Understanding the GC

[Steven Schveighoffer]

On Fri, 01 Feb 2013 02:07:00 -0500, monarch_dodra <monarchdodra at gmail.com>  
wrote:

> On Thursday, 31 January 2013 at 23:53:26 UTC, Steven Schveighoffer wrote:
>>
>> A destructor should ONLY be used to free up resources other than GC  
>> allocated memory.  Because of that, it's generally not used.
>>
>> It should be used almost as a "last resort".
>>
>> For example, a class that holds a file descriptor should have both a  
>> destructor (which closes the descriptor) and a manual close method.   
>> The former is to clean up the file descriptor in case nobody thought to  
>> close it manually before all references were gone, and the latter is  
>> because file descriptors are not really managed by the GC, and so  
>> should be cleaned up when they are no longer used.
>>
>> This kind of gives us a paradox, since the class is managed via the GC,  
>> how do you know it's no longer used (that is, how do you know this is  
>> the last reference to it)?  That is really up to the application  
>> design.  But I wouldn't recommend relying on the GC to clean up your  
>> descriptors.
>>
>> -Steve
>
> I've actually run into this very issue: I was iterating on files,  
> opening them, and placing the descriptor in GC-allocated RAII data. I  
> can't remember if class or struct, but not a big issue. Come to think  
> about it, I think I was using "File", but allocating them because I  
> thought they were classes `auto f = new File("my file", "r")`.
>
> After running for a second, my program halts, because an exception was  
> thrown trying to open a new file:
> "Cannot open file: Too many open file handles".
>
> It was basically: Sure, the GC will destroy and close files for you...  
> if you forget... eventually...
>
> I ended up closing them in scope(exit) blocks. Problem immediately  
> solved. Or I could have stopped allocating my File's on the heap.
>
> Either way, it shows you shouldn't rely on the GC for deterministic  
> destruction.

Actually, that's a different problem.  File is a struct, and structs do  
NOT have their destructor run by the GC.  Only Objects do.

This is a GC limitation, since structs do not contain a pointer to their  
typeinfo like classes do, and there is no provision for storing a pointer  
to the typeinfo of a block.  It could be fixed by a more precise GC.   
AIUI, we have something like that coming, but I've been hearing that for  
more than a year ;)

-Steve