extending classes and implementing interfaces C++ style

[interec]

I have some code in Java that I need to translate into C++. My Java
code defines a class hierarchy as follows:

// interface IA
public interface IA
{
public abstract void doA();
}

// interface IB
public interface IB extends interface IA
{
public abstract void doB();
}

// concrete class JA conforming to interface IA
public class JA implements IA
{
public void doA() { ... }
}

// concrete class JB extending from JA (so that JB knows about method
doA() ) and
// conforms to interface IB (which is derived from interface IA)
public class JB extends JB implements IB
{
public void doB() { ...}
}

How do I translate these classes into C++ (specially class JB),
without duplicating code in C++ equivalent classes of JA and JB and
while making sure that JA and JB conform to their respecting
interfaces IA and IB.

Thanks

-- Albert
Webmaster, InterEC.NET

 

[interec]

Here is what I have so far:


// interface IA
class IA
{
public:
IA() {}
virtual ~IA() {}

void doA() throw() = 0;
}

// interface IB
class IB: public IA
{
public:
IB() {}
virtual ~IB() {}

void doB() throw() = 0;
}

// concrete class JA conforming to interface IA
class JA: public IA
{
public:
JA() {}
virtual ~JA()

void doA throw() { ... }
}

// concrete class JB conforming to interface IB
class JA: public JA, public IB
{
public JB() {}
virtual ~JB()

void doB throw() { ... }
}

When I compile this, I get an error message saying that doA is
abstract in class JA????

 

[Ondra Holub]

(e-mail address removed) napsal:

Here is what I have so far:


// interface IA
class IA
{
public:
IA() {}
virtual ~IA() {}

void doA() throw() = 0;
}

// interface IB
class IB: public IA
{
public:
IB() {}
virtual ~IB() {}

void doB() throw() = 0;
}

// concrete class JA conforming to interface IA
class JA: public IA
{
public:
JA() {}
virtual ~JA()

void doA throw() { ... }
}

// concrete class JB conforming to interface IB
class JA: public JA, public IB
{
public JB() {}
virtual ~JB()

void doB throw() { ... }
}

When I compile this, I get an error message saying that doA is
abstract in class JA????

class JA: public JA, public IB is nonsense (I think it is typo) -
class cannot inherit from itself.

doA is abstract, because it is declared as void doA() throw() = 0;,
but I miss there key word virtual (in C++ are not all methods virtual
as in some other languages).

 

[Alan Johnson]

I have some code in Java that I need to translate into C++. My Java
code defines a class hierarchy as follows:

// interface IA
public interface IA
{
public abstract void doA();
}

// interface IB
public interface IB extends interface IA
{
public abstract void doB();
}

// concrete class JA conforming to interface IA
public class JA implements IA
{
public void doA() { ... }
}

// concrete class JB extending from JA (so that JB knows about method
doA() ) and
// conforms to interface IB (which is derived from interface IA)
public class JB extends JB implements IB
{
public void doB() { ...}
}

How do I translate these classes into C++ (specially class JB),
without duplicating code in C++ equivalent classes of JA and JB and
while making sure that JA and JB conform to their respecting
interfaces IA and IB.

Thanks

-- Albert
Webmaster, InterEC.NET

Assuming that last class was supposed to be:
public class JB extends JA implements IB

I think this is what you want.

class IA
{
public:
virtual ~IA() {}
virtual void doA() = 0 ;
} ;

class IB : virtual public IA
{
public:
virtual void doB() = 0 ;
} ;

class JA : virtual public IA
{
public:
virtual void doA()
{
std::cout << "JA::doA()" << std::endl ;
}
} ;

class JB : public JA, public IB
{
public:
virtual void doB()
{
std::cout << "JB::doB()" << std::endl ;
}
} ;

Some notes:
The virtual keyword is not required everywhere I used it. Once you make
a function virtual in a base class, it remains virtual in derived
classes whether you declare it as such or not.

The "virtual" in virtual inheritance doesn't mean the same thing as the
"virtual" in virtual member functions. If you don't already understand
virtual inheritance, I suggest looking it up rather than confusing
yourself by making assumptions about what it does. Also, see Dave
Rahardja's thread from earlier today "Virtual Inheritance and
interfaces" for some debate about how it applies to your situation.

The virtual destructor in IA isn't required, but it is a Good Idea.
Also, there are some good arguments for why you should have at least one
virtual function in a class that is not defined inline. Since the
destructor of IA is the only function with a definition, it might make
sense not to define it inline as I did.

 

[=?iso-8859-1?q?Erik_Wikstr=F6m?=]

Here is what I have so far:
[snip]

When I compile this, I get an error message saying that doA is
abstract in class JA????

Not in JB? I had to make a few changes to get your code through the
compiler but I never got that error, what I did get was an ambiguity
in JB if I tried to call doA(), since there's a doA() in both JA and
in IA (which JB inherits from indirectly through IB). This can be
solved with a using statement, see code below. This could probably be
solved in some other way, I'm not a big fan of large inheritance
hierarchies myself so I've never had this problem. There was a thread
just the other day about virtual public inheritance which might be
worth checking out.

