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