[fpc-devel] apple properties // Re: Help/Guidance please: Dwarf support for properties

[Jonas Maebe]

On 02/05/2024 10:56, Martin Frb via fpc-devel wrote:
> https://github.com/llvm/llvm-project/blob/main/llvm/docs/SourceLevelDebugging.rst#debugging-information-format-1
> 
>> Objective C properties exist separately from class members.  A 
>> property can be
>> defined only by "setter" and "getter" selectors, and be calculated 
>> anew on each
>> access.  Or a property can just be a direct access to some declared ivar.
> 
> I don't know if in Apple terms "ivar" has any special meaning?

ivar = instance var. Basically a regular field (as opposed to a 
class/static field), i.e. the equivalent of a "property size: longint 
read fsize write fsize;"

> Nor if there is more to "calculated anew on each access"?

I think they mean that on each access, the getter/setter gets called again.

> ---------------
> 
> I don't have background on the Apple properties.

It's not Apple, but Objective-C.

> property goes from the member to the property strikes me as odd. (first 
> example of dwarf)
> It indicates, that the property is only meant to be found when starting 
> at the member?

Objective-C also knows the concept of "computed properties", which are 
not tied to a particular field. I don't immediately see how the proposal 
deals/would deal with that.

> Also the Apple spec uses strings (names) to refer to the getter/setter.
> I don't know if there is a particular underlying need. It's more costly 
> than a reference. Also It gets problematic if the getter is in an 
> embedded record, or if there are overloaded functions, ....

It doesn't use a symbol name, but a selector name. Objective-C method 
dispatching mostly does not happen using a VMT, but by looking up a 
(hash of a) selector in a table. "Calling a method" is called "sending a 
message to" in Objective-C lingo; it's also how it can fairly 
transparently support working with remote objects.

There are various optimisations to make this less costly than a naive 
implementation when running on a single system. But that is the reason 
why it uses a string. As Objective-C is an extension of C rather than 
C++, it does not support overloading either (although due to the way its 
method names are split up when calling, it can support something 
syntactically similar).


Jonas