[Bug libstdc++/59182] New: can't convert from void (T::*)() to std::function<void()>

[Bug libstdc++/59182] New: can't convert from void (T::*)() to std::function<void()>

[sequoiahead at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59182

            Bug ID: 59182
           Summary: can't convert from void (T::*)() to
                    std::function<void()>
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sequoiahead at gmail dot com

Created attachment 31242
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=31242&action=edit
gcc -v output

I'm trying to compile following piece of code:

#include <functional>

struct Func {
    void func() {}
    void pfunc(int p) {}
};

int main(int argc, char** argv) {
    Func fInstance;
    std::function<void(int)> voidMemberParamFunc = std::bind(&Func::pfunc,
fInstance, std::placeholders::_1);
    std::function<void()> voidMemberFunc = std::bind(&Func::func, fInstance,
std::placeholders::_1); //compilation fails here
}

via "g++ -MMD -MP -D_DEBUG -Werror -Wall -g -std=c++11 -fno-rtti -o "main.o" -c
"main.cpp"

and receive following error:
main.cpp:24:96: error: conversion from 'std::_Bind_helper<false, void
(Func::*)(), Func&, const std::_Placeholder<1>&>::type {aka
std::_Bind<std::_Mem_fn<void (Func::*)()>(Func, std::_Placeholder<1>)>}' to
non-scalar type 'std::function<void()>' requested
  std::function<void()> voidMemberFunc = std::bind(&Func::func, fInstance,
std::placeholders::_1); //compilation fails here

The problem is that the compiler is able to convert void Func::pfunc(int) to
std::function<void(int)>, but can't convert void Func::func() to
std::function<void()>

[Bug libstdc++/59182] can't convert from void (T::*)() to std::function<void()>

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59182

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> ---
(In reply to sequoiahead from comment #0)
> The problem is that the compiler is able to convert void Func::pfunc(int) to
> std::function<void(int)>, but can't convert void Func::func() to
> std::function<void()>

No, that's not what's happening.

std::bind(&Func::pfunc, fInstance, std::placeholders::_1) returns a function
object, call it B1, that takes a single argument, as indicated by the
placeholder. B1 is convertible to function<void(int)>, because that type takes
also a single argument, so will pass its argument to B1.

B1 is not convertible to function<void()> because you have to call a
function<void()> with no arguments, and B1 cannot be called with no arguments.

std::function is rejecting your program because it's invalid.  std::bind
doesn't do the same checking, so doesn't reject it, but your program is still
invalid.

This compiles OK:

std::function<void()> voidMemberFunc = std::bind(&Func::func, fInstance);

This bind expression creates a function object that takes no arguments, so can
be stored in a function<void()>.