『阅读笔记』设计模式4

组合模式

利用一个包含当前类指针变量的基类，并让这个类指向父节点来使不同类实例之间可以互相组合成一个集合，有点像并查集，调用的时候分组合类和非组合类进行操作，代码直接看书里的，写的很好

#include <algorithm>
#include <iostream>
#include <list>
#include <string>
/**
 * The base Component class declares common operations for both simple and
 * complex objects of a composition.
 */
class Component {
  /**
   * @var Component
   */
 protected:
  Component *parent_;
  /**
   * Optionally, the base Component can declare an interface for setting and
   * accessing a parent of the component in a tree structure. It can also
   * provide some default implementation for these methods.
   */
 public:
  virtual ~Component() {}
  void SetParent(Component *parent) {
    this->parent_ = parent;
  }
  Component *GetParent() const {
    return this->parent_;
  }
  /**
   * In some cases, it would be beneficial to define the child-management
   * operations right in the base Component class. This way, you won't need to
   * expose any concrete component classes to the client code, even during the
   * object tree assembly. The downside is that these methods will be empty for
   * the leaf-level components.
   */
  virtual void Add(Component *component) {}
  virtual void Remove(Component *component) {}
  /**
   * You can provide a method that lets the client code figure out whether a
   * component can bear children.
   */
  virtual bool IsComposite() const {
    return false;
  }
  /**
   * The base Component may implement some default behavior or leave it to
   * concrete classes (by declaring the method containing the behavior as
   * "abstract").
   */
  virtual std::string Operation() const = 0;
};
/**
 * The Leaf class represents the end objects of a composition. A leaf can't have
 * any children.
 *
 * Usually, it's the Leaf objects that do the actual work, whereas Composite
 * objects only delegate to their sub-components.
 */
class Leaf : public Component {
 public:
  std::string Operation() const override {
    return "Leaf";
  }
};
/**
 * The Composite class represents the complex components that may have children.
 * Usually, the Composite objects delegate the actual work to their children and
 * then "sum-up" the result.
 */
class Composite : public Component {
  /**
   * @var \SplObjectStorage
   */
 protected:
  std::list<Component *> children_;

 public:
  /**
   * A composite object can add or remove other components (both simple or
   * complex) to or from its child list.
   */
  void Add(Component *component) override {
    this->children_.push_back(component);
    component->SetParent(this);
  }
  /**
   * Have in mind that this method removes the pointer to the list but doesn't
   * frees the
   *     memory, you should do it manually or better use smart pointers.
   */
  void Remove(Component *component) override {
    children_.remove(component);
    component->SetParent(nullptr);
  }
  bool IsComposite() const override {
    return true;
  }
  /**
   * The Composite executes its primary logic in a particular way. It traverses
   * recursively through all its children, collecting and summing their results.
   * Since the composite's children pass these calls to their children and so
   * forth, the whole object tree is traversed as a result.
   */
  std::string Operation() const override {
    std::string result;
    for (const Component *c : children_) {
      if (c == children_.back()) {
        result += c->Operation();
      } else {
        result += c->Operation() + "+";
      }
    }
    return "Branch(" + result + ")";
  }
};
/**
 * The client code works with all of the components via the base interface.
 */
void ClientCode(Component *component) {
  // ...
  std::cout << "RESULT: " << component->Operation();
  // ...
}

/**
 * Thanks to the fact that the child-management operations are declared in the
 * base Component class, the client code can work with any component, simple or
 * complex, without depending on their concrete classes.
 */
void ClientCode2(Component *component1, Component *component2) {
  // ...
  if (component1->IsComposite()) {
    component1->Add(component2);
  }
  std::cout << "RESULT: " << component1->Operation();
  // ...
}

/**
 * This way the client code can support the simple leaf components...
 */

int main() {
  Component *simple = new Leaf;
  std::cout << "Client: I've got a simple component:\n";
  ClientCode(simple);
  std::cout << "\n\n";
  /**
   * ...as well as the complex composites.
   */

  Component *tree = new Composite;
  Component *branch1 = new Composite;

  Component *leaf_1 = new Leaf;
  Component *leaf_2 = new Leaf;
  Component *leaf_3 = new Leaf;
  branch1->Add(leaf_1);
  branch1->Add(leaf_2);
  Component *branch2 = new Composite;
  branch2->Add(leaf_3);
  tree->Add(branch1);
  tree->Add(branch2);
  std::cout << "Client: Now I've got a composite tree:\n";
  ClientCode(tree);
  std::cout << "\n\n";

  std::cout << "Client: I don't need to check the components classes even when managing the tree:\n";
  ClientCode2(tree, simple);
  std::cout << "\n";

  delete simple;
  delete tree;
  delete branch1;
  delete branch2;
  delete leaf_1;
  delete leaf_2;
  delete leaf_3;

  return 0;
}

装饰模式

其实就是多继承一层，来定制类的行为，总感觉和前面学的桥接模式和适配器模式有点像呢

class base {
public:
  virtual op() const = 0;
};

class D : public base {
public:
  virtual op() const {
    std::cout << base_->op() << std::endl;
  }
protected:
  base* base_;
};

class D_A : public D {
public:
  virtual op() const {
    D::op();
    // D_A op...
  }
};

class D_B : public D {
public:
  virtual op() const {
    D::op();
    // D_B op...
  }
};