Add value member to allocator_is_always_equal

[edsavage]

In order to use boost::allocator_is_always_equal with e.g. boost::unordered_set a value member is required to exist in the struct, else a compilation error is encountered:

C:\usr\local\include\boost-1_77\boost/unordered/unordered_set.hpp(170,1): error C2039: 'value': is not a member of 'boost::allocator_is_always_equal<Alloc,void>'
        with
        [
            Alloc=std::allocator<std::string>
        ]
        BOOST_NOEXCEPT_IF(value_allocator_traits::is_always_equal::value&&

Encountered with boost version 1.77, on OS Windows Server 2016 with compiler version MSVC 19.29.30136 for x64

[Lastique]

value must have an initializer. And its type should presumably be bool, not type.

[Lastique]

Also, if you're going to add it, it must be added to all code branches. And while we're at it, we might as well add it to the other traits.

[pdimov]

The documentation for allocator_is_always_equal clearly states that it only has type, so the error must be on the unordered side. What is the code that gives the error?

[edsavage]

The documentation for allocator_is_always_equal clearly states that it only has type, so the error must be on the unordered side. What is the code that gives the error?

C:\usr\local\include\boost-1_77\boost/unordered/unordered_set.hpp(170,1): error C2039: 'value': is not a member of 'boost::allocator_is_always_equal<Alloc,void>'
        with
        [
            Alloc=std::allocator<std::string>
        ]
        BOOST_NOEXCEPT_IF(value_allocator_traits::is_always_equal::value&&
^
C:\usr\local\include\boost-1_77\boost/unordered/detail/implementation.hpp(1474): note: see declaration of 'boost::allocator_is_always_equal<Alloc,void>'
        with
        [
            Alloc=std::allocator<std::string>
        ]
        typedef boost::allocator_is_always_equal<Alloc> is_always_equal;
C:\usr\local\include\boost-1_77\boost/unordered/unordered_set.hpp(170,1): error C2039: 'value': is not a member of 'boost::allocator_is_always_equal<Alloc,void>'
        with
        [
            Alloc=std::allocator<std::string>
        ]
        BOOST_NOEXCEPT_IF(value_allocator_traits::is_always_equal::value&&
^
C:\usr\local\include\boost-1_77\boost/unordered/detail/implementation.hpp(1474): note: see declaration of 'boost::allocator_is_always_equal<Alloc,void>'
        with
        [
            Alloc=std::allocator<std::string>
        ]
        typedef boost::allocator_is_always_equal<Alloc> is_always_equal;

so stemming from

#if !defined(BOOST_NO_CXX11_RVALUE_REFERENCES)
      unordered_set& operator=(unordered_set&& x)
        BOOST_NOEXCEPT_IF(value_allocator_traits::is_always_equal::value&&
            boost::is_nothrow_move_assignable<H>::value&&
              boost::is_nothrow_move_assignable<P>::value)
      {    
        table_.move_assign(x.table_, boost::unordered::detail::true_type());
        return *this;
      }    
#endif

in unordered_set.hpp line 170

[pdimov]

That's the error message itself but what's the source code that gives it?

[edsavage]

That's the error message itself but what's the source code that gives it?

Sorry, I added the source code in an edit to the above comment.

[pdimov]

I'm trying

#include <boost/unordered_set.hpp>

int main()
{
    boost::unordered_set<int> s1;
    boost::unordered_set<int> s2;
    s2 = std::move( s1 );
}

but it seems to work fine.

[pdimov]

I'm not sure where boost::allocator_is_always_equal is even coming from, because as far as I can see Unordered doesn't use it (yet).

[edsavage]

@pdimov Yes, that is the case at the moment. But in an effort to quiet a deprecation warning related to the use of std::allocator::is_always_equal (see https://github.com/boostorg/unordered/pull/52) I've made use of boost::allocator_is_always_equal. This was initially suggested on a mailing list a while back - https://groups.google.com/g/boost-developers-archive/c/3MOWuQgEEy4/m/boIwsJsdBAAJ but never followed up.

[pdimov]

Try replacing

        typedef BOOST_UNORDERED_DEFAULT_TYPE(std::allocator_traits<Alloc>,
          is_always_equal,
          BOOST_UNORDERED_DEFAULT_TYPE(Alloc, is_always_equal,
            typename boost::is_empty<Alloc>::type)) is_always_equal;

with

        typedef typename boost::allocator_is_always_equal<Alloc>::type is_always_equal;

instead.

[edsavage]

Thanks @pdimov , trying that now..

Update: The suggested change by @pdimov has fixed the compilation error encountered in https://github.com/boostorg/unordered/pull/52. I'll close this PR and update that one accordingly. Thanks all!

[edsavage]

Closing as a better solution has been found.

[Lastique]

Is there a rationale for not having the value member? This is unlike other type traits with a value result.

[pdimov]

This is not a trait with a value result.

[Lastique]

allocator_is_always_equal is a yes or no trait, its type member is supposed to have the value member. Realistically, it is true_type or false_type or something that looks like one. It might as well have a value result instead. Same with allocator_propagate_* traits.

[pdimov]

No, allocator_is_always_equal<A> is a type transformation trait that returns A::is_always_equal, similarly to how allocator_size_type<A> is a type transformation trait that returns A::size_type.

[Lastique]

I realize that's how it works now. I'm saying it would make more sense if it was a value trait and gave the result right away rather than require you to use an extra layer of indirection.

[pdimov]

allocator_is_always_equal_t<A> is the value trait that gives the result straight away.

[glenfe]

The pull request is wrong, because I don't want these types to have a value. They are type traits which give you the equivalent of allocator_traits<A>::facility (which give you types - Alloc::facility if it exists, or some other type).