[L-4]

Both defer and RAII have proven to be useful, but RAII has also proven to be quite harmful in cases, in the limit introducing a lot of hidden control flow.

I think that defer is actually limited in ways that are good - I don't see it introducing surprising control flow in the same way.

[kibwen]

Defer is also hidden control flow. At the end of every block, you need to read backwards in the entire block to see if a defer was declared in order to determine where control will jump to. Please stop pretending that defer isn't hidden control flow.

> RAII has also proven to be quite harmful in cases

The downsides of defer are much worse than the "downsides" of RAII. Defer is manual and error-prone, something that you have to remember to do every single time.

[sparkie]

Defer is a restricted form of COMEFROM with automatic labels. You COMEFROM the end of the next `defer` block in the same scope, or from the end of the function (before `return`) if there is no more `defer`. The order of execution of defer-blocks is backwards (bottom-to-top) rather than the typical top-to-bottom.

    puts("foo");
    defer { puts("bar"); }
    puts("baz");
    defer { puts("qux"); }
    puts("corge");
    return;

Will evaluate:

    puts("foo");
    puts("baz");
    puts("corge");
    puts("qux");
    puts("bar");
    return;

[vlowther]

That is the most cursed description I have seen on how defer works. Ever.

[sparkie]

This is how it would look with explicit labels and comefrom:

    puts("foo");
    before_defer0:
    comefrom after_defer1;
    puts("bar");
    after_defer0:
    comefrom before_defer0;
    puts("baz");
    before_defer1:
    comefrom before_ret;
    puts("qux");
    after_defer1:
    comefrom before_defer1;
    puts("corge");
    before_ret:
    comefrom after_defer0;
    return;

---

`defer` is obviously not implemented in this way, it will re-order the code to flow top-to-bottom and have fewer branches, but the control flow is effectively the same thing.

In theory a compiler could implement `comefrom` by re-ordering the basic blocks like `defer` does, so that the actual runtime evaluation of code is still top-to-bottom.

[fauigerzigerk]

But of course what you call "surprising" and "hidden" is also RAII's strength.

It allows library authors to take responsibility for cleaning up resources in exactly one place rather than forcing library users to insert a defer call in every single place the library is used.

[gpderetta]

RAII also composes.