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Comment by K0nserv | Hacker News Reader

> In c the callers isn’t choosing typically. The author of some library or api decides this for you.

Tbf this applies to Rust too. If the author writes

   fn foo(bar: Box<dyn BarTrait>)

they have forced the caller into dynamic dispatch.

Had they written

   fn foo(bar: impl BarTrait)

the choice would've remained open to the caller

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Right, but almost all APIs in Rust use something like

    fn foo(bar: impl BarTrait)

and AFAIK it isn't possible to write that in C (though C++ does allow this kind of thing).

C++ you either use templates or classes and virtuals. In either case the caller doesn't get to decide.

Interesting, there isn't some way to have a template that is polymorphic over virtuals?

In C++ you do it the other way around, have a single class that is polymorphic over templates. The name of this technique within C++ is type-erasure (that term means something else outside of C++).

Examples of type erasure in C++ are classes like std::function and std::any, and normally you need to implement the type erasure manually, but there are some library that can automate it to a degree, such as [1], but it's fairly clumsy.

[1] https://www.boost.org/doc/libs/latest/doc/html/boost_typeera...

It's neat this is a thing I guess, but I agree it looks fairly clumsy compared to the Rust answer.

how do apis typically manage to actually « use » the « bar » of your example, such as storing it somewhere, without enforcing some kind of constraints ?

"BarTrait" is the constraint.

This is monomorphized for every type you pass in, in short.

If you need to store the value then you have no choice but to take in a dyn trait.

Depending on exactly what you mean, this isn't correct. This syntax is the same as <T: BarTrait>, and you can store that T in any other generic struct that's parametrized by BarTrait, for example.

> you can store that T in any other generic struct that's parametrized by BarTrait, for example

Not really. You can store it on any struct that specializes to the same type of the value you received. If you get a pre-built struct from somewhere and try to store it there, your code won't compile.

Can you show me what you’re talking about? I don’t understand what you mean. I’ll add a code example of what I mean in a bit.

Here's what I'm talking about: https://play.rust-lang.org/?version=stable&mode=debug&editio...

sure about that?

the struct in which it is stored, could be generic as well

I'm addressing the intent of the original question.

No one would ask this question in the case where the struct is generic over a type parameter bounded by the trait, since such a design can only store a homogeneous collection of values of a single concrete type implementing the trait; the question doesn't even make sense in that situation.

The question only arises for a struct that must store a heterogeneous collection of values with different concrete types implementing the trait, in which case a trait object (dyn Trait) is required.

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