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Section: 24.2.4 [sequence.reqmts] Status: New Submitter: Casey Carter Opened: 2019-09-17 Last modified: 2022-04-24
Priority: 3
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Discussion:
24.2.4 [sequence.reqmts] paragraph 3 says that the names i
and j
denote
"iterators that meet the Cpp17InputIterator requirements and refer to elements implicitly
convertible to value_type
". Ignoring for the moment that this is an occurrence of LWG
3105(i) — we really mean that *i
and *j
must be implicitly
convertible to value_type
— this requirement seems to be completely extraneous.
i
and j
are used in three places in the requirements table:
The range constructors X(i, j)
and X u(i, j)
, which require that the
container's value type is Cpp17EmplaceConstructible into the container from *i
;
implicit conversion is neither necessary nor sufficient.
The range insert overload a.insert(p, i, j)
which also requires Cpp17EmplaceConstructible,
as well as the capability to move elements around for vector
and deque
; again,
implicit conversion is neither necessary nor sufficient. It would be useful / performant
here to require that the container's value type is assignable from *i
, which may have
been the intent of the implicit conversion requirement — would doing so be too breaking?
The range assign overload a.assign(i, j)
which requires both
Cpp17EmplaceConstructible as above and that it can assign the result of dereferencing an
iterator directly to the container's value type; again, implicit conversion is not useful here.
We should strike the implicit conversion requirement since it is not useful and only serves to confuse readers of the Standard (see e.g. here).
[2019-10-31 Issue Prioritization]
Priority to 3 after reflector discussion.
Previous resolution [SUPERSEDED]:
This wording is relative to N4830.
Modify 24.2.4 [sequence.reqmts] as indicated:
-3- In Tables 76 and 77,
X
denotes a sequence container class,a
denotes a value of typeX
containing elements of typeT
,u
denotes the name of a variable being declared,A
denotesX::allocator_type
if the qualified-idX::allocator_type
is valid and denotes a type (13.10.3 [temp.deduct]) andallocator<T>
if it doesn't,i
andj
denote iterators that meet the Cpp17InputIterator requirementsand refer to elements implicitly convertible to,value_type
[i, j)
denotes a valid range, […]
[2022-04-24; Daniel rebases wording on N4910]
Proposed resolution:
This wording is relative to N4910.
Modify 24.2.4 [sequence.reqmts] as indicated:
-3- In this subclause,
(3.1) — […]
[…]
(3.5) —
i
andj
denote iterators that meet the Cpp17InputIterator requirementsand refer to elements implicitly convertible to,value_type
[…]