介紹
將同性質的類的個體差異另外拉出來定義一個類，不同實作的此類讓引入它的原類表示出不同的類型。

以下範例取自Game Programming Pattern一書:

多個怪物子類的行為透過繼承並覆寫Monster基類來達成。但若怪物子類很多時，一個個覆寫會很麻煩，且造成繼承階層肥大，這時將這些覆寫部分拉出去各自實作再引入回去。

原本要一個個覆寫怪物的行為:

class Monster
{
public:
  virtual ~Monster() {}
  virtual const char* getAttack() = 0;

protected:
  Monster(int startingHealth)
  : health_(startingHealth)
  {}

private:
  int health_; // Current health.
};

class Dragon : public Monster
{
public:
  Dragon() : Monster(230) {}

  virtual const char* getAttack()
  {
    return "The dragon breathes fire!";
  }
};

class Troll : public Monster
{
public:
  Troll() : Monster(48) {}

  virtual const char* getAttack()
  {
    return "The troll clubs you!";
  }
};

將這些覆寫部分拉出去各自實作(範例為Breed類)，再引入回去:

class Breed
{
public:
  Breed(int health, const char* attack)
  : health_(health),
    attack_(attack)
  {}

  int getHealth() { return health_; }
  const char* getAttack() { return attack_; }

private:
  int health_; // Starting health.
  const char* attack_;
};

class Monster
{
public:
  Monster(Breed& breed)
  : health_(breed.getHealth()),
    breed_(breed)
  {}

  const char* getAttack()
  {
    return breed_.getAttack();
  }

private:
  int    health_; // Current health.
  Breed& breed_;
};

Ref:
https://gameprogrammingpatterns.com/type-object.html