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by jandrewrogers 1460 days ago
Okay, yeah, I would broadly agree with this. I think most people use template-based polymorphism, which is pretty flexible in practice. The use of virtual methods is verboten for many common use cases of C++, due to the necessity of being in paged memory, so constructions for dealing with polymorphism without virtual methods are commonplace. And std::variant is a bit of a hot mess.
1 comments
strictfp 1460 days ago
> The use of virtual methods is verboten for many common use cases of C++, due to the necessity of being in paged memory

I wasn't aware of this. Maybe I'm just out of the loop. Do you know where one can I learn more about this? I'm desperately trying to reduce the number of virtual calls in our codebase, but I'm hitting the aforementioned problems.

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jandrewrogers 1460 days ago
It is a design problem endemic to database engines and probably file systems. A design requirement for most of the dynamic runtime data structures is that they can be directly paged to storage, either in whole or in part, and be paged from storage in an arbitrarily distant future on different machines with different compilers. In order to make this work, all data types used in pageable data structures must 1) have a size and alignment that is not compiler-dependent so that page types always have a size that is a strict multiple of the I/O page size and 2) not contain any pointers. This precludes vtables.

This has traditionally been managed with CRTP, tagged unions, etc with some scaffolding to make it convenient and compliant with strict aliasing rules. Ideally, almost all of the dynamic polymorphism is pulled up to the level of the page types, an opaque blob of I/O friendly complex data structure, minimizing the amount you have to do. It is also important to note that JIT-ing has replaced many of the use cases for dynamic dispatch e.g. adding user-defined schemas at runtime.

None of which may apply to your use case. Some things inherently require an unfortunate amount of dynamic dispatch.

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heisjustsosmart 1460 days ago
Virtual methods are about dispatch tables (which are built at compile time) and have nothing to do with paging. Nothing. At all.
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