[Bug libstdc++/52917] [DR 2048] explicitly stated return type in std::mem_fn cannot be compiled
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52917 Jonathan Wakely redi at gcc dot gnu.org changed: What|Removed |Added Status|SUSPENDED |RESOLVED Resolution|--- |INVALID --- Comment #5 from Jonathan Wakely redi at gcc dot gnu.org --- The standard agrees with libstdc++ now, so this is invalid.
[Bug libstdc++/52917] [DR 2048] explicitly stated return type in std::mem_fn cannot be compiled
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52917
--- Comment #3 from freunddeslichts at web dot de 2012-04-10 09:54:28 UTC ---
Ok, I didn't know about the defect report and resolution yet.
I must admit that I quite like the int() syntax.
I added a remark about the defect and a short example to
http://en.cppreference.com/w/cpp/utility/functional/mem_fn
And I noted that the most c++11-ish code would be anyway:
auto y = [] (X x) {return x.get();};
[Bug libstdc++/52917] [DR 2048] explicitly stated return type in std::mem_fn cannot be compiled
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52917
--- Comment #4 from Jonathan Wakely redi at gcc dot gnu.org 2012-04-10
11:23:51 UTC ---
(In reply to comment #3)
Ok, I didn't know about the defect report and resolution yet.
I must admit that I quite like the int() syntax.
It's a peculiarity of the C++ grammar, the function parameter R T::*pm is a
pointer to member, with T deduced as X and R deduced as the function type
int()
If it helps, consider that you can declare X::get() like this:
#include functional
struct X
{
int a;
typedef int func_type();
func_type get;
};
int X::get() { return a; }
And then you can use the typedef with mem_fn:
auto pm = std::mem_fnX::func_type(X::get);
I added a remark about the defect and a short example to
http://en.cppreference.com/w/cpp/utility/functional/mem_fn
N.B. That page has a heading Execptions
And I noted that the most c++11-ish code would be anyway:
auto y = [] (X x) {return x.get();};
In most cases yes, but mem_fn has the advantage it always returns the same
type, whereas two lambda expressions produce two different closure types even
if the expressions are identical, and mem_fn might be easier to use in
late-specified return types.
[Bug libstdc++/52917] [DR 2048] explicitly stated return type in std::mem_fn cannot be compiled
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52917
Jonathan Wakely redi at gcc dot gnu.org changed:
What|Removed |Added
Status|UNCONFIRMED |SUSPENDED
Last reconfirmed||2012-04-09
Summary|explicitly stated return|[DR 2048] explicitly stated
|type in std::mem_fn cannot |return type in std::mem_fn
|be compiled |cannot be compiled
Ever Confirmed|0 |1
--- Comment #1 from Jonathan Wakely redi at gcc dot gnu.org 2012-04-09
21:18:11 UTC ---
(In reply to comment #0)
(2) needs the return type to diffrentiate between the const and the non-const
getter.
Or you can do
auto y = std::mem_fn((int (X::*)())X::get );
Which also works in this case where specifying the return type doesn't help:
struct X {
int get() { return 0; }
int get() const { return 1; }
};
The specification for mem_fn is
template class R, class T /*unspecified*/ mem_fn(R T::* pm);
template class R, class T, class... Args /*unspecified*/ mem_fn(R (T::*
pm)(Args...));
[ ... some more overloads for const, volatile and references]
Maybe not for long: http://cplusplus.github.com/LWG/lwg-active.html#2048
The libstdc++ implementation exactly matches that issue's proposed resolution,
which has been voted Tentatively Ready.
(2) should result in an instantiation of the second version with
sizeof...(Args)==0, because argument to the first version is a pointer to
member data, not pointer to member function.
No, it's a pointer to member, which can match either a pointer to member data
or pointer to member function.
If the second overload exists (as it presumably does in libc++) then it's a
better match than the first overload, but the first overload is still viable.
Both g++-4.7 and clang++-3.1 (when compiling with libstdc++) try to
instantiate
the first version by mistake.
Not by mistake, by design, that overload can match any pointer to member.
I'm suspending this because I believe the resolution to DR 2048 would make
libstdc++ correct.
[Bug libstdc++/52917] [DR 2048] explicitly stated return type in std::mem_fn cannot be compiled
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52917 --- Comment #2 from Jonathan Wakely redi at gcc dot gnu.org 2012-04-09 21:35:07 UTC --- (In reply to comment #1) Or you can do auto y = std::mem_fn((int (X::*)())X::get ); Or auto y = std::mem_fn int()(X::get ); Which should also work with either libstdc++ or libc++
