This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115d. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2024-10-26
[Adopted at the February, 2016 meeting.]
The intent was for PODs in C++11 to be a superset of C++03 PODs. Consequently, in the following example, C should be a POD but isn't:
struct A { const int m; A& operator=(A const&) = default; // deleted and trivial, so A is a // POD, as it would be in 2003 // without this explicit op= decl }; static_assert(__is_trivially_copyable(A), ""); struct B { int i; B& operator=(B &) & = default; // non-trivial B& operator=(B const&) & = default; // trivial }; struct C { const B m; C& operator=(C const& r) = default; // deleted (apparently), but non-trivial (apparently) /* Notionally: C& operator=(C const& r) { (*this).m.operator=(r.m); return *this; } */ }; static_assert(!__is_trivially_copyable(C), "");
This is because of the following text from 11.4.5.3 [class.copy.ctor] paragraph 25:
for each non-static data member of X that is of class type (or array thereof), the assignment operator selected to copy/move that member is trivial;
In this case, overload resolution fails, so no assignment operator is selected, so C::operator=(const C&) is non-trivial.
(See also issue 1928.)
Additional note, November, 2014:
See paper N4148.
Additional note, October, 2015:
Moved from "extension" to "open" status, along with issue 1928, to allow reconsideration by CWG. It has been suggested that the triviality of a deleted function should be irrelevant, since it cannot be used in any event. A possible change to implement that, more conservative than the one proposed in N4148, would be:
A trivially copyable class is a class that:
has no non-trivial, non-deleted copy constructors (11.4.5.3 [class.copy.ctor]),
has no non-trivial, non-deleted move constructors (11.4.5.3 [class.copy.ctor]),
has no non-trivial, non-deleted copy assignment operators (12.4.3.2 [over.ass], 11.4.5.3 [class.copy.ctor]),
has no non-trivial, non-deleted move assignment operators (12.4.3.2 [over.ass], 11.4.5.3 [class.copy.ctor]),
has at least one non-deleted copy or move constructor or assignment operator, and
has a trivial, non-deleted destructor (11.4.7 [class.dtor]).
Proposed resolution (October, 2015):
Change Clause 11 [class] paragraph 6 as follows:
A trivially copyable class is a class
that:
has no non-trivial copy constructors (11.4.5.3 [class.copy.ctor]),
has no non-trivial move constructors (11.4.5.3 [class.copy.ctor]),
has no non-trivial copy assignment operators (12.4.3.2 [over.ass], 11.4.5.3 [class.copy.ctor]),
has no non-trivial move assignment operators (12.4.3.2 [over.ass], 11.4.5.3 [class.copy.ctor]), andwhere each copy constructor, move constructor, copy assignment operator, and move assignment operator (11.4.5.3 [class.copy.ctor], 12.4.3.2 [over.ass]) is either deleted or trivial,
that has at least one non-deleted copy constructor, move constructor, copy assignment operator, or move assignment operator, and
that has a trivial, non-deleted destructor (11.4.7 [class.dtor]).