C / C++ / MFC

25-Sep-07 23:16

I have a base class with a virtual method, from this base there are multiple derived classes, but more than one needs to overide that virtual method.

class A{<br />
    virtual void OnSth();<br />
}<br />
<br />
class B : public A<br />
{<br />
    virtual void OnSth();<br />
}<br />
<br />
class C : public B<br />
{<br />
}<br />
<br />
class D : public C<br />
{<br />
    virtual void OnSth();<br />
}

In the snippet above when class A calls OnSth() I need it to call the overide in both class B and class D. Everything I have tried only results in the class B's method being called. What am I doing wrong?

Waldermort

Russell^'25-Sep-07 23:51

Russell^'

25-Sep-07 23:51

WalderMort wrote:
What am I doing wrong?

seems nothing. Smile | :)

Try to overide OnSth also in class C

void C::OnSth(){
  B::OnSth();
}

Russell

Cedric Moonen26-Sep-07 1:01

Cedric Moonen

26-Sep-07 1:01

WalderMort wrote:
In the snippet above when class A calls OnSth() I need it to call the overide in both class B and class D.

I don't really understand that part of your question. Do you mean that when something outside your class calls the OnSth function, it should be redirected to D::OnSth (standard behavior of virtual functions) ?

Because if you call OnSth from inside class A, it cannot call both B::OnSth and D::OnSth, it will only call the function in the most derived class.

Cédric Moonen
Software developer

Charting control [v1.2]

toxcct26-Sep-07 1:09

toxcct

26-Sep-07 1:09

Cedric Moonen wrote:
it will only call the function in the most derived class

yes, and that's his problem... D's method should be executed, but it's actually the method from B class that is called.

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Waldermort26-Sep-07 3:47

26-Sep-07 3:47

The problem was that the derived method in class D was not being called and instead calling that of class B.

I realise only one method may be called, but in reality I need both methods in classes B and D to be called. I suppose the only way to do this is to call B::OnSth() explicitly from class D or to add a new virtual method in class B.

The type of behaviour I'm looking for is when class A, internally, calls DoSth(), then the code should call the method in class B THEN call the method in class D. Not exactly C++ standard I know.

As for the main problem, I fixed it.

Waldermort

Roger Stoltz26-Sep-07 5:06

Roger Stoltz

26-Sep-07 5:06

Perhaps this could be a design issue...

Basic OO design gives that a child class is a specialization of the base class and inherits the behavior of the base class. It's like saying "a rectangle is a shape" and shape would be the base class of rectangle. I guess you already know this.
But the implication is that when you derive from a class you also derive its behavior, i.e. the implementation of a certain method in case it's declared as virtual.
Calling a base class implementation of a method is quite alright, it just means that you want the behavior of the base class and want to add a little something to it.
From my understanding you want to go the other way around, i.e. calling the child class implementation from the base class and this creates some problems. Not that they cannot be solved, but there's a possibility that you're trying to solve the problem using the wrong tool.

Perhaps it's the observer design pattern[^] you really need. It's the same pattern used in the doc/view architecture when the document notifies all views about a change in the document.

"It's supposed to be hard, otherwise anybody could do it!" - selfquote
"High speed never compensates for wrong direction!" - unknown

Waldermort26-Sep-07 5:19

26-Sep-07 5:19

Roger Stoltz wrote:
Perhaps it's the observer design pattern[^] you really need

That sounds like exactly what I am looking for. I have several windows, each represented by a class, that display certain aspects of a main class. When something changes in the base class, one or more windows may need to update their content.

After quick glance at a few articles relating to the topic, it seems a little difficult to implement. Guess I have some reading to do.

Thanks for pointing out this, new to me, design pattern.

Waldermort

Roger Stoltz26-Sep-07 5:29

Roger Stoltz

26-Sep-07 5:29

WalderMort wrote:
I have several windows, each represented by a class, that display certain aspects of a main class. When something changes in the base class, one or more windows may need to update their content.

Yep, that sounds like a school book situation for an observer pattern. Wink | ;)

Perhaps you won't need to do a fully fledged implementation if you don't separate the data from the presentation: you could have a static list in the top-most class containing the "observers" and when the observers need to be notified you simply traverse the list calling the virtual "notify" method of each observer.

Just a thought... :->

"It's supposed to be hard, otherwise anybody could do it!" - selfquote
"High speed never compensates for wrong direction!" - unknown

Waldermort26-Sep-07 5:37

26-Sep-07 5:37

Sounds like a suitable solution, for now anyway.

Funny thing is I started off with a similar method, but used callback methods. I had to change from this because the more code I wrote, the more complicated it became trying to manage each instance.

Waldermort

Waldermort26-Sep-07 6:28

26-Sep-07 6:28

A little further reading on the subject makes it all the more clearer.

