Builder pattern

When would you use the Builder Pattern?

原文的这个回答非常好，下面是java版的代码

public class Pizza {
  private int size;
  private boolean cheese;
  private boolean pepperoni;
  private boolean bacon;

  public static class Builder {
    //required
    private final int size;

    //optional
    private boolean cheese = false;
    private boolean pepperoni = false;
    private boolean bacon = false;

    public Builder(int size) {
      this.size = size;
    }

    public Builder cheese(boolean value) {
      cheese = value;
      return this;
    }

    public Builder pepperoni(boolean value) {
      pepperoni = value;
      return this;
    }

    public Builder bacon(boolean value) {
      bacon = value;
      return this;
    }

    public Pizza build() {
      return new Pizza(this);
    }
  }

  private Pizza(Builder builder) {
    size = builder.size;
    cheese = builder.cheese;
    pepperoni = builder.pepperoni;
    bacon = builder.bacon;
  }
}

Pizza pizza = new Pizza.Builder(12)
                       .cheese(true)
                       .pepperoni(true)
                       .bacon(true)
                       .build();

c++版

class Pizza;

class Pizza
{
    int size;
    bool cheese;
    bool pepperoni;
    bool bacon;
public:
    class Builder
    {
        //required
        int size_;

        //optional
        bool cheese_ { false };
        bool pepperoni_ { false };
        bool bacon_ { false };
        public:
        friend class Pizza;
        Builder(int value)
        {
            size_ = value;
        }

        Builder& cheese(bool value)
        {
            cheese_ = value;
            return *this;
        }

        Builder& pepperoni(bool value)
        {
            pepperoni_ = value;
            return *this;
        }

        Builder& bacon(bool value)
        {
            bacon_ = value;
            return *this;
        }

        Pizza* build()
        {
            return new Pizza(*this);
        }
    };
private:
    Pizza(Builder& builder)
    {
        size = builder.size_;
        cheese = builder.cheese_;
        pepperoni = builder.pepperoni_;
        bacon = builder.bacon_;
    }
};

int main()
{
    Pizza* pizza = Pizza::Builder(12).cheese(true).pepperoni(true).bacon(true).build();
}

oodesign Builder Pattern

维基百科Builder pattern

The Builder is a design pattern designed to provide a flexible solution to various object creation problems in object-oriented programming. The intent of the Builder design pattern is to separate the construction of a complex object from its representation. It is one of the Gang of Four design patterns.