Open Radar

 
With the excellent addition of “defer” to Swift, this seems like the perfect opportunity to also offer an @defer directive in Objective-C.  This is actually simple enough to express currently as a macro in Objective-C, but built in compiler support would provide the powerful construct as a built in feature of the language just like Swift has.  Below I’ve outlined how one must go about doing a defer today in Objective-C (which requires a header include/import) and how making @defer a directive would be a very natural addition to the language.


// some helper declarations

#define _custom_macro_concat(a, b) a##b
#define custom_macro_concat(a, b) _custom_macro_concat(a, b)
typedef void(^custom_defer_block_t)();
NS_INLINE void custom_deferFunc(__strong custom_defer_block_t *blockRef)
{
    custom_defer_block_t actualBlock = *blockRef;
    actualBlock();
}


// the core macro

#define custom_defer(deferBlock) \
__strong custom_defer_block_t custom_macro_concat(__custom_stack_defer_block_, __LINE__) __attribute__((cleanup(custom_deferFunc), unused)) = deferBlock

// use via macro in code

#include <custom_defer.h>

- (void)foo
{
    FILE *file = fopen(…);
    custom_defer(^{
        if (file) {
            fclose(file);
        }
    });

    // continue code where any stack exit will lead to the defer being executed
}

// if we had an @defer directive instead

- (void)foo
{
    FILE *file = fopen(…);
    @defer(^{
        if (file) {
            fclose(file);
        }
    });

    // continue code where any stack exit will lead to the defer being executed
}