Gohan Gohan - 2 months ago 6
C++ Question

Inherit interfaces which share a method name

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?

Answer

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.