Overriding the virtual functions with different return types raises error with private inheritance

问题

In the following code I got the following compilation errors:

1>c:\users\mittamani\desktop\06-10\over_riding_test\over_riding_test\over_riding_test.cpp(33) : error C2555: '_D1::fun': overriding virtual function return type differs and is not covariant from 'Base2::fun'
1>        c:\users\mittamani\desktop\06-10\over_riding_test\over_riding_test\over_riding_test.cpp(28) : see declaration of 'Base2::fun'
1>        'Base1' : base class is not accessible
1>c:\users\mittamani\desktop\06-10\over_riding_test\over_riding_test\over_riding_test.cpp(37) : error C2555: '_D2::fun': overriding virtual function return type differs and is not covariant from 'Base2::fun'
1>        c:\users\mittamani\desktop\06-10\over_riding_test\over_riding_test\over_riding_test.cpp(28) : see declaration of 'Base2::fun'
1>        'Base1' : base class is not accessible
1>Build log was saved at "file://c:\Users\mittamani\Desktop\06-10\Over_riding_Test\Over_riding_Test\Debug\BuildLog.htm"
1>Over_riding_Test - 2 error(s), 0 warning(s)

Here is the code:

class Base1{
public:
    Base1(){}
    virtual ~Base1(){}
};

class D1:Base1{

};

class D2:Base1{

};

class Base2{
public:
    Base2(){}
    virtual ~Base2(){}
    virtual Base1 * fun() = 0;
};

class _D1:Base2{
public:
    D1* fun(){}
};

class _D2:Base2{
public:
    D2* fun(){}
};

By the way, I am fresher to C++.. plz help..thanks in advance..

回答1:

Assuming you are trying to use covariance of return types, your attempts are valid except for the fact that the types must be using a public inheritance:

class D1:public Base1{
//       ~~~~~^

};

class D2:public Base1{
//       ~~~~~^    
};

class _D1 : Base2{
public:
    D1* fun(){} // ok now, D1 inherits publicly from Base1
};

class _D2 : Base2{
public:
    D2* fun(){} // ok now, D2 inherits publicly from Base1
};

Just like you can't cast D2* to Base1* unless you are using a public inheritance, the same applies here.

DEMO

---

Alternatively, you would have to make those classes to be friends, so that they have access to the private base class:

class _D1;
class _D2;

class D1 : Base1{
    friend class _D1;
};

class D2 : Base1{
    friend class _D2;
};

---

C++ Standard reference:

§ 10.3 Virtual functions [class.virtual]

7. The return type of an overriding function shall be either identical to the return type of the overridden function or covariant with the classes of the functions. If a function D::f overrides a function B::f, the return types of the functions are covariant if they satisfy the following criteria:

   — both are pointers to classes, both are lvalue references to classes, or both are rvalue references to classes

   — the class in the return type of B::f is the same class as the class in the return type of D::f, or is an unambiguous and accessible direct or indirect base class of the class in the return type of D::f

   — both pointers or references have the same cv-qualification and the class type in the return type of D::f has the same cv-qualification as or less cv-qualification than the class type in the return type of B::f.

回答2:

The return type of an overriding function shall be either identical to the return type of the overridden function or covariant with the classes of the functions. If a function D::f overrides a function B::f, the return types of the functions are covariant if they satisfy the following criteria:

-both are pointers to classes or references to classes
-the class in the return type of B::f is the same class as the class in the return type of D::f or, is an unambiguous direct or indirect base class of the class in the return type of D::f and is accessible in D
-both pointers or references have the same cv-qualification and the class type in the return type of D::f has the same cv-qualification as or less cv-qualification than the class type in the return type of B::f.

回答3:

You must change all the fun()s to return Base1.

来源：https://stackoverflow.com/questions/26354807/overriding-the-virtual-functions-with-different-return-types-raises-error-with-p