Designing interface class in a good way

[The Cool Giraffe]

I'm designing an ABC and in connection to that i have run
into some "huh!" and "oh...". Let me put it as a list.

1. Since the class will only contain bodies of the methods,
only the header file is needed. There will be no definitions
provided until i derive the ABC. True or false?

2. Since i'll have two different classes (both derived from
the original ABC) i'll use the following syntax in my main
class using the derivation.
AbstractClass* obj;
obj = DerivedClassA ();
Is that correct? Do i need "new" somewhere? I'm a Java
boy really so i like using "new" every now and then...

3. I have looked through the whole of the internet (almost*)
for a simple syntax example that will show how to set up
such a derivation scheme but to no avail. Any pointers?

*almost = until i got tired after about 2 hours

 

[Victor Bazarov]

I'm designing an ABC and in connection to that i have run
into some "huh!" and "oh...". Let me put it as a list.

1. Since the class will only contain bodies of the methods,
only the header file is needed. There will be no definitions
provided until i derive the ABC. True or false?

The "body of the method" is its *definition*. So, false.

2. Since i'll have two different classes (both derived from
the original ABC) i'll use the following syntax in my main
class using the derivation.
AbstractClass* obj;
obj = DerivedClassA ();

What's "DerivedClassA"?

Is that correct? Do i need "new" somewhere? I'm a Java
boy really so i like using "new" every now and then...

We C++ boys use "new" every now and then too, whether we like
it or not.

3. I have looked through the whole of the internet (almost*)
for a simple syntax example that will show how to set up
such a derivation scheme but to no avail. Any pointers?

groups.google.com.

V

 

[Jerry Coffin]

I'm designing an ABC and in connection to that i have run
into some "huh!" and "oh...". Let me put it as a list.

1. Since the class will only contain bodies of the methods,
only the header file is needed. There will be no definitions
provided until i derive the ABC. True or false?

I think you mean it will contain only declarations of the methods, not
definitions. A declaration has no body, so to speak.

This is not necessarily the case: at least as I've always used the term,
an abstract base class contains at least one pure virtual function (I.e.
which is declared but not defined). Other functions may or may not be
defined in the base class.

If you want, you can certainly create an abstract base class that's
simliar to a Java interface -- i.e. that only declares functions, but
doesn't define any of them. C++, however, doesn't _require_ anything
like this.

2. Since i'll have two different classes (both derived from
the original ABC) i'll use the following syntax in my main
class using the derivation.
AbstractClass* obj;
obj = DerivedClassA ();
Is that correct? Do i need "new" somewhere? I'm a Java
boy really so i like using "new" every now and then...

Use of new (or lack thereof) is related primarily to lifetime. If you're
creating an object that needs to live after execution exits from the
current scope, chances are you'll need to use new to create it. If you
want the object to cease to exist when execution exits from the current
scope, new is probably unnecessary and may be counterproductive.

Good use of an ABC doesn't require that when you create the object you
use a pointer (or reference) to the base class. Consider something like:

class base { /* whatever */ };
class derived : public base { /* whatever */ };

void some_func(base &b) { /* use object */ }

int main() {
derived x;
some_func(x);
return 0;
}

This allows some_func to operate any derivative of base, even though
were we create base, we're just creating an automatic object.

3. I have looked through the whole of the internet (almost*)
for a simple syntax example that will show how to set up
such a derivation scheme but to no avail. Any pointers?

Perhaps if you told us what you're really trying to accomplish we could
provide more help. Right now, it's not clear what sort of "derivation
scheme" you really want.

 

[The Cool Giraffe]

Jerry Coffin wrote/skrev/kaita/popisal/schreibt :

(e-mail address removed) says...

I think you mean it will contain only declarations of the methods, not
definitions. A declaration has no body, so to speak.

This is not necessarily the case: at least as I've always used the
term, an abstract base class contains at least one pure virtual
function (I.e. which is declared but not defined). Other functions
may or may not be defined in the base class.

If you want, you can certainly create an abstract base class that's
simliar to a Java interface -- i.e. that only declares functions, but
doesn't define any of them. C++, however, doesn't _require_ anything
like this.

