Allocators have been in wide use long before the 2020s but I agree there does seem to be a resurgent interest lately. Although I would argue it's part of a more broad trend of focusing data driven design. Which makes sense because accessing main memory is one of the slowest things your program can do.
They do, but not necessarily together with generic, standard containers.
When you find yourself wanting a nonstandard allocator, you usually want it because you want it to have some interesting property. It's not necessarily trivial to fit that into the interface of something like `std::vector`, or `std::unordered_map`, etc.
Here's my take as a game developer: 99% of use cases for custom allocators are scratch allocators for doing stuff within a frame. 95% of those are much easier to serve by just amortizing allocations by storing things in an `std::vector` (or equivalent) that gets cleared every frame. You can use linear storage to back many interesting data structures, including queues, ring buffers, priority queues, binary heaps, etc., and that's more than enough for a large number of systems in a game.
The overwhelming majority of the time, more complex data structures (like hash maps etc.) have a longer lifetime than the current frame, because the whole point of using them in the first place is to amortize lookup time across frames.
The important part is a language where the standard library isn't special, and Rust has this property, too. So in domains where things like per-frame allocators are useful, you can still have them. That capability just isn't cluttering up the more common path where that isn't useful.
I haven't used the relatively new C++17 polymorphic_allocator, though, maybe it fixes this.
What does that mean?
Zig doesn't force or even tends to prefer one way or another. If I want unassuming heap allocations that can be reclaimed in any order, there's an allocator for that. If I want an arena to discard at the end of something like a request or a video frame, there's an allocator for that. If I want to use a fixed backing buffer for the allocations, there's an allocator for that.
The point is that the standard library doesn't assume one or the other, which seems good if the problem is that "real world software just isn't that simple" in the more general sense that there's no one-size-fits-all allocation strategy.
The automatic solutions are usually pretty good and usually the right place to start. But if performance is a priority, you want options.
BTW, “per-frame arena” is part of a general pattern of a repeated interval of work doing significant allocation. This is really common in software of all kinds… servers that process requests (like web servers and database servers) and typical command line tools.
I haven’t tried to replicate this for myself. And, even assuming for the sake of argument that it was definitely true back then, a lot can happen in 20 years. But still, it does speak to wanting options when performance really is critical.
You can generalize this far beyond per-frame semantics. Think per-http-request, per-pubsub-message.
Per each, you can create a new virtual.Arena, set it as your context.temp_allocator and use it for the entirety of the request or message. Afterwards, throw it away.
Sometimes using a global one isn't the best thing (it's often a good idea to specialise base on allocation size, reuse and lifetimes), but I've used them quite a bit in C++ over the past 16 years doing HPC for graphics, rendering and simulation, so calling them only useful for "toy problems" likely shows you just haven't found a need for them in what you've been doing.
this goes back decades, its not just a feature of the 2020s. as a systems programmer I always want this, and the idea that allocator state should be completely hidden and implicit is shortsighted.
but yes, it is a bit onerous to pass around allocator(s). the real complaint that I have is that if 'malloc' is global and a compiler primitive, then we can do things like coalesce allocations and have compiler managed lifetimes when appropriate.
explicit allocators are an important lever, its not clear to me that we could never find a way to make them possible without the minor downsides.