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Notes on splatter/varargs and CRTP

static_polymorphism
Bradlee Speice 2020-09-03 00:58:45 -04:00
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@ -682,16 +682,37 @@ mostly please just use concepts.
# Templated splatter
Rust can't handle arbitrary numbers of template parameters. Can use macros, but I should investigate
`typename...` types.
Rust can't handle arbitrary numbers of template parameters. Macros can (see `vec![]`), and you could
use a macro to define the implementation of free functions, but methods need to know exactly what
the arguments are. Also, no concept of SFINAE or type inspection in Rust macros.
Common pattern to implement
Good example of how to demonstrate Rust not being able to use splatter templates: can't emplace back
on a vector. Have to construct and move. In general, don't think it's possible to "construct" into a
pre-existing address; could use same variation of unsafe to say "I know how large i am, I know my
layout, I have a pointer to where I begin, set it all up," but even that would have to be defined on
the struct, `Vec` can't forward args to this initializer method.
That said, open question as to whether the move vs. construct-in-place/placement new matters given
an optimizing compiler: https://stackoverflow.com/a/36919571
# Potentially excluded
Some ideas related to traits, but that I'm not sure sufficiently fit the theme. May be worth
investigating in a future post?
# CRTP
Might not need to be an extensive section? CRTP lets bases reference children. Rust has no struct
inheritance, but some CRTP stuff can be used with traits.
Review of the examples Wikipedia gives:
- Static polymorphism: Traits are allowed to declare that implementors define an `implementation()`
and then provide default implementations of other methods that use it (without virtual calls). Not
a common pattern though; use composition, not inheritance. https://godbolt.org/z/Md55e7
- Object counter: I don't think Rust has a way to accomplish this; traits aren't allowed to hold
data.
- Polymorphic chaining: Feel free to return `Self`, `&Self`, etc., builder patterns aren't new.
## Visibility
Worth acknowledging that C++ can do interesting things with `protected`, `friend`, and others, that