[Bug c++/56556] New: Wshadow warns for private members in base classes

[Bug c++/56556] New: Wshadow warns for private members in base classes

[david at doublewise dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56556

             Bug #: 56556
           Summary: Wshadow warns for private members in base classes
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: david@doublewise.net


The following code warns due to -Wshadow:

class Base {
private:
    int x;
};

class Derived : public Base {
    Derived(int x) {}
};

Perhaps we should consider not having this warn? In my particular case, I have
a Base class with a private member variable, a class Derived that derives from
Base, and a class EvenMoreDerived that derives from Derived. In a member
function of EvenMoreDerived, I am trying to use a variable with the same name
as the private member variable of Base.

The only situation I can think of where this could theoretically be useful is
if there is some kind of overload resolution issue, but due to derived classes
hiding member functions of base classes I don't imagine that would be an issue.

[Bug c++/56556] Wshadow warns for private members in base classes

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56556

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> 2013-03-07 00:46:12 UTC ---
The main reason for the warning here is namelookup happens before access
control happens.  So even if change the name of x usage in Derived, you will
start to get weird error messages.

[Bug c++/56556] Wshadow warns for private members in base classes

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56556

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|RESOLVED                    |NEW
         Resolution|INVALID                     |---
   Last reconfirmed|                            |2022-02-04
           Severity|normal                      |enhancement
           Keywords|                            |diagnostic

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The problem with -Wshadow=local is that it doesn't warn about this either:

class Derived {
        int x;
        Derived(int x) {}
};

And you might want a warning there, just not for the case where 'x' is a
private member in some base.

I think I agree with David that this shouldn't warn in the first place, you
shouldn't need to work around it.

I understand Andrew's explanation, but that's the reason GCC behaves like this
today, not a reason it *should* behave like this.

[Bug c++/56556] Wshadow warns for private members in base classes

[trash.paradise at protonmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56556

--- Comment #4 from Gaël Bonithon <trash.paradise at protonmail dot com> ---
It would be nice if this could be fixed. I'm currently experimenting with
porting a GTK text editor to Qt6, and it seems that because of this, I can't
name a local variable 'data' in a method of a QWidget derived class without
causing a warning:

mousepad-window.cpp:3740:12: warning: declaration of 'data' shadows a member of
'MousepadWindow' [-Wshadow]
 3740 |   gpointer data = GINT_TO_POINTER (gtk_text_buffer_get_modified
(active->buffer));
      |            ^~~~
In file included from /usr/include/qt6/QtWidgets/qmainwindow.h:8,
                 from /usr/include/qt6/QtWidgets/QMainWindow:1,
                 from ../mousepad/mousepad-window.hpp:27:
/usr/include/qt6/QtWidgets/qwidget.