It shouldn't be too much hassle to create a CSubject and CObserver containing only the virtual methods I need. Then it's a simple matter of deriving my existing code from these new classes, and have the observers call CSubject::HeyTellMeWhenSomethingChangesWillYa()

Waldermort

Roger Stoltz26-Sep-07 8:58

Roger Stoltz

26-Sep-07 8:58

WalderMort wrote:
It shouldn't be too much hassle to create a CSubject and CObserver containing only the virtual methods I need.

Nope.

WalderMort wrote:
Then it's a simple matter of deriving my existing code from these new classes, and have the observers call CSubject::HeyTellMeWhenSomethingChangesWillYa()

Yes, and now that you strike me as empowered I really suggest you do it this way instead of a lazy shortcut like the one I suggested earlier Blush | :O

, because in this case you would create reusable base classes that you can benefit from later. You will also get a clear understanding of the observer design pattern, which will also help you later since the pattern is quite common.

Go 'n get'em boy! Yiee-ha! Big Grin | :-D

"It's supposed to be hard, otherwise anybody could do it!" - selfquote
"High speed never compensates for wrong direction!" - unknown

Waldermort27-Sep-07 5:49

27-Sep-07 5:49

Roger Stoltz wrote:
in this case you would create reusable base classes that you can benefit from later

I have to agree there, it's always useful to create re-usable code and add it to a common folder somewhere. I have got to the point now that I am re-using so much code, that I had to create a single header file just to #include my most used snippets.

As for this Subject Observer pattern. Most of the examples/articles I found simply pass a this pointer back to the observer through a single virtual method. In my eyes this is somewhat over the top, each observer wouldn't know what type of update or even if the update is relevent, but still have to call back to the subject to check what changed.

I took a different approach, much the same as you would normaly use virtual methods. I created a single base class, complete with a list of all virtual calls a class would need. This way the method can be over-ridden directly and whatever updated data is already there.

Infact I may even write an article on it. To think of all those projects where I could have used this, but instead took the far more complicated route of multiple derived types and callbacks...

Again, thanks for introducing this.

Waldermort

Drawing a rectangle

Maynka25-Sep-07 23:12

Maynka

25-Sep-07 23:12

i have put a picture control inside a dialog box and in the picture control i have put a bitmap
Now i want to draw a rectangle(only boundary) inside this bitmap.Further on right side of rectangle i want to put numbers -1 -0.5 0 0.5 1 starting from top to bottom.

Please sugget how can i do this.
Regards

Re: Drawing a rectangle

Hamid_RT25-Sep-07 23:29

Hamid_RT

25-Sep-07 23:29

You can use of CDC::Rectangle function.Is this your question?

Re: Drawing a rectangle

Russell^'25-Sep-07 23:55

Russell^'

25-Sep-07 23:55

CDC::Rectangle(...) and CDC::DrawText(...)
Smile | :)

Russell

size of char *

baerten25-Sep-07 22:42

baerten

25-Sep-07 22:42

Hi everybody,

i have a class which stores char arrays, for example char name[50];
If i write int test = sizeof(inst->name); test has the value 50, which his OK.

Now i like to send a the char array to a function, there'in it has the form char*
and i sizeof() of this char* returns only the size of the pointer or something like this.

How can i find the number of allocated chars from this char* ?

Big thanks ! Smile | :)

Naveen25-Sep-07 22:52

Naveen

25-Sep-07 22:52

baerten wrote:
How can i find the number of allocated chars from this char*

you can use _msize() to find the size of buffer allocated in heap. I.e the function will work in the secnarios like

char *pszName = new char[50];<br />
fun( pszName );

but not for

char szName[50];<br />
fun( szName );

So to solve such pblms we usually pass the size of array also to such functions.

nave

[OpenedFileFinder]

Cedric Moonen25-Sep-07 22:59

Cedric Moonen

25-Sep-07 22:59

You should prefer strlen which will check the real size of your null-terminated string and not the size of the 'container'. So, if you copy "Hello" in your buffer, strlen will return 5 and not 50.
Is that what you are looking for ? If no, then you should maybe elaborate a little bit more.

Cédric Moonen
Software developer

Charting control [v1.2]

Waldermort25-Sep-07 23:08

25-Sep-07 23:08

short answer, you can't. When passing arrays around, it is always good practice to also send the number of items, or use one of the STL templates like std::vector

Waldermort

David Crow26-Sep-07 2:51

David Crow

26-Sep-07 2:51

Have you not ever used functions that take a buffer and the size of that buffer as arguments?

"A good athlete is the result of a good and worthy opponent." - David Crow

"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne

Nemanja Trifunovic26-Sep-07 3:37

Nemanja Trifunovic

26-Sep-07 3:37

That's called "array decay" Smile | :)

Just google for the term and you'll find the explanation.

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