Interfaces, multiple inheritence, and ambiguous access

[John]

Hi,

I am trying to create a class heirarchy similar to the following:

// base interface
class ICar {
public: virtual void start() = 0;
};

// add members to that interface, but retain base capabilities
class IChevy : public ICar {
public: virtual void breakdown() = 0;
};

and here are the implementations:

class Car : public ICar {
public: virtual void start { // start the car }
};

class Chevy :
public IChevy, // we want to create a Chevy
public Car { // but don't want to have to re-implement car from
scratch
public: virtual void breakdown() { start(); }
};

using these class definitions, consider the following main function:

void main(int argc, char * argv)
{
ICar * car = new Car();
car->start();

ICar * car2 = new Chevy();
car2->start();
}

The compiler complains that it cannot instantiate Chevy because it is
abstract. The compiler apparently cannot pick up the implementation of
ICar::start() from the base class of Car. Fair enough. I was hoping to
avoid duplication of code, but I grit my teeth and add the following
to Chevy:

virtual void start() { Car::start(); }

I now get a complaint about the line ICar * car2 = new Chevy(). The
compiler says "ambiguous conversions from 'Chevy *' to 'ICar *'". If,
while playing around and trying to fix the problem, I delete the last
two lines of main() and replace them with the following:

void main(int argc, char * argv)
{
ICar * car = new Car();
car->start();

IChevy * chevy = new Chevy();
chevy->start();
chevy->breakdown();
}

The compiler is happy, but the linker is not. It says "inheritence
error LNK2001: unresolved external symbol "public: virtual void
__thiscall IChevy::breakdown(void)" (?breakdown@IChevy@@UAEXXZ)"".

I do not understand why the linker is failing. I am using MSVC++ 7.0
(.net).

The above is, obviously, a reduction of a broader problem I'm having
with larger classes, but contains the same fundamental design. The
bottom line is that I would like to be able to define interfaces that
inherit from each other. Each interface needs to be implemented
because concrete instances of those implementations are used. However,
I don't want to have to duplicate implementation code in sub classes
if possible. But I am hitting compiler and/or linker errors at every
turn, and hacking my original structure to make things work.

What am I failing to understand here? Is there a clean way to have a
heirarchy of interfaces and implementations such as that listed above?

Thanks in advance for the help.

John

[Gianni Mariani]

John wrote:

Hi,

I am trying to create a class heirarchy similar to the following:

// base interface
class ICar {
public: virtual void start() = 0;
};

// add members to that interface, but retain base capabilities
class IChevy : public ICar {
try:

class IChevy : public virtual ICar {
public: virtual void breakdown() = 0;
};

and here are the implementations:

class Car : public ICar {
public: virtual void start { // start the car }
};

class Chevy :
public IChevy, // we want to create a Chevy
public Car { // but don't want to have to re-implement car from
scratch

and again:

public virtual Car {
look up virtual inheritance.