That is *extremely* clear - thanks! m. On Jun 29, 2013, at 9:42 AM, Jens Alfke <j...@mooseyard.com> wrote:
> > On Jun 29, 2013, at 9:20 AM, Matt Neuburg <m...@tidbits.com> wrote: > >> I'm trying to come up with a pithy explanation, suitable for beginners, of >> why a "static" variable doesn't need a "__block" specifier in order for a >> block to assign to it. The word "captured", as you suggest, may be useful >> here, since this embodies the notion that an automatic variable is copied as >> const (and so can't be assigned to) in the context of the block. Thx - m. > > The block is going to run in its own stack frame, like a regular function. > But there’s no normal way for a function to access a local variable of > another function (the enclosing function of the block.) So enclosing > variables used by the block either have to be copied in the block’s stack > frame, or referenced through pointers back to the enclosing stack frame. The > latter is clearly more expensive, both because of the pointer dereference and > because there’s extra work that has to be done to rescue that variable if the > block can survive past the return of its caller (because the stack frame will > be gone later on.) That’s why it’s optional, and you have to use __block to > get that behavior. > > None of this is an issue with a static or global variable because it doesn’t > live on the stack or belong to some enclosing function. It’s just got a > single location that’s valid for the lifetime of the program. (Even if you > declared it inside a function: that only affects where the variable can be > referred to, not its lifetime.) > > —Jens -- matt neuburg, phd = m...@tidbits.com, http://www.apeth.net/matt/ pantes anthropoi tou eidenai oregontai phusei Programming iOS 6! http://shop.oreilly.com/product/0636920029717.do RubyFrontier! http://www.apeth.com/RubyFrontierDocs/default.html TidBITS, Mac news and reviews since 1990, http://www.tidbits.com _______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
