Question

我继承了两个类TFoamIntegrand和IBaseFunctionMultiDim，它们都是抽象类。我遇到的问题是我希望能够使用克隆成员函数，对于IBaseFunctionMultiDim来说，它是一个没有参数的纯虚函数，对于TFoamIntegrand，它不是TFoamIntegrand中的纯虚函数，而是有一个参数（带有默认值）。

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;  
    ...
}

我可以编译的唯一方法是：

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

当然，这会产生关于使用参数隐藏Clone的警告。我尝试了以下但是我得到了协变返回类型错误。

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

根据我对协变返回类型的理解，我认为上面的最后一个版本是正确的。我在这里错过了什么？

提前致谢！

编辑：这是一个完整的例子：

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());}
};

它编译。更改为注释行给出：

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

编译：

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);
      }
};

但我确实删除了代码中似乎没有相关性的内容。

C ++多重继承，虚方法覆盖问题和协变返回类型

1 个答案: