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by quotemstr
1712 days ago
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> But, because Rust's traits have semantics not just syntax the actual effect is different. Don't concepts give C++ the same semantic power? What am I missing? Concepts give C++ generic code exactly the same nominative constraints you're describing, yes? Besides: even without concepts, you can use tag types and traits to similar effect. Not everything needs to be duck-typed, even in C++17! One of my main problems with using Rust instead of C++ is losing metaprogramming power. I really like the template system. |
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Not really. But, it's on purpose, and, in their context it makes sense. These aren't idiots, they know there are however many bajillion lines of C++ code and if Concepts doesn't work with that code it's useless.
In Rust, you have to explicitly write implementations of traits. If the author of stupid::Bicycle doesn't implement nasty::Smells and the author of nasty::Smells didn't implement it for stupid::Bicycle either, then a stupid::Bicycle just isn't nasty::Smells.
Even "marker traits" where the implementation is empty, are still explicit. If you have a type that implements Clone but you didn't say it is Copy, then it isn't Copy, even though maybe the type would qualify as Copy if you had asked. Rust will always move this type, because you didn't say copying it was OK.
This is where the semantic strength comes from, and it was fine in Rust because Rust "always" did this. There isn't a bajillion lines of Rust code that doesn't know about traits, and so it's OK to require this.
In C++ they realistically couldn't do that. So, next best thing is to say that a Concept is modelled on syntax. You mentioned duck-typing, and that's what concepts are assumed to be for.
Now, of course you can use tag types, for a home grown Concept it actually might be viable, if you've only got two users and they both know you personally you can just tell them to use the tag type and they'll both add it. But for the C++ standard library Concepts that was not practical. So, that's not how Concepts is being taught or has been used in practice, unlike Rust's traits.