// interface IA
class IA
{
public:
virtual void doA() throw() = 0;
};

// interface IB
class IB: public IA
{
public:
virtual void doB() throw() = 0;
};

// concrete class JA conforming to interface IA
class JA: public IA
{
public:
JA() { }
virtual ~JA();
void doA() throw() { }
};

// concrete class JB conforming to interface IB
class JB: public JA, public IB
{
public:
using JA::doA;

JB() { }
virtual ~JB();
void doB() throw() {doA();}
};

 

[Grizlyk]

Wrong way in C++. Do not use private inheritance in C++ without concrete
request, because it is nearly always much better to use composition here:

template< class IA>
class JA
{
private:
IA ia;
}

Here the same:

template< class IB>
class JB: public interface_JB
{
private:
IB ib;
}

 

[Grizlyk]

Maybe i was wrong with Java directives, but i hope my reasons are clear -
use composition instead of C++ multiple inheritance:

// interface IA
class IA
{
public:
virtual void doA() throw() = 0;
};

// interface IB
class IB: public IA
{
public:
virtual void doB() throw() = 0;
};

// concrete class JA conforming to interface IA
class JA: public IA
{
public:
JA() { }
virtual ~JA();
void doA() throw() { }
};

// concrete class JB conforming to interface IB
class JB: public JA, public IB

template<class T=JA>
class JB: public IB
{
T ja;

{
public:
using JA::doA;

//forwarding here
void doA() throw() {ja.doA();}

JB() { }
virtual ~JB();
void doB() throw() {doA();}

//implement here
void doB() throw();

 

[Alan Johnson]

Wrong way in C++. Do not use private inheritance in C++ without concrete
request, because it is nearly always much better to use composition here:

template< class IA>
class JA
{
private:
IA ia;
}

Where did anybody use private inheritance?

Here the same:

template< class IB>
class JB: public interface_JB
{
private:
IB ib;
}

In your example, what is interface_JB? And what is IB? I can't really
tell what point you are trying to make. Runtime polymorphism can't be
achieved with templates and composition in C++.

 

[Pete Becker]

(e-mail address removed) wrote:

class JB : public JA, public IB
{
public:
virtual void doB()
{
std::cout << "JB::doB()" << std::endl ;
}
} ;

This works for this particular example, but in general, C++ classes that
are intended to emulate Java's (misguided) interfaces should be used as
virtual bases (as in the definition of IB). So:

class JB : public JA, public virtual IB

--

-- Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com)
Author of "The Standard C++ Library Extensions: a Tutorial and
Reference." (www.petebecker.com/tr1book)

 

[Pete Becker]

template< class IB>
class JB: public interface_JB
{
private:
IB ib;
}

The goal is to be able to pass objects of the derived type to a function
like this:

void f(const IB&);

Using IB to define a member doesn't support that.

--

-- Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com)
Author of "The Standard C++ Library Extensions: a Tutorial and
Reference." (www.petebecker.com/tr1book)

 

[Grizlyk]

Where did anybody use private inheritance?

Yes, i forget, that Java directives "class JA implements IA" does not mean
"private inheritance". Look my message dated "12:45":
It means, that independent from Java directives, the Java classes can be
implemented without multiple inheritance.

Runtime polymorphism can't be achieved with templates and composition in
C++.

What do you mean as "runtime polymorphism"? The following expression

can not be treated as "runtime polymorphism" (for C++ at least), because
"class JB " will be created at compile time or am I wrong?

As I could understand, the problem is appearence of multiple inheritence for
direct Java -> C++ casting and OP wants to avoid it (take in account, I can
not see whole thread), if yes:
1. In all cases any kind (ordinary or multiple) of "inheritance of
implementation" _can_ be replaced by composition (static or dynamic).
2. In most cases any kind (ordinary or multiple) of "inheritance of
implementation" _must_ be replaced by composition (static or dynamic),
because composition (instead of inheritance) increase ability of "reusage"
of classes.

Somebody implements the Java classes on C++ like this:
Here he has used multiple inheritance.

In your example, what is interface_JB?

In the example "interface_JB" is "IB".

This is my example of static implementation with the help of composition

template<class IA=JA>
class JB: public IB
{
IA ia;
public:
//forwarding
void doA() throw() {ia.doA();}


//to implement
void doB() throw();//{ia.doA();}
};

This is my example of dynamic implementation with the help of composition

template< class IA>
class JB: public IB
{
private:
IA *ia;

public:
//forwarding
void doA() throw() {ia->doA();}


//to implement
void doB() throw();//{ia->doA();}

public:
JB( IA *p):ia(p){}
};

Note, that explicit throw specification (as "throw(expr)") is not safe in
current C++ implementation, in the following example:

void doA() throw() { throw 0; }
try{ doA(); }catch(...){}

"throw 0;" will terminate your program.

 

[Grizlyk]

The goal is to be able to pass objects of the derived type to a function
like this:

void f(const IB&);

Using IB to define a member doesn't support that.

No, I was assuming "IB" is derived from "interface_JB". Look here
(message dated "16:03")

 

[Grizlyk]

Where did anybody use private inheritance?

Here

"public JA" is really "inheritance of implementation".

 

[Dave Rahardja]

I have some code in Java that I need to translate into C++. My Java
code defines a class hierarchy as follows:

In general, Java's interface definitions can be implemented in C++ as a class
with nothing but pure virtual functions (and a public virtual destructor that
does nothing).

Java:

public interface I
{
public abstract void fn();
}

C++:

class I // Interface
{
public:
virtual void fn() = 0;
virtual ~I() {}
};

Implementing a Java interface is analogous to virtual public inheritance in
C++.

Java:

public class A implements I
{
public void fn() { /* ... */ }
}

C++:

class A: public virtual I
{
public:
virtual void fn() { /* ... */ }
};


I started a thread about the pros and cons of performing this analogous
exercise in C++ about a week ago. Do a search for my name and the topic
"Virtual inheritance and interfaces".

-dr