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by tialaramex 1415 days ago
comex mentions this above. It's a nice hack, but it doesn't actually fix the problem I was talking about, you can give this attribute to a data member, which allows you to do the ZST marker type trick that is available in other languages, but you can't apply the attribute (at least, not as documented) to a type itself, those empty tuples are obliged to take up one byte each.
1 comments
tsimionescu 1414 days ago
But why not use an empty base class instead? That seems both more idiomatic and simpler. And since C++ supports multiple inheritance, there's no limit to how many empty base classes you could have.
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tialaramex 1414 days ago
Using an empty base class is idiomatic C++ for solving the problem comex is talking about (marker types) although it has some pretty annoying consequences. Whether it's "simpler" probably depends on whether to you "simpler" means, "how I'm used to in C++".

But it doesn't have any bearing on the problem I was talking about, if you were to instantiate your empty class it has non-zero size. C++ just doesn't have ZSTs.

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tsimionescu 1414 days ago
Ok, I misunderstood the thread a bit and was only commenting on the marker issue. I do thing that a type-level marker is "simpler" in a more general sense than checking whether a struct has a particular field, even if that field takes up no space in the struct.

Now, related to ZSTs, I think the main reason why C++ doesn't have this is that C++ really doesn't have any good support for constant values. Sure, you can mark something `const` but that rarely means too much - specifically, it can never add any new semantics to a type, it only removes some options.

One consequence of this is your example - a `const vector<T>` can't be a simple counter of how many T are in the vector even if the T type has a single possible value: the language can't really use the fact that the array is `const` to change its layout.

An even worse consequence is that a `const vector<int> * const` (a const pointer to a const vector of T) is not covariant (it can't be initialized with a const pointer to a const vector of a subtype of T), even though it should be: the language just won't use the fact that it is `const` in this way.

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