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[SUGGESTION] unsafe_narrow is not a good name for forced upcasts. #510
Description
If you have e.g. expression templates, then the upcast to the implementing class needs to be done with unsafe_narrow.
Maybe an alias of unsafe_narrow like unsafe_cast, 'unsafe_pointer_cast` or something like that could be introduced.
Example: https://cpp2.godbolt.org/z/jb1MY3Mcq
Exp : <Impl> type = {
cast: (this) -> forward Impl = cpp2::unsafe_narrow<Impl>(this&)*; // unsafe_narrow does not really express what this cast is.
}
A : type = {
this: Exp<A> = ();
foo: (this) = std::cout << "foo" << std::endl;
}
main: () -> int = {
a: A = ();
e: *Exp<A> = a&;
e*.cast().foo();
return 0;
}
Activity
JohelEGP commentedon Jun 13, 2023
I call this
static_downcast: https://johelegp.github.io/jegp/static.downcast.Inspired by https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-casts-named,
I created my own more specific one.
MaxSagebaum commentedon Jun 13, 2023
Yes, that is also possible.
JohelEGP commentedon Jun 13, 2023
By the way,
I noticed in your other issue (and this one)
that you mixed up the terms upcast and downcast.
Then, I tried to find it in the specification for
static_cast,but it wasn't mentioned, so I just left it unsaid.
Now, I did find other examples.
These use "upcast" when converting from derived to base:
https://eel.is/c++draft/class.cdtor#example-3 and https://eel.is/c++draft/class.cdtor#example-1.
MaxSagebaum commentedon Jun 13, 2023
Thanks for pointing this out. I knew, that this can also be seen the other way round. I will start using it in terms of the type system and not the functionality.
JohelEGP commentedon Jun 19, 2023
cpp2::unsafe_narrowis modeled aftergsl::narrow_cast.gsl::narrow_castseems to be modeled aftergsl::narrowbased on its uses in the C++ Core Guidelines.
Although
gsl::narrow_cast<T>(x)is specified as beingstatic_cast<T>(x)[ Note:
static_cast<T>(x)is illegal Cpp2. -- end note ]it's not clear if those should work with non-arithmetic types:
See isocpp/CppCoreGuidelines#1845 and linked issues (and their linked issues/PRs).
MaxSagebaum commentedon Jun 19, 2023
Ok,
cpp2::unsafe_narrowshould be equally handled as the gsl one. But there should be additional casts in cpp2.