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Section: 28.2 [numeric.requirements] Status: C++20 Submitter: Tim Song Opened: 2018-07-05 Last modified: 2021-02-25
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28.2 [numeric.requirements] contains some very old wording that hasn't been changed since C++98 except for issue 2699. As a result, it is at once over- and under-restrictive. For example:
We can significantly clean up this wording by using the existing named requirements. For ease of review, the following table provides a side-by-side comparison of the current and proposed wording.
|A C++ program shall instantiate these components only with a type T that satisfies the following requirements: [Footnote … ]||A C++ program shall instantiate these components only with a type T
that satisfies the
|(1.1) — T is not an abstract class (it has no pure virtual member functions);||Cpp17DefaultConstructible|
|(1.2) — T is not a reference type;
(1.3) — T is not cv-qualified;
|Implied by "cv-unqualified object type"|
|(1.4) — If T is a class, it has a public default constructor;||Cpp17DefaultConstructible|
|(1.5) — If T is a class, it has a public copy constructor with the signature T::T(const T&);||Cpp17CopyConstructible|
|(1.6) — If T is a class, it has a public destructor;||Cpp17Destructible|
|(1.7) — If T is a class, it has a public copy assignment operator whose signature is either T& T::operator=(const T&) or T& T::operator=(T);||Cpp17CopyAssignable|
|(1.8) — If T is a class, its assignment operator, copy and default constructors, and destructor
shall correspond to each other in the following sense:
(1.8.1) — Initialization of raw storage using the copy constructor on the value of T(), however obtained, is semantically equivalent to value-initialization of the same raw storage.
(1.8.2) — Initialization of raw storage using the default constructor, followed by assignment, is semantically equivalent to initialization of raw storage using the copy constructor.
(1.8.3) — Destruction of an object, followed by initialization of its raw storage using the copy constructor, is semantically equivalent to assignment to the original object.
[Note: […] — end note]
|These requirements are implied by Cpp17CopyConstructible and Cpp17CopyAssignable's requirement that the value of the copy is equivalent to the source.|
|(1.9) — If T is a class, it does not overload unary operator&.||omitted now that we have std::addressof|
[2019-01-20 Reflector prioritization]
Set Priority to 0 and status to Tentatively Ready
This wording is relative to the post-Rapperswil 2018 working draft.
Edit 28.2 [numeric.requirements] p1 as indicated, striking the entire bulleted list:
-1- The complex and valarray components are parameterized by the type of information they contain and manipulate. A C++ program shall instantiate these components only with a type T that satisfies the
followingrequirements :[Footnote: … ]
(1.1) — T is not an abstract class (it has no pure virtual member functions); […] (1.9) — If T is a class, it does not overload unary operator&.
Edit 126.96.36.199 [valarray.access] p3-4 as indicated:
const T& operator(size_t n) const; T& operator(size_t n);
-1- Requires: n < size().-2- Returns: […] -3- Remarks: The expression
&a[i+j] == &a[i] + j evaluates to true for all size_t i and size_t j such that i+j < a.size(). -4- The expression &a[i] != &b[j] evaluates to true for any two arrays a and b and for any size_t i and size_t j such that i < a.size() and j < b.size(). [Note: […] — end note ]