Re: Tutorials enhancement proposal : First Steps in GNUstep GUI Programming (2) : NSWindow, NSButton

[David Chisnall]

On 10/06/2020 10:42, Patrick Cardona via Discussion list for the GNUstep 
programming environment wrote:
> Ref : First Steps in GNUstep GUI Programming (2) : NSWindow,  NSButton
> Edition : First Edition: July 2000; Last Updated: January 2008
> URL : http://www.gnustep.it/nicola/Tutorials/WindowsAndButtons/index.html
>
> Hi Nicola (Pero),
> Hi All,
>
> I wish first to thank You, Nicola, for the great tutorials You made.
> They helped Me to understand and begin my learning pretty well.
>
> Trying to play and understand the final code in the Tutorial 'First 
> Steps in GNUstep GUI Programming (2) : NSWindow,  NSButton', I was 
> facing to some warnings and bugs. I think You should modify the final 
> code listing at page 9, in the '@implementation' section like this :
>
> (1) In the method 'dealloc' :
>
> - (void) dealloc
> {
>    RELEASE (myWindow);
>    /* According to a warning while making the app */
>    [super dealloc];
> }

Note that this -dealloc method is not required at all with ARC.

> (2) In the method 'createWindow' :
>
> - (void) createWindow
> {
> ...
> myWindow = [NSWindow alloc];
>
>    /* Don't make a assignment of 'myWindow' again...
>    myWindow = */
>    /* So I kept only the message... */
>    [myWindow initWithContentRect: rect
>                         styleMask: styleMask
>                         backing: NSBackingStoreBuffered
>                         defer: NO];
>
>   /* Here, the contentView was obviously missing : I added it... */
> [myWindow setContentView: myButton];
> ...
> }

As Richard says, the semantics of init-family methods mean that this 
doesn't do what you think it does.

An init-family method consumes the receiver and returns a new owning 
reference to the return value.  In the common case, the the receiver 
will be the same as the return value and so this cancels out.  In the 
case of things like NSWindow, it is quite likely that they are 
different.  NSWindow +alloc is likely to return a singleton placeholder 
and do the real allocation when you call one of the init-family methods 
that

That is why it is considered good style to never assign the result of 
alloc to a variable and always chain the call as [[SomeClass alloc] 
initWithSomeInitFamilyMethod: ...].  For more information about method 
families, see this:

https://www.informit.com/articles/article.aspx?p=1745875&seqNum=4

Note that without ARC, your version may generate memory management 
errors.  With ARC, it will just have surprising behaviour: the newly 
created window created by the init-family method will be deallocated 
immediately and the value assigned to myWindow will point to the 
placeholder.

> I Hope this could be helpful for other Beginners like Me. ;-)

I would *strongly* recommend any beginner use ARC from the outset. 
Thinking in terms of ownership and consume operations is a far better 
memory model than thinking in terms of reference count manipulation.

ARC also makes memory management work cleanly in Objective-C++, so 
things like std::vector<NSMutableString*> work correctly (and will 
typically be faster than NSMutableArray<NSMutableString*>).

David