Hi Tom,
> On Sep 14, 2021, at 9:53 AM, Tom Doan via Cocoa-dev
> wrote:
>
> I have a multiple platform application (Windows, Mac, GTK) that is
> primarily organized using C++ with Objective-C used for the
> Mac-specific interface. I recently switched to use ARC (as we are
> using Scintilla which recently switched to ARC). However, I am
> getting a zombied release of an NSWindow instance. So far as I
> can tell, the memory handling of this seemed to be fine pre-ARC.
> Unfortunately, because it's an NSWindow, the Instruments output
> for it has 100's of toolbox calls, so it's very hard to tell where the
> extra release is. What should I be looking for?
>
> Just to describe this, there's an EWindow C++ structure which has
> the NSWindow * as a member (under Mac OS; it's an HWND under
> Windows and Widget under GTK). It's after the delete of the
> EWindow that things go south. I'm assuming that ARC is putting in a
> release, but I haven't really seen any good description of how ARC
> interacts with C++. A release there seems fine---the question is
> where is the earlier (apparently erroneous) release.
ARC will insert code at the end of your C++ destructor to release any strong id
members.
One possible wrinkle is the default release-when-close behavior of NSWindow.
By default, when an NSWindow is `-close`d (which can only happen once in its
lifetime), AppKit sends it a `-release`. The idea is that this release
balances the initial `+alloc`.
However, this is somewhat incompatible with ARC, which doesn’t know about this
special behavior and will attempt to insert its own release to balance the
alloc.
Here is an example of a program that works perfectly fine under MRR but crashes
under ARC:
#import
NSWindow *window;
int main(void) {
@autoreleasepool {
window = [[NSWindow alloc]
initWithContentRect:NSMakeRect(0., 0., 100., 100.)
styleMask:NSWindowStyleMaskTitled backing:NSBackingStoreBuffered defer:NO];
[window makeKeyAndOrderFront:nil];
[window close];
window = nil;
}
}
You can disable this behavior by changing the `releasedWhenClosed` property of
the window to `NO`:
[window setReleasedWhenClosed:NO];
Matt
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