Since the standard range adaptors are specified to derive from the empty
class view_base, having their first data member store the underlying
view is suboptimal, for if the underlying view also derives from
view_base then the two view_base subobjects will be adjacent; this
prevents the compiler from applying the empty base optimization to elide
away the storage for these two empty bases.
This patch improves the situation by declaring the _M_base data member
last instead of first in each range adaptor that has more than one data
member, so that the empty base optimization can apply in more cases.
libstdc++-v3/ChangeLog:
* include/std/ranges (filter_view): Declare the data member
_M_base last instead of first, and adjust constructors' member
initializer lists accordingly.
(transform_view): Likewise.
(take_view): Likewise.
(take_while_view): Likewise.
(drop_view): Likewise.
(drop_while_view): Likewise.
(join_view): Likewise.
(split_view): Likewise (and tweak nearby formatting).
(reverse_view): Likewise.
* testsuite/std/ranges/adaptors/sizeof.cc: Update expected
sizes.
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