专业描述 生成器模式是一种创建型设计模式， 使你能够分步骤创建复杂对象。 该模式允许你使用相同的创建代码生成不同类型和形式的对象。
生成器模式结构
生成器 （Builder） 接口声明在所有类型生成器中通用的产品构造步骤。
具体生成器 （Concrete Builders） 提供构造过程的不同实现。 具体生成器也可以构造不遵循通用接口的产品。
产品 （Products） 是最终生成的对象。 由不同生成器构造的产品无需属于同一类层次结构或接口。
主管 （Director） 类定义调用构造步骤的顺序， 这样你就可以创建和复用特定的产品配置。
客户端 （Client） 必须将某个生成器对象与主管类关联。 一般情况下， 你只需通过主管类构造函数的参数进行一次性关联即可。 此后主管类就能使用生成器对象完成后续所有的构造任务。 但在客户端将生成器对象传递给主管类制造方法时还有另一种方式。 在这种情况下， 你在使用主管类生产产品时每次都可以使用不同的生成器。
实现方法
清晰地定义通用步骤， 确保它们可以制造所有形式的产品。 否则你将无法进一步实施该模式。
在基本生成器接口中声明这些步骤。
为每个形式的产品创建具体生成器类， 并实现其构造步骤。
不要忘记实现获取构造结果对象的方法。 你不能在生成器接口中声明该方法， 因为不同生成器构造的产品可能没有公共接口， 因此你就不知道该方法返回的对象类型。 但是， 如果所有产品都位于单一类层次中， 你就可以安全地在基本接口中添加获取生成对象的方法。
考虑创建主管类。 它可以使用同一生成器对象来封装多种构造产品的方式。
客户端代码会同时创建生成器和主管对象。 构造开始前， 客户端必须将生成器对象传递给主管对象。 通常情况下， 客户端只需调用主管类构造函数一次即可。 主管类使用生成器对象完成后续所有制造任务。 还有另一种方式， 那就是客户端可以将生成器对象直接传递给主管类的制造方法。
只有在所有产品都遵循相同接口的情况下， 构造结果可以直接通过主管类获取。 否则， 客户端应当通过生成器获取构造结果。
使用示例： 当你需要创建一个可能有许多配置选项的对象时， 该模式会特别有用。
识别方法： 生成器模式可以通过类来识别， 它拥有一个构建方法和多个配置结果对象的方法。 生成器方法通常支持方法链 （例如 someBuilder->setValueA(1)->setValueB(2)->create() ）。
C++算法概念示例：
/**
* 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;
}
执行结果：
Standard basic product:
Product parts: PartA1
Standard full featured product:
Product parts: PartA1, PartB1, PartC1
Custom product:
Product parts: PartA1, PartC1