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usefulcat • yesterday at 4:14 PM • 1 reply • view on HN

That is a different approach, but I don't think you've demonstrated why it's better. Seems like that approach forces you to introduce a new scope for every resource, which might otherwise be unnecessary:

    using (var resource1 = acquire() {
        using (var resource2 = acquire()) {
            using (var resource3 = acquire()) {
                // use resources here..
            }
        }
    }
Compared to:

    var resource1 = acquire();
    defer { release(resource1); }
    var resource2 = acquire();
    defer { release(resource2); }
    var resource3 = acquire();
    defer { release(resource3); }
    // use resources here
Of course if you want the extra scopes (for whatever reason), you can still do that with defer, you're just not forced to.

Replies

sparkie • yesterday at 4:42 PM

While the macro version doesn't permit this, if it were built-in syntax (as in C#) we can write something like:

    using (auto res1 = acquire1(); free(res1))
    using (auto res2 = acquire2(); free(res2))
    using (auto res3 = acquire3(); free(res3)) 
    {
        // use resources here
    } 
    // free(res3); free(res2); free(res1); called in that order.
The argument for this approach is it is structural. `defer` statements are not structural control flow: They're `goto` or `comefrom` in disguise.

---

Even if we didn't want to introduce new scope, we could have something like F#'s `use`[1], which makes the resource available until the end of the scope it was introduced.

    use auto res1 = acquire1() defer { free(res1); };
    use auto res2 = acquire2() defer { free(res2); };
    use auto res3 = acquire3() defer { free(res3); };
    // use resources here
In either case (using or use-defer), the acquisition and release are coupled together in the code. With `defer` statements they're scattered as separate statements. The main argument for `defer` is to keep the acquisition and release of resources together in code, but defer statements fail at doing that.

[1]:https://learn.microsoft.com/en-us/dotnet/fsharp/language-ref...