クラス設計の苦境

Question

私は、以下の例のような状況でクラスを設計するためのアドバイスをしたいと思います。 BおよびCオブジェクトには、数値コンポーネントがある場合とない場合があります。問題は、NumberComponentコンストラクタを選択的に呼び出す方法がないことです。これを設計するより良い方法はありますか？より良いデザインがありますが

class NumberComponent 
{ 
public: 
//Objects which don't have a number component just store a null pointer. 
//Any time we do anything here, we have to make sure p_int isn't null. 
NumberComponent() : p_int(0) { } 
NumberComponent(int x) { p_int = new int(x); } 
~NumberComponent() { delete p_int; } 

void DoSomething() { if(p_int) ++(*p_int); } 

//In real situation, this will be another complex class object. 
//Using an int* here to keep this class simple for example. 
int* p_int; 
}; 

//B objects may or may not have a number component. 
class B : public NumberComponent 
{ 
public: 
//If hasNumberComponent is false, we'd like to call the default constructor. 
//If hasNumberComponent is true, we'd like to call NumberComponent(int) constructor. 
B(int x, bool hasNumberComponent) {} 

int value; 
}; 

//C objects may or may not have a number component. 
class C : public NumberComponent 
{ 
public: 
//If hasNumberComponent is false, we'd like to call the default constructor. 
//If hasNumberComponent is true, we'd like to call NumberComponent(int) constructor. 
C(int x, bool hasNumberComponent) {} 

int value; 
}; 

int main() 
{ 
//myList contains NumberComponent* to B and C objects 
std::list<NumberComponent*> myList; 

myList.push_back(new B(5, true)); 
myList.push_back(new C(3, true)); 

for(std::list<NumberComponent*>::iterator i = myList.begin(); i != myList.end(); ++i) 
    (*i)->DoSomething(); 

//These objects don't have a Number component. 
B b(2, false); 
C c(1, false); 

return 0; 
} 

Answer 1

、あなたはこのようなあなたの当面の問題を解決することができます：悪いデザインの多くがここにあります

class NumberComponent 
{ 
public: 
NumberComponent(int x,bool hasNumberComponent) 
    : p_int(hasNumberComponent ? new int(x) : 0) { } 
~NumberComponent() { delete p_int; } 

void DoSomething() { if(p_int) ++(*p_int); } 

//In real situation, this will be another complex class object. 
//Using an int* here to keep this class simple for example. 
int* p_int; 
}; 

//B objects may or may not have a number component. 
class B : public NumberComponent 
{ 
public: 
B(int x, bool hasNumberComponent) : NumberComponent(x,hasNumberComponent) {} 

int value; 
}; 

//C objects may or may not have a number component. 
class C : public NumberComponent 
{ 
public: 
C(int x, bool hasNumberComponent) : NumberComponent(x,hasNumberComponent) {} 

int value; 
}; 

Answer 2

。あなたはそれがために発明されたもののために継承を使用するために考えた：

class B 
{ 
    public: 
     B(int x): 
     m_val(x) 
     { 
     } 

     virtual void doSomething() 
     { 
      //something 
     } 

    private: //might be protected as well 
     m_val; 
}; 

class BWithNumberComponent : public B 
{ 
    public: 
    BWithNumberComponent(int x): 
     B(x), 
     m_numberComponent(x) 
    { 
    } 

    virtual void doSomething() 
    { 
     //something else 
    } 

    private: 
    NumberComponent m_numberComponent; 
};