On: Implementing Go's defer keyword in C++

[tcbawo]

It seems that this might be for only limited cases. I'm not that familiar with Go, but I suspect that due to coroutines, the deferral may not execute for quite some time, while in C++ this could be limited to the single-threaded flow of the calling method (unless you manually move the scoped object out of the call). To match Go's behavior, you would need to attach the object lifetime to whatever future/asynch mechanism you were using. One very nice implementation (if heavyweight) is Seastar's then chaining: http://docs.seastar-project.org/master/md_doc_tutorial.html

[lucio → tcbawo]

go's "defer" is like java/net "finally" for a try/catch enclosing the entire function.

pro: you can push more than one "defer" in the same function.

con: "finally" puts the cleanup code at the end of the function. With "defer" you have it scattered at the beginning.

[nothrabannosir → lucio]

con: "finally" puts the cleanup code at the end of the function. With "defer" you have it scattered at the beginning.

Quite commonly an advantage, e.g.:

    f, err := openFile('...')
    if err != nil {
        ...
    }
    defer f.Close()

is how it's commonly used. This makes sense because the cleanup lives next to the initialization of the actual thing it's cleaning up. In a way it becomes part of the initialization, itself.

Same with locks and release. Now you don't have to scan across the entire function body to make sure your inits and cleanups match up.

[albinofrenchy → nothrabannosir]

There is a substantial issue with this pattern for things like files -- the operating system imposes a max limit on the number of file descriptors you can have open.

Defer gives no guarantees about when that thing is going to run -- just that it is after the function returns. Because of this, you can get erratic behavior if you open and defer closing a lot; essentially openFile just stops working at some point.

This can happen with any limited resource and isn't theoretical; I had a friend who ran into this in long running process with a lot of IO.

[nothrabannosir → albinofrenchy]

Defer statements run exactly at the moment of a function return (in lifo order). They are equivalent to wrapping the rest of the function body in a try / finally. They have to, since you can change the return value in a defer (by using named return values).

A "defer" statement invokes a function whose execution is deferred to the moment the surrounding function returns, either because the surrounding function executed a return statement, reached the end of its function body, or because the corresponding goroutine is panicking.

https://golang.org/ref/spec#Defer_statements

There might have been something else going on with your friend's code. Maybe the function he was using didn't return before running out of file descriptors?

[albinofrenchy → nothrabannosir]

I somehow completely misunderstood that from the Go documentation when first reading it; thank you for clarifying that.

There must have been some other leaking going on. He was using a largish library when he saw the leak and eventual crash so maybe it was due to improper pooling or C api calls? I'll have to ask him more about it.

[int_19h → albinofrenchy]

Isn't it guaranteed to run at the exact point where the function in which "defer" was used returns?

[albinofrenchy → int_19h]

I was wrong, apparently it is. Really wish there was a way to edit that comment now.

[karmakaze → lucio]

> con: "finally" puts the cleanup code at the end of the function. With "defer" you have it scattered at the beginning.

I would consider this a 'pro' as each action and defer are paired, executes in reverse order and a conditional action/defer doesn't need a second matching check.