從基類實例化派生對象

Question

試圖創建一個驅動程序類型類，其中，下面的Base是在實例化時傳遞類型的驅動程序。在這種情況下，類型2用於構造正確的派生對象。

我的編譯器在「Class Base」行上拋出了一個Declaration語法錯誤。

我的最終目標是能夠做到這一點：

Base *B; 

    B = new Base(2); 
    if(B) 
    { 
     B->DoStuff(); 
     B->DoMoreStuff(); 
     delete B; 
    } 

這裏是我的代碼將無法編譯...

class Base 
{ 
public: 

    Base(int h); 
    virtual ~Base(); 

private: 
    int hType; 
    Base *hHandle; 
}; 


class Derived1 : public Base 
{ 
public: 
    Derived1(); 
    virtual ~Derived1(); 

}; 

class Derived2 : public Base 
{ 
public: 
    Derived2(); 
    virtual ~Derived2(); 

}; 

Base::Base(int h) 
{ 
    hType = h; 

    switch(h) 
    { 
     case 1: 
      hHandle = new Derived1; 
     break; 

     case 2: 
      hHandle = new Derived2; 
     break; 

    } 
} 


Derived1::Derived1():Base(1) 
{ 
    printf("\nDerived1 Initialized\n\n"); 
} 

Derived2::Derived2():Base(2) 
{ 
    printf("\nDerived2 Initialized\n\n"); 
} 

下面是更新後的代碼，以顯示完整的源代碼。我想我現在明白爲什麼它不會編譯。正如下面指出的，我有一個無限循環呼叫'新'

#include <stdio.h> 

class Base 
{ 
public: 

    Base(); 
    Base(int h); 
    Create (int h); 
    virtual ~Base(); 

private: 
    int hType; 
    Base *hHandle; 
}; 


class Derived1 : public Base 
{ 
public: 
    Derived1(); 
    virtual ~Derived1(); 

}; 

class Derived2 : public Base 
{ 
public: 
    Derived2(); 
    virtual ~Derived2(); 

}; 

Base::Base() 
{ 

} 

Base::Base(int h) 
{ 
    Create(h); 
} 

Base::Create(int h) 
{ 
    hType = h; 

    switch(h) 
    { 
     case 1: 
      hHandle = new Derived1; 
     break; 

     case 2: 
      hHandle = new Derived2; 
     break; 

    } 
} 

Derived1::Derived1() 
{ 
    printf("\nDerived1 Initialized\n\n"); 
} 

Derived2::Derived2() 
{ 
    printf("\nDerived2 Initialized\n\n"); 
} 

Answer 1

它看起來像你試圖做class factory。

我建議你在Base中有一個返回Derived1或Derived2的靜態方法。

class Base 
{ 
public: 
    static Base* Create(int); 
    virtual void DoStuff() = 0; 
} 

class Derived1 : public Base 
{ 
    Derived1() 
    { 
     printf("\nDerived1 Initialized\n\n"); 
    } 

    virtual void DoStuff() 
    { 
    } 
} 

class Derived2 : public Base 
{ 
    Derived2() 
    { 
     printf("\nDerived2 Initialized\n\n"); 
    } 

    virtual void DoStuff() 
    { 
    } 
} 

Base* Base::Create(int n) 
{ 
    if (n==1) 
     return new Derived1(); 
    else if (n==2) 
     return new Derived2(); 
    else 
     return nullptr; 
} 

void main() 
{ 
    Base* B = Base::Create(2); 
    if(B) 
    { 
     B->DoStuff(); 
     delete B; 
    } 
}