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Subject: Re: C++17 std::hash disabled specializations
From: Ville Voutilainen (ville.voutilainen_at_[hidden])
Date: 2020-04-15 10:58:16


On Wed, 15 Apr 2020 at 18:43, Daniel Krügler via Std-Discussion
<std-discussion_at_[hidden]> wrote:
>
> Am Mi., 15. Apr. 2020 um 16:13 Uhr schrieb Edward Diener via
> Std-Discussion <std-discussion_at_[hidden]>:
> >
> > In C++17 if a std::hash specialization for a key is not provided and is
> > therefore considered a disabled specialization, is it a compiler error
> > if an unordered associate container is instantiated with that key ?
> >
> > As an example:
> >
> > struct MyStruct
> > {
> > int x;
> > long y;
> > MyStruct(int a, long b) : x(a), y(b) {}
> > };
> >
> > /* No std::hash specialization is provided for MyStruct */
> >
> > MyStruct my1(1,2);
> >
> > std::unordered_set<MyStruct> oname;
> > oname.insert(my1);
> >
> > Should the code cause a compilation error in C++17 and above ?

It should cause a compilation error in C++11 and above.

>
> The standard says for a disabled specialization:
>
> "these values are false: is_default_constructible_v<H>,
> is_copy_constructible_v<H>, is_move_constructible_v<H>,
> is_copy_assignable_v<H>, and is_move_assignable_v<H>. Disabled
> specializations of hash are not function object types (23.14)."
>
> For unordered associative containers, C++17 imposes the requirement
>
> "Hash shall be a unary function
> object type such that the
> expression hf(k) has type
> size_t."
>
> For the default constructor the requirements
>
> "hasher and
> key_equal are
> DefaultConstructible."
>
> are imposed.
>
> Technically these are pre-conditions whose violation would lead to UB.
> But given the above mentioned characteristics of disabled hash
> specializations I would expect sane implementations to make the code
> ill-formed, e.g. when the constructor of the set is invoked or the
> when insert is called.

I would expect *correct* implementations to make the code ill-formed,
as those operations
are specified to invoke operations of the hash specialization, and
those operations
are ill-formed for a disabled hash.


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