How to override a virtual member function which has the same name in two base classes
c++, gcc, multiple-inheritance, visual-c++
Solution
This problem doesn't come up very often. The solution I'm familiar with was designed by Doug McIlroy and appears in Bjarne Stroustrup's books (presented in both Design & Evolution of C++ section 12.8 and The C++ Programming Language section 25.6). According to the discussion in Design & Evolution, there was a proposal to handle this specific case elegantly, but it was rejected because "such name clashes were unlikely to become common enough to warrant a separate language feature," and "not likely to become everyday work for novices."
Not only do you need to call `Name()` through pointers to base classes, you need a way to say which `Name()` you want when operating on the derived class. The solution adds some indirection:
class Interface1{
public:
virtual void Name() = 0;
};
class Interface2{
public:
virtual void Name() = 0;
};
class Interface1_helper : public Interface1{
public:
virtual void I1_Name() = 0;
void Name() override
{
I1_Name();
}
};
class Interface2_helper : public Interface2{
public:
virtual void I2_Name() = 0;
void Name() override
{
I2_Name();
}
};
class RealClass: public Interface1_helper, public Interface2_helper{
public:
void I1_Name() override
{
printf("Interface1 OK?\n");
}
void I2_Name() override
{
printf("Interface2 OK?\n");
}
};
int main()
{
RealClass rc;
Interface1* i1 = &rc;
Interface2* i2 = &rc;
i1->Name();
i2->Name();
rc.I1_Name();
rc.I2_Name();
}
Not pretty, but the decision was it's not needed often.
Problem
There are two base classes have same function name. I want to inherit both of them, and over ride each method differently. How can I do that with separate declaration and definition (instead of defining in the class definition)? ``` #include <cstdio> class Interface1{ public: virtual void Name() = 0; }; class Interface2 { public: virtual void Name() = 0; }; class RealClass: public Interface1, public Interface2 { public: virtual void Interface1::Name() { printf("Interface1 OK?\n"); } virtual void Interface2::Name() { printf("Interface2 OK?\n"); } }; int main() { Interface1 *p = new RealClass(); p->Name(); Interface2 *q = reinterpret_cast<RealClass*>(p); q->Name(); } ``` I failed to move the definition out in VC8. I found the Microsoft Specific Keyword __interface can do this job successfully, code below: ``` #include <cstdio> __interface Interface1{ virtual void Name() = 0; }; __interface Interface2 { virtual void Name() = 0; }; class RealClass: public Interface1, public Interface2 { public: virtual void Interface1::Name(); virtual void Interface2::Name(); }; void RealClass::Interface1::Name() { printf("Interface1 OK?\n"); } void RealClass::Interface2::Name() { printf("Interface2 OK?\n"); } int main() { Interface1 *p = new RealClass(); p->Name(); Interface2 *q = reinterpret_cast<RealClass*>(p); q->Name(); } ``` but is there another way to do this something more general that will work in other compilers?