If you look at the following code, I think that both the lines in main()
should call the initializer_list
constructor for InitSomething
. gcc outputs 22
as I expected, but clang just outputs a single 2
. Is clang wrong?
I am compiling with -std=c++14
.
#include <iostream>
struct InitSomething {
explicit InitSomething(int) { std::cout << '1'; }
InitSomething(std::initializer_list<int> ) { std::cout << '2'; }
operator int() { return 1; }
};
int main() {
InitSomething init_something{1};
InitSomething init_something_else{init_something};
}
The output of clang++ --version
(I am on a mac) is
Apple LLVM version 7.3.0 (clang-703.0.31)
Target: x86_64-apple-darwin15.5.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
and the output of g++ --version
on the other platform I mentioned is
g++ (GCC) 4.8.5 20150623 (Red Hat 4.8.5-4)
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
DR1467's title concerns aggregates, but the wording it added to [dcl.init.list]/3 as the first bullet is not limited to aggregates:
If T
is a class type and the initializer list has a single element
of type cv U
, where U
is T
or a class derived from T
, the
object is initialized from that element (by copy-initialization for
copy-list-initialization, or by direct-initialization for
direct-list-initialization).
However, DR2137 walked back from this wording (replacing "a class type" with "an aggregate class"), and so this bullet no longer applies to non-aggregates like InitSomething
.
Instead, [dcl.init.list]/3.6 applies, like it did pre-DR1467:
Otherwise, if T is a class type, constructors are considered.
The applicable constructors are enumerated and the best one
is chosen through overload resolution ([over.match],
[over.match.list]).
And [over.match.list] makes clear that initializer-list constructors are preferred if at all viable:
When objects of non-aggregate class type T
are list-initialized such
that [dcl.init.list] specifies that overload resolution is performed
according to the rules in this section, overload resolution selects
the constructor in two phases:
- Initially, the candidate functions are the initializer-list constructors ([dcl.init.list]) of the class
T
and the argument list
consists of the initializer list as a single argument.
- If no viable initializer-list constructor is found, overload resolution is performed again, where the candidate functions are all
the constructors of the class
T
and the argument list consists of
the elements of the initializer list.
All this talk about ranking implicit conversion sequences is irrelevant because the non-initializer-list constructors are not even candidates.
Clang is implementing the wording after DR1467 and before DR2137.