Why objective-c's block cannot capture values dynamically? - objective-c

Question

I wanna implement defer in objective-c. And here's my code:
/**
RAII : ABC->~ABC
*/
#interface DeferImpl_ : NSObject
/**
* init with a callback
*
* #return
*/
-(instancetype) initWithCallback:(void(^)())callback;
/**
* a callback
*/
#property(nonatomic, copy) void(^callback)();
#end
/**
* Defer
*
* #param X { statement; statement; ... }
*
* #return
*/
#define DEFER(X) [[DeferImpl_ alloc] initWithCallback:^X]
#define SAFE_INVOKE(x) do{if(x){(x)();}}while(0)
#implementation DeferImpl_
/**
* invoke callback
*/
-(void) dealloc {
SAFE_INVOKE(self.callback);
}
-(instancetype) initWithCallback:(void(^)())callback {
self = [super init];
self.callback = callback;
return self;
}
#end
The implementation is simple and seems easy to use.
But it's buggy!.
Here comes what I feel frustrated.
int main(void)
{
NSInteger count = 0;
DEFER({
NSLog(#"Defer: %#", #(count));
});
count = 123;
NSLog(#"Before defer block!");
return 0;
}
Log is:
2017-01-12 17:31:32.401 test[73724:18571479] Defer: 0
2017-01-12 17:31:32.402 test[73724:18571479] Before defer block!
So is there anyone can tell me why count still 0 in the block?

Answer 1

You have hidden most of your work into macros (not a good programming style!) and your are missing the basic point - DEFER calls the block immediately, before even reaching count = 123.
If you don't assign the result of [DeferImpl_ alloc] to any variable, it will get released immediately, also immediately calling the block.
Your current code is the same as writing directly:
NSInteger count = 0;
NSLog(#"Defer: %#", #(count));
count = 123;
NSLog(#"Before defer block!");
If you change your code to:
NSInteger count = 0;
id x = DEFER({
NSLog(#"Defer: %#", #(count));
});
count = 123;
NSLog(#"Before defer block!");
Your result will be:
Before defer block!
Defer: 0
Now, why the value is still zero? Because blocks capture variables by value. To capture the reference, you will have to add __block:
__block NSInteger count = 0;
id x = DEFER({
NSLog(#"Defer: %#", #(count));
});
count = 123;
NSLog(#"Before defer block!");
Before defer block!
Defer: 123
See Blocks and Variables