[Bug libstdc++/57283] New: missing typedefs in specialized std::mem_fn for member variables

[Bug libstdc++/57283] New: missing typedefs in specialized std::mem_fn for member variables

[record.nctu.cis91 at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57283

            Bug ID: 57283
           Summary: missing typedefs in specialized std::mem_fn for member
                    variables
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: record.nctu.cis91 at gmail dot com

Created attachment 30118
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30118&action=edit
code-reproduce-this-issue.cpp

I found that gcc complain that "no type named ‘argument_type’ in ‘class
std::_Mem_fn<int cls::*>’", with the code in attachment.

Then I try to locate the problem and find there is no any typedefs for
argument_type.
So I guess it's missing the required typedefs.
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idW276

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-05-15
                 CC|                            |jamborm at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.

[Bug libstdc++/57283] missing typedefs in specialized std::mem_fn for member variables

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57283

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The standard is clear, the typedef is not required when wrapping a pointer to
member data, see [func.memfn]

You can use a lambda expression in C++11:

    [](const cls& c) { return !c.value; };

In C++14 you will be able to write a generic lambda to negate arbitrary
function objects, replacing std::not1 completely:

    [](auto f) { return !f(); };

[Bug libstdc++/57283] missing typedefs in specialized std::mem_fn for member variables

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57283

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Lin Yi-Li from comment #2)
> Sorry I missed that 'pm is a pointer to member function...'.
> 
> I know that C++11 has lambda, but using std::mem_fn is sometimes much clear.

You can still use std::mem_fn, the problem is with std::not1, which only works
with the C++98 function object protocol.

> But according to standard, value from std::mem_fn for variable will not be
> able to work with std::not1. Even if the usage is similiar to the one for
> member function takes no argument.

It's similar but not the same, for a member function the result_type is the
return type and the argument_type is dictated by the cv-qualifiers of the
member function, i.e. for a const member function the argument_type is 'const
T*', but for a member variable the result_type depends on whether you call it
with a const or non-const argument.

Should mem_fn(&cls::value) have result_type=int& and argument_type=cls*
or result_type=const int& and argument_type=const cls*?
The result of mem_fn(&cls::value) is polymorphic.

> So is it possible to be an enhancement?

It might be possible to generalize std::unary_negate, but that's for the
standard committee not GCC's bugzilla.

[Bug libstdc++/57283] missing typedefs in specialized std::mem_fn for member variables

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57283

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
N.B. you can also do it with a nested std::bind expression and std::logical_not

std::bind(std::logical_not<int>(),
          std::bind(std::mem_fn(&cls::value), std::placeholders::_1))

in C++14 that means you can provide a working not1 replacement:

template<typename F>
auto not1(F f)
{
    return std::bind(std::logical_not<>(), std::bind(f,
std::placeholders::_1));
}

[Bug libstdc++/57283] missing typedefs in specialized std::mem_fn for member variables

[record.nctu.cis91 at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57283

--- Comment #5 from Lin Yi-Li <record.nctu.cis91 at gmail dot com> ---
(In reply to Jonathan Wakely from comment #4)
> N.B. you can also do it with a nested std::bind expression and
> std::logical_not
> 
> std::bind(std::logical_not<int>(),
>           std::bind(std::mem_fn(&cls::value), std::placeholders::_1))
> 
> in C++14 that means you can provide a working not1 replacement:
> 
> template<typename F>
> auto not1(F f)
> {
>     return std::bind(std::logical_not<>(), std::bind(f,
> std::placeholders::_1));
> }

I understand the problem of std::mem_fn + member variable now.
Thank you for your explanation.