[LLVMdev] Error prone default memory capturing convention of blocks.

Iaroslav Pavlov iaroslav.pavlov at gmail.com
Tue Oct 2 08:35:29 PDT 2012

Hi guys,

I've been using blocks for a while and found that current behavior is error prone. So I am going to propose to you the better one.


1) The __weak variables in blocks are very common pattern. So having any implicit default behavior makes thing worse.
2) Some stupid mistakes like: 

__weak typeof (self) theSelf = self;

...^ {
	theSelf.blabla = ..
	[self blabla];  // ups, calling self directly is an error
        _ivar = … // or implicit call to self.

Retain cycles are very hard to find. I am not saying about trivial ones, that compiler can find, or the Leaks tool. Recently I have 3 retain cycles in my program, no one appeared in Leaks. So the main point is prevent such errors at first place, not to rely on that tool will find it. 


Any variable captured in block must have explicit qualifier to specify memory capturing semantic: __retain, __weak, __unsafe_unretained. Qualifier is obligatory, no defaults allowed.

Code will look something like this:

NSString* theLocalVariable = …

...^ {
	__weak self;
	self.blabla  =  ...

	[self blabla:(__retain theLocalVariable)];

	self.blabla = … // compile error: missing memory capture semantic qualifier.

For __block qualifier I propose to leave old behavior (leave the default __retain semantic), because retain cycles with write-to variables are uncommon.

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