I really, really do not like what that could do to the readability of code if it was used liberally.
It's like GOTOs, but worse, because it's not as visible.
C++'s destructors feel like a better/more explicit way to handle these sorts of problems.
It's like GOTOs, but worse, because it's not as visible.
C++'s destructors feel like a better/more explicit way to handle these sorts of problems.
Exactly. Both C++ RAII (constructors/destructors) and C23 defer are awful. They make the behavior implicit; in effect they tie an indefinitely large (and growing) baggage to scope termination without the source code reflecting that baggage.
Cascading gotos (or the arrow pattern) are much better. Cleanup code execution is clearly reflected by location in the source code; the exit path can be easily stepped through in a debugger.
C++ destructors are great for this, but are not possible in C. Destructors require an object model that C does not have.
> It's like GOTOs, but worse, because it's not as visible.
> C++'s destructors feel like a better/more explicit way to handle these sorts of problems.
But what C++ gives you is the same thing:
> It's like GOTOs, but worse, because it's not as visible.
!
The whole point of syntactic sugar is for the machinery to be hidden, and generated assembly will generally look like goto spaghetti even when your code doesn't.
What this implementation of defer does under the covers is not interesting unless you're trying to make it portable (e.g., to older MSVC versions that don't even support C99 let alone C23 or GCC local function extensions) or efficient (if this one isn't).
And when the machinery fails, you'll not only have the machinery to debug, but the syntactic sugar too.
If we only need permanent sub-objects, then we set those up gradually, and build an error path in reverse order (with gotos or with the arrow pattern); upon success, the sub-objects' ownership is transferred to the new super-object, and the new super-object is returned, just before the error path is reached. Otherwise, the suffix of the error path that corresponds to the successful prefix of the construction path is executed (rollback). This approach cannot be rewritten with "defer" usefully. Normally you'd defer the rollback step for a sub-object immediately after its successful construction step, but this rollback step (= all rollback steps) will run upon successful exit too. So you need a separate flag for neutering all the deferred actions (all deferred actions will have to check the flag).
If we only need temporaries (and no permanent sub-objects), then (first without defer, again) we build the same code structure (using cascading gotos or the arrow pattern), but upon success, we don't return out of the middle of the function; instead, we store the new object outwards, and fall through to the rollback path. IOW, the rollback steps are used (and needed) for the successfully constructed temporaries regardless of function success or failure. This can be directly expressed with "defer"s. The problem is of course that the actual rollback execution order will not be visible in the source code; the compiler will organize it for you. I dislike that.
If we need both temporaries and permanent sub-objects, then we need the same method as with temporaries, except the rollback steps of the permanent sub-objects need to be restricted to the failure case of the function. This means that with either the cascading gotos or the arrow pattern, some teardown steps will be protected with "if"s, dependent on function return value. Not great, but still quite readable. With defer, you'll get a mix of deferred actions some of which are gated with ifs, and some others of which aren't. I find that terrible.
There are other options, but none of them is better, IMO. You can use nested functions:
char * p = malloc(10);
if(p!=0) {
doTheRealWork(p);
};
free(p);
You can also (more readable than that first alternative, IMO) wrap code in a single-iteration for or while loop and then use break to exit it:
char *p = null;
for(int i=0;i==0;i=1) {
p = malloc(10);
if(p==0) break;
…
}
free(p);