c++ - 派生类中需要构造函数参数的成员对象如何初始化？

Question

我想在我的 Base 类中包含一个 Composed 对象，并让不同的派生类对其进行不同的配置。但是 Composed 类需要构造函数中的参数，我无法控制它的架构。

编译器报告错误，因为 Composed 的参数应该在 Base 类的初始化时传递。但我只希望它在派生类中被初始化。这种情况怎么办？

class Base{
  public:
    Base(); //Error
  protected:
    Composed comp;
};
class Composed{
  public:
    Composed(int id):id(id);
  private:
    int id;
};
class Derived_1:public Base{
  public:
    Derived():comp(1234){};
};    
class Derived_2:public Base{
  public:
    Derived():comp(2345){};
};   

Answer 1

你必须将该参数传递给 Base，然后传递给 comp:

class Composed{
  public:
    Composed(int id):id(id);
  private:
    int id;
};
class Base{
  public:
    Base(int id):comp(id){}
  protected:
    Composed comp;
};
class Derived_1:public Base{
  public:
    Derived():Base(1234){};
};    
class Derived_2:public Base{
  public:
    Derived():Base(2345){};
};