Cocoa Internals/Memory Management
All Objective-C objects are reference counted objects. Once the count reaches zero - the object is freed.
The modern Objective-C (or Swift) recommends (strongly, fiercely) to use Automatic-Reference-Counting (ARC) support. FPC support for Objective-C was started way before ARC became highly recommended and was part of the language. Right now Objective-C 2.0 syntax provides the language level support for ARC, while FPC doesn't have those (yet?). All Cocoa Widgetset code cannot doesn't rely on ARC and using reference and dereference manually.
Memory Management
As mentioned earlier - all Cocoa objects are reference counted objects. They should be allocated via a class constructor (typically alloc()) and be released via release() method (rather than calling a "destructor" dealloc).
Consider the following code example:
A new sub-class of NSObject is declared, overriding its alloc (constructor) and dealloc (destructor) methods. Overridden methods are providing a logging output.
program project1;
{$mode delphi}{$H+}
{$modeswitch objectivec1}
{$modeswitch objectivec2}
uses CocoaAll;
type
{ NSMyObject }
NSMyObject = objcclass(NSObject)
public
class function alloc: id; override;
procedure dealloc; override;
end;
{ NSMyObject }
class function NSMyObject.alloc: id;
begin
Result:=inherited alloc;
writeln('alloc: ', PtrUInt(Result));
end;
procedure NSMyObject.dealloc;
begin
writeln('dealloc: ', PtrUInt(Self));
inherited dealloc;
end;
procedure MakeObject;
var
obj : NSMyObject;
begin
obj:=NSMyObject.alloc.init;
obj.release;
end;
begin
MakeObject;
MakeObject;
MakeObject;
end.
The output of calling such process would be:
alloc: 3206800 dealloc: 3206800 alloc: 3206800 dealloc: 3206800 alloc: 3206800 dealloc: 3206800
This is happening, because whenever an object is allocated, it's reference count is set to 1. Calling release method reduced the reference counter by 1. If reference count becomes zero, objc runtime - is calling objects destructor - dealloc.
Leaking
If MakeObject procedure is modified as following:
procedure MakeObject;
var
obj : NSMyObject;
begin
obj:=NSMyObject.alloc.init;
// removed the line with .release call
end;
the output of the application changes to:
alloc: 2146560 alloc: 2146288 alloc: 2146304
That looks like a memory leak via ObjC objects, because allocated objects were not released.
ObjC Object Leaks
Free Pascal debugging unit heaptrc is only able to track leaking of memory allocated via Pascal Run-time memory management. It's not able to track allocations made via C-runtime/Obj-C memory manager.
Apple provides an external tools to keep tracking of ObjC allocations.
Xcode Instruments
One of the
See Also
- Cocoa Internals
- Advanced Memory Management - at Apple Developer