reset(){
        this->product= new Product1();
    }
    /**
     * All production steps work with the same product instance.
     */

    void ProducePartA()const override{
        this->product->parts_.push_back("PartA1");
    }

    void ProducePartB()const override{
        this->product->parts_.push_back("PartB1");
    }

    void ProducePartC()const override{
        this->product->parts_.push_back("PartC1");
    }

    /**
     * Concrete Builders are supposed to provide their own methods for
     * retrieving results. That's because various types of builders may create
     * entirely different products that don't follow the same interface.
     * Therefore, such methods cannot be declared in the base Builder interface
     * (at least in a statically typed programming language). Note that PHP is a
     * dynamically typed language and this method CAN be in the base interface.
     * However, we won't declare it there for the sake of clarity.
     *
     * Usually, after returning the end result to the client, a builder instance
     * is expected to be ready to start producing another product. That's why
     * it's a usual practice to call the reset method at the end of the
     * `getProduct` method body. However, this behavior is not mandatory, and
     * you can make your builders wait for an explicit reset call from the
     * client code before disposing of the previous result.
     */

    /**
     * Please be careful here with the memory ownership. Once you call
     * GetProduct the user of this function is responsable to release this
     * memory. Here could be a better option to use smart pointers to avoid
     * memory leaks
     */

    Product1* GetProduct() {
        Product1* result= this->product;
        this->Reset();
        return result;
    }
};

/**
 * The Director is only responsible for executing the building steps in a
 * particular sequence. It is helpful when producing products according to a
 * specific order or configuration. Strictly speaking, the Director class is
 * optional, since the client can control builders directly.
 */
class Director{
    /**
     * @var Builder
     */
    private:
    Builder* builder;
    /**
     * The Director works with any builder instance that the client code passes
     * to it. This way, the client code may alter the final type of the newly
     * assembled product.
     */

    public:

    void set_builder(Builder* builder){
        this->builder=builder;
    }

    /**
     * The Director can construct several product variations using the same
     * building steps.
     */

    void BuildMinimalViableProduct(){
        this->builder->ProducePartA();
    }
    
    void BuildFullFeaturedProduct(){
        this->builder->ProducePartA();
        this->builder->ProducePartB();
        this->builder->ProducePartC();
    }
};
/**
 * The client code creates a builder object, passes it to the director and then
 * initiates the construction process. The end result is retrieved from the
 * builder object.
 */
/**
 * I used raw pointers for simplicity however you may prefer to use smart
 * pointers here
 */
void ClientCode(Director& director)
{
    ConcreteBuilder1* builder = new ConcreteBuilder1();
    director.set_builder(builder);
    std::cout << "Standard basic product:\n"; 
    director.BuildMinimalViableProduct();
    
    Product1* p= builder->GetProduct();
    p->ListParts();
    delete p;

    std::cout << "Standard full featured product:\n"; 
    director.BuildFullFeaturedProduct();

    p= builder->GetProduct();
    p->ListParts();
    delete p;

    // Remember, the Builder pattern can be used without a Director class.
    std::cout << "Custom product:\n";
    builder->ProducePartA();
    builder->ProducePartC();
    p=builder->GetProduct();
    p->ListParts();
    delete p;

    delete builder;
}

int main(){
    Director* director= new Director();
    ClientCode(*director);
    delete director;
    return 0;    
}

Output.txt: Resultados da execução

Standard basic product:
Product parts: PartA1

Standard full featured product:
Product parts: PartA1, PartB1, PartC1

Custom product:
Product parts: PartA1, PartC1

Builder em outras linguagens

Padrões de Projeto: Builder em Java Padrões de Projeto: Builder em C# Padrões de Projeto: Builder em PHP Padrões de Projeto: Builder em Python Padrões de Projeto: Builder em Ruby Padrões de Projeto: Builder em Swift Padrões de Projeto: Builder em TypeScript