[fulafel]

There's always performance hacks to be found in big apps, in exchange for implementation complexity and man-hours. And there's always a next bottleneck to be found. Performnace bugs are subject to an extended form of survivorship bias where they become bigger when the preceding perf bottlenecks get fixed.

[mehrdadn]

I've refreshed and seen three major revisions of your comment after writing a full reply to the first one, and I can't say I've been able to clearly follow where you're coming from and where you're going with any of them. I would suggest that if you feel the need for such drastic revisions, it may be worth reconsidering whether whatever you're trying to argue is really compelling.

[slededit]

His advice is sage and you should look past whatever deficiencies you find in his form of communication. Simply put there's always bigger fish to fry, and virtual methods are a quick and easy way to implement test stubs. Especially when the compiler devirtualizes.

[mehrdadn]

Well I had a response to that originally but his edits kind of changed how much sense my reply made in response. Here's what I had:

Performance is only one aspect of it. It also reduces code bloat, reducing the program's size footprint. Most tests (yes, I know, not all, but most) should not make it into the final binary users are running. I also don't see what's "hacky" about making a foo.test.cc file when I want an alternate implementation for foo.cc. It seems to be quite a positive and clear way to document the fact that an implementation is only needed for testing, and vice-versa. And not only that, but it reduces compile (& link) times, since you only need to compile one of the two implementations for each use case.

[slededit]

Yea all things being equal I'd choose your approach. But in an existing project that did things this way - it would be the last thing I'd touch. This is of course barring extreme circumstances - which profiling and other tools would identify.

Unfortunately a lot of C++ developers don't really understand how the linker works, and because of that the build system is black magic.

[mehrdadn]

Yeah, I wasn't suggesting they immediately dispatch a team for transforming everything into this. But at least from now on they could try to get people to use this pattern, and make the move gradually.

[slededit]

I want to reiterate what I think the broader point is - If this is the most pressing problem you have a very highly functioning team. There's only so much bandwidth for change and I frequently see it get spent poorly.

[fulafel]

Sorry! That's a risk you have when following comments tightly :)

I'll reconstruct my first take from memory if you have comments on it:

Current CPUs are pretty good at predicting indirect branches, and it's hard to tell beforehand which virtual calls still turn out to be perf problems - and it's wasted effort and a fallible process to attempt their compllete elimination beforehand.

[mehrdadn]

Right, and I posted my reply to that here: https://news.ycombinator.com/item?id=17504680