Is it possible to control the size of an array using the type parameter of a generic?

Question

What follows is just used as an example, and not valid Rust code.

struct Vec<T: Sized, Count> {     a: [T; Count] } 

Something like it is possible in C++ templates, but I haven't seen it in Rust.

Answer 1

Rust 1.51

Use const generics:

struct Vec<T: Sized, const COUNT: usize> {     a: [T; COUNT], } 

Previous versions

RFC 2000 — const generics introduces support for this and progress is tracked in issue #44580.

If you look at the design of Rust, you will notice that it started first by tackling the hardest problems (memory-safe, data-race free) but there are otherwise lots of areas where it is "incomplete" (compared to what could be achieved).

In particular, generic structures and functions started out somewhat limited:

• lack of Higher Kinded Types (HKT)
• lack of non-type parameters => arrays are special-cased, and implementing a trait for an array is a known issue, the work-around being to implement it for a few different dimensions
• lack of variadic parameters => tuples are special-cased, and implementing a trait for all tuples is similarly difficult

For the moment, not all of these are implemented, not because they are not desired but simply because time was lacking. The idea of Rust 1.0 was not to release a final product that would not evolve, but a stable base from which to start; some or maybe all will come.