CODEWITHSUNDEEP
c++inheritance
whit2333
whit2333

Reputation: 53

C++ Multiple inheritance, virtual method override problems, and covariant return type

I am inheriting two classes,TFoamIntegrand, and IBaseFunctionMultiDim, which are both abstract classes. The problem I have is that I want to be able to use the Clone Member function which for IBaseFunctionMultiDim is a pure virtual with no arguments, and for TFoamIntegrand it is not a pure virtual in TFoamIntegrand but has an argument (with a default value).

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;  
    ...
}

The only way I can make it compile is to do:

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

Of course this produces the warnings about hiding the Clone with the argument. I have tried the following but I get covariant return type errors.

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

I thought that the last version above was the correct one, according to my understanding of covariant return types. What am I missing here?

Thanks in advance!

EDIT: Here is a complete example:

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());}
};

It compiles. Changing to the commented line gives:

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 {

Upvotes: 1

Views: 416

Answers (1)

Yakk - Adam Nevraumont
Yakk - Adam Nevraumont

Reputation: 275260

This compiles:

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);
      }
};

but I did remove stuff from your code that did not seem to be relevant.

Upvotes: 1

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