C++多繼承，虛方法重載問題和協變返回類型

Question

我繼承了兩個類，TFoamIntegrand和IBaseFunctionMultiDim，它們都是抽象類。我遇到的問題是我希望能夠使用克隆成員函數，該函數對於IBaseFunctionMultiDim是一個沒有參數的純虛擬，對於TFoamIntegrand，它不是TFoamIntegrand中的純虛擬，而是具有參數（具有默認值）。

class TObject { 
    public: 
     virtual TObject *Clone(const char *newname="") const; 
    ... 
}; 

class TFoamIntegrand : public TObject { 
public: 
    virtual Double_t Density(Int_t ndim, Double_t *) = 0; 
    ... 
}; 

class IBaseFunctionMultiDim { 
    public: 
     /** 
     Clone a function. 
     Each derived class must implement his version of the Clone method 
     */ 
     virtual IBaseFunctionMultiDim * Clone() const = 0; 
    ... 
} 

我可以把它編譯的唯一方法就是要做到：

class InSANEDiffXSec : public TFoamIntegrand, public ROOT::Math::IBaseFunctionMultiDim { 
    public: 
     virtual ROOT::Math::IBaseFunctionMultiDim* Clone() const ; 
    ... 
} 

當然，這產生大約隱藏克隆與參數的警告。我已經嘗試了以下，但我得到協變返回類型錯誤。

virtual ROOT::Math::IBaseFunctionMultiDim* Clone(const char * newname = "") const ; 
    virtual InSANEDiffXSec* Clone(const char * newname = "") const ; 

根據我對協變返回類型的理解，我認爲上面的最後一個版本是正確的。我在這裏錯過了什麼？

在此先感謝！

編輯：這是一個完整的例子：

class A { 
    public: 
    virtual A* Clone(const char * name="") { return(new A());} 
}; 

class B { 
    public: 
    virtual B* Clone() = 0; 
}; 

class C : public A { 
    public: 
    virtual double Density(int ndim, double *) = 0; 
}; 

class D : public C, public B { 
    public: 
     virtual double Density(int ndim, double *x) { 
     return 0.0; 
     } 
     //virtual D* Clone(const char * name="") { return(new D());} 
     virtual B* Clone() { return(new D());} 
}; 

它編譯。更改爲註釋行給出：

test.h: In member function ‘virtual D* D::Clone(const char*)’: 
test.h:24:62: error: cannot allocate an object of abstract type ‘D’ 
     virtual D* Clone(const char * name="") { return(new D());} 
                  ^
test.h:19:7: note: because the following virtual functions are pure within ‘D’: 
class D : public C, public B { 
    ^
test.h:11:17: note:  virtual B* B::Clone() 
     virtual B* Clone() = 0; 
       ^
test.cxx: In function ‘int main()’: 
test.cxx:7:18: error: cannot allocate an object of abstract type ‘D’ 
    D * d = new D(); 
       ^
In file included from test.cxx:2:0: 
test.h:19:7: note: since type ‘D’ has pure virtual functions 
class D : public C, public B { 

Answer 1

這編譯：

class TObject { 
    public: 
     virtual TObject *Clone(const char *newname="") const = 0; 
}; 

class IBaseFunctionMultiDim { 
    public: 
     virtual IBaseFunctionMultiDim * Clone() const = 0; 
}; 

class InSANEDiffXSec : 
    public TObject, 
    public IBaseFunctionMultiDim 
{ 
    public: 
     virtual InSANEDiffXSec* Clone(const char* newname) const 
     { 
     return nullptr; 
     } 
     virtual InSANEDiffXSec* Clone() const 
     { 
     return Clone(nullptr); 
     } 
}; 

，但我沒有從你的代碼，這似乎並不相關刪除的東西。