So, since i only will provide the declarations of the members,
hence, only providing virtual methods - is it possible to only
provide the header file, then? Or do i have to provide a cpp
file anyway and if so what does it contain?

Perhaps if you told us what you're really trying to accomplish we
could provide more help. Right now, it's not clear what sort of
"derivation scheme" you really want.

Here i have created an example for what i think i'd like to
ask about. Please point out the errors and/or iffy spots.
The cout is working (includes to that and std:: are implicit).

// file Base.h
class Base {
public:
Base ();
virtual void doSome ();
};

// file Base.cpp
// empty or non-existing

// file DeriA.h
// empty or non-existing

// file DeriA.cpp
#include "Base.h"
class DeriA : public Base {
public:
DeriA () {}
void doSome () { cout << "Test A"; }
};

// file DeriB.h
// empty or non-existing

// file DeriB.cpp
#include "Base.h"
class DeriB : public Base {
public:
DeriB () {}
void doSome () { cout << "Test B"; }
};

// file Test.h
// empty or non-existing

// file Test.cpp
#include "Base.h"
class Test {
public:
Base base;
void assignA () { base = DeriA (); }
void assignB () { base = DeriB (); }
};

Above anything you might have to say i'd like to
explicitly ask about the following.

1. Is my not-using destructors a problem?
2. What exactly should i include in Test-class?
3. I've read that Base base; will create an object of that
class. On the other hand, it's abstract, so it won't do it. Does
it mean that i'm supposed to declare a handle base to take
care of the problem? Is it Base& base; or perhaps should i
use Base* base; instead?

 

[paul.joseph.davis]

Jerry Coffin wrote/skrev/kaita/popisal/schreibt :







So, since i only will provide the declarations of the members,
hence, only providing virtual methods - is it possible to only
provide the header file, then? Or do i have to provide a cpp
file anyway and if so what does it contain?




Here i have created an example for what i think i'd like to
ask about. Please point out the errors and/or iffy spots.
The cout is working (includes to that and std:: are implicit).

// file Base.h
class Base {
public:
Base ();
virtual void doSome ();

};

// file Base.cpp
// empty or non-existing

// file DeriA.h
// empty or non-existing

// file DeriA.cpp
#include "Base.h"
class DeriA : public Base {
public:
DeriA () {}
void doSome () { cout << "Test A"; }

};

// file DeriB.h
// empty or non-existing

// file DeriB.cpp
#include "Base.h"
class DeriB : public Base {
public:
DeriB () {}
void doSome () { cout << "Test B"; }

};

// file Test.h
// empty or non-existing

// file Test.cpp
#include "Base.h"
class Test {
public:
Base base;
void assignA () { base = DeriA (); }
void assignB () { base = DeriB (); }

};

Above anything you might have to say i'd like to
explicitly ask about the following.

1. Is my not-using destructors a problem?
2. What exactly should i include in Test-class?
3. I've read that Base base; will create an object of that
class. On the other hand, it's abstract, so it won't do it. Does
it mean that i'm supposed to declare a handle base to take
care of the problem? Is it Base& base; or perhaps should i
use Base* base; instead?

Your lack of destructors is a problem. Classes with virtual functions
should have virtual destructors. The base class can be a simple empty
function if need be.


For Base.* you either need pure virtual specifiers or an
implementation:

If you're shooting for pure abstract ( To mimick java's interface
type ) you probably don't need a constructor.

class Base {
public:
virtual Base() {}
virtual void doSome () = 0 ;
} ;

DeriA.h and DeriB.h must not be empty and contain a declaration of the
class. Ie:

DeriA.h:

class DeriA {
public:
DeriA ();
virtual ~DeriA() ;
virtual void doSome () ;
};

Test.h can be empty.

Test.cpp needs to include DeriA.h and DeriB.h to use those classes
though.

HTH,
Paul Davis

 

[throatslasher]

Your lack of destructors is a problem. Classes with virtual functions
should have virtual destructors. The base class can be a simple empty
function if need be.

Somebody makes this overgeneralization every month or two. Please
think before you write!