http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52917
Bug #: 52917
Summary: explicitly stated return type in std::mem_fn cannot be
compiled
Classification: Unclassified
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: freunddeslic...@web.de
#include <functional>
struct X {
int a;
int& easy() {return a;}
int& get() {return a;}
const int& get() const {return a;}
};
int main(void)
{
auto x = std::mem_fn (&X::easy); // (1) works
auto y = std::mem_fn <int&>(&X::get ); // (2) ERROR!!
auto z = std::mem_fun<int&>(&X::get ); // (3) deprecated in c++11, but works:
return 0;
}
################################################################################
(2) needs the return type to diffrentiate between the const and the non-const
getter.
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]
(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".
Both g++-4.7 and clang++-3.1 (when compiling with libstdc++) try to instantiate
the first version by mistake.
This definitely a library error, because clang when compiling with libc++
correctly instantiates the second version of mem_fn.
##################################################################################
g++-4.7 -std=c++0x bug.cpp
bug.cpp: In function ‘int main()’:
bug.cpp:17:39: error: no matching function for call to ‘mem_fn(<unresolved
overloaded function type>)’
bug.cpp:17:39: note: candidate is:
In file included from bug.cpp:3:0:
/usr/include/c++/4.7/functional:821:5: note: template<class _Tp, class _Class>
std::_Mem_fn<_Tp _Class::*> std::mem_fn(_Tp _Class::*)
/usr/include/c++/4.7/functional:821:5: note: template argument
deduction/substitution failed:
/usr/include/c++/4.7/functional: In substitution of ‘template<class _Tp, class
_Class> std::_Mem_fn<_Tp _Class::*> std::mem_fn(_Tp _Class::*) [with _Tp =
int&; _Class = <missing>]’:
bug.cpp:17:39: required from here
/usr/include/c++/4.7/functional:821:5: error: creating pointer to member
reference type ‘int&’
bug.cpp:17:39: error: unable to deduce ‘auto’ from ‘<expression error>’
####################################################################################
clang++ -std=c++0x bug.cpp
bug.cpp:17:12: error: no matching function for call to 'mem_fn'
auto y = std::mem_fn <int&>(&X::get ); // (2) ERROR!!
^~~~~~~~~~~~~~~~~~
/usr/lib/gcc/i686-linux-gnu/4.7/../../../../include/c++/4.7/functional:820:5:
note: candidate template ignored: substitution failure [with _Tp = int &]
mem_fn(_Tp _Class::* __pm)
^
1 error generated.
