上課設計的困境

Question

我想在類似於下面的例子的情況下得到一些設計類的建議。 B和C對象可能有也可能沒有數字組件。問題是沒有辦法有選擇地調用一個NumberComponent構造函數。有沒有更好的方法來設計這個？

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

雖然有更好的設計，你可以解決你眼前的問題是這樣的：

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

有很多不好的設計在這裏。你是否考慮過使用繼承來發現它：

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; 
};