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mort96 • yesterday at 9:10 AM • 2 replies • view on HN

I feel like C people, out of anyone, should respect the code gen wins of defer. Why would you rely on runtime conditional branches for everything you want cleaned up, when you can statically determine what cleanup functions need to be called?

In any case, the biggest advantage IMO is that resource acquisition and cleanup are next to each other. My brain understands the code better when I see "this is how the resource is acquired, this is how the resource will be freed later" next to each other, than when it sees "this is how this resource is acquired" on its own or "this is how the resource is freed" on its own. When writing, I can write the acquisition and the free at the same time in the same place, making me very unlikely to forget to free something.

Replies

sparkie • yesterday at 2:58 PM

Defer might be better than nothing, but it's still a poor solution. An obvious example of a better, structural solution is C#'s `using` blocks.

    using (var resource = acquire()) {

    } // implicit resource.Dispose();
While we don't have the same simplicity in C because we don't use this "disposable" pattern, we could still perhaps learn something from syntax and use a secondary block to have scoped defers. Something like:

    using (auto resource = acquire(); free(resource)) {

    } // free(resource) call inserted here.
That's no so different to how a `for` block works:

    for (auto it = 0; it < count; it++) {

    } // automatically inserts it++; it < count; and conditional branch after secondary block of for loop.
A trivial "hack" for this kind of scoped defer would be to just wrap a for loop in a macro:

    #define using(var, acquire, release) \
        auto var = (acquire); \
        for (bool var##_once = true; var##_once; var##_once = false, (release))

    using (foo, malloc(szfoo), free(foo)) {
        using (bar, malloc(szbar), free(bar)) {
            ...
        } // free(bar) gets called here.
    } // free(foo) gets called here.
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qsera • yesterday at 10:45 AM

>"this is how the resource is acquired, this is how the resource will be freed later"

Lovely fairy tale. Now can you tell me how you love to scroll back and examine all the defer blocks within a scope when it ends to understand what happens at that point?

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