Implementing the ‘defer’ statement from Go in Objective-C?
- by zoul
Hello! Today I read about the defer statement in the Go language:
A defer statement pushes a function
call onto a list. The list of saved
calls is executed after the
surrounding function returns. Defer is
commonly used to simplify functions
that perform various clean-up actions.
I thought it would be fun to implement something like this in Objective-C. Do you have some idea how to do it? I thought about dispatch finalizers, autoreleased objects and C++ destructors.
Autoreleased objects:
@interface Defer : NSObject {}
+ (id) withCode: (dispatch_block_t) block;
@end
@implementation Defer
- (void) dealloc {
block();
[super dealloc];
}
@end
#define defer(__x) [Defer withCode:^{__x}]
- (void) function
{
defer(NSLog(@"Done"));
…
}
Autoreleased objects seem like the only solution that would last at least to the end of the function, as the other solutions would trigger when the current scope ends. On the other hand they could stay in the memory much longer, which would be asking for trouble.
Dispatch finalizers were my first thought, because blocks live on the stack and therefore I could easily make something execute when the stack unrolls. But after a peek in the documentation it doesn’t look like I can attach a simple “destructor” function to a block, can I?
C++ destructors are about the same thing, I would create a stack-based object with a block to be executed when the destructor runs. This would have the ugly disadvantage of turning the plain .m files into Objective-C++?
I don’t really think about using this stuff in production, I’m just interested in various solutions. Can you come up with something working, without obvious disadvantages? Both scope-based and function-based solutions would be interesting.
