[tialaramex]

> Do you really want an error returned from push_back?

For most people, no, you definitely want it to just work or explode, which is indeed what happens in normal Rust, and, not coincidentally, the actual effect when this exception happens in your typical C++ application after it is done with all the unwinding and discovers there is no handler (or that the handler was never tested and doesn't actually somehow cope).

But, sometimes that is what you wanted, and Linus has been very clear it's what he wants in the kernel he created.

For such purposes Rust has Vec::try_reserve() and Vec::push_within_capacity() which let us express the idea that we'd like more room and to know if that wasn't possible, and also if there was no room left for the thing we pushed we want back the thing we were trying to push - which otherwise we don't have any more.

There is no analogous C++ API, std::vector just throws an exception and good luck to you AFAIK.

[dataflow]

> For such purposes Rust has Vec::try_reserve() and Vec::push_within_capacity() [...] There is no analogous C++ API, std::vector just throws an exception and good luck to you AFAIK.

https://godbolt.org/z/6xE6jr3zr ?

[tialaramex]

I guess this is an attempt at Vec::push_within_capacity ? Your function takes a reference and then tries to copy the referenced object into the growable array. But of course nobody said this object can be copied - after all we want it back if it won't fit so perhaps it's unique or expensive to make.

[dataflow]

> I guess this is an attempt at Vec::push_within_capacity?

Sure, yes. It's trivial to change to try_reserve if that's what you want. (There are other solutions for that as well, but they're more complicated and better for other situations.)

> Your function takes a reference and then tries to copy the referenced object into the growable array. But of course nobody said this object can be copied - after all we want it back if it won't fit so perhaps it's unique or expensive to make

Just add extend it to allow moves then? It's pretty trivial. (Are you familiar with move semantics in C++?)

[tialaramex]

But how? I did attempt this before I replied, but of course after not long I had inexplicable segfaults and we're not in a thread about those problems with C++

I can't see how to make that work, but I also can't say for sure it's impossible all I can tell you is that I was genuinely trying and all I got for my trouble was a segfault that I don't understand and couldn't fix.

Edited to add: In case it helps the signature we want is:

    pub fn push_within_capacity(&mut self, value: T) -> Result<(), T>

If you're not really a Rust person, this takes a value T, not a reference, not a magic ultra-hyper-reference, nor a pointer, it's taking the value T, the value is gone now, which just isn't a thing in C++, then it's returning either Ok(()) which signifies that this worked, or Err(T) thus giving back the T because we couldn't push it.

[dataflow]

I'm sorry I don't think I understand the problem you're trying to illustrate. I'm not sure why you're emphasizing value vs. reference, but even if that's what you want, this works just fine: https://godbolt.org/z/P8EGPYWW5

[tialaramex]

Well the good news is that now I realise the biggest problem in my previous attempt was that I forgot C++ types which can't be copy constructed also by default can't be moved, so I'd actually made it impossible to use my example type. I still don't know why I had segfaults, but I don't care now.

I agree that your new code does roughly what you'd do in C++ if you wanted this, but you get to the same place as before -- if for example you try commenting out your allocation failure boolean, the code just blows up now.

There are lots of APIs like this which make sense in Rust but not in C++ because if you write them in Rust the programmer is going to handle edge cases properly, but in C++ the programmer just ignores the edge cases so why bother.