ldionne added inline comments.
================
Comment at: libcxx/include/memory:1645
- template <class _Tp>
+ template <class _SourceTp, class _DestTp>
_LIBCPP_INLINE_VISIBILITY
----------------
Quuxplusone wrote:
> ldionne wrote:
> > Coming at it from a slightly different angle, I would think this is what we
> > want:
> >
> > ```
> > template <class _SourceTp, class _DestTp,
> > class _RawSourceTp = typename remove_const<_SourceTp>::type,
> > class _RawDestTp = typename remove_const<_DestTp>::type>
> > _LIBCPP_INLINE_VISIBILITY static typename enable_if<
> >
> > // We can use memcpy instead of a loop with construct if...
> > is_trivially_move_constructible<_DestTp>::value &&
> > // - the Dest is trivially move constructible, and
> > is_same<_RawSourceTp, _RawDestTp>::value &&
> > // - both types are the same modulo constness, and either
> > (__is_default_allocator<allocator_type>::value ||
> > // + the allocator is the default allocator (and we know `construct` is
> > just placement-new), or
> > !__has_construct<allocator_type, _DestTp*, _SourceTp const&>::value),
> > // + the allocator does not provide a custom `construct` method (so we'd
> > fall back to placement-new)
> > void>::type
> > __construct_range_forward(allocator_type&, _SourceTp* __begin1, _SourceTp*
> > __end1, _DestTp*& __begin2)
> > {
> > ptrdiff_t _Np = __end1 - __begin1;
> > if (_Np > 0)
> > {
> > _VSTD::memcpy(const_cast<_RawDestTp*>(__begin2), __begin1, _Np *
> > sizeof(_DestTp));
> > __begin2 += _Np;
> > }
> > }
> > ```
> >
> > And then we should have
> >
> > ```
> > template <class _Tp>
> > struct __is_default_allocator : false_type { };
> >
> > template <class _Tp>
> > struct __is_default_allocator<_VSTD::allocator<_Tp> > : true_type { };
> > ```
> >
> > Does this make sense?
> >
> > Also, I'm not sure I understand why we use `const_cast` on the destination
> > type. It seems like we should instead enforce that it is non-const? But
> > this is a pre-existing thing in the code, this doesn't affect this review.
> >
> I agree that it is wrong to express the check in terms of
> `is_same<allocator_type, allocator<...>>`; it should be expressed in terms of
> a trait which is satisfied by `std::allocator<T>`-for-any-T. @ldionne
> expressed it in terms of `__is_default_allocator<A>`. I continue to ask that
> it be expressed in terms of `__has_trivial_construct<A, _DestTp*,
> _SourceTp&>`, where libc++ specializes
> `__has_trivial_construct<std::allocator<_Tp>, ...>` if need be.
>
> Orthogonally, the condition `__has_construct<allocator_type, _DestTp*,
> _SourceTp const&>` is wrong because it has an extra `const`. It is
> conceivable — though of course implausible/pathological — for `construct(T*,
> T&)` to exist and do something different from `construct(T*, const T&)`.
> I continue to ask that it be expressed in terms of
> `__has_trivial_construct<A, _DestTp*, _SourceTp&>`, where libc++ specializes
> `__has_trivial_construct<std::allocator<_Tp>, ...>` if need be.
Would you be OK with us applying this fix and then generalizing
`__is_default_allocator` into `__has_trivial_construct` as a followup? I
suspect we'll have more discussion around that generalization and I'd like for
us to fix this bug because I find PR37574 somewhat concerning and I'd like for
it to be fixed soon (like within a couple of days).
> Orthogonally, the condition `__has_construct<allocator_type, _DestTp*,
> _SourceTp const&>` is wrong because it has an extra const. It is conceivable
> — though of course implausible/pathological — for `construct(T*, T&)` to
> exist and do something different from `construct(T*, const T&)`.
Good catch. IIUC, `__has_construct<allocator_type, _DestTp*, _SourceTp&>` would
work?
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D48342/new/
https://reviews.llvm.org/D48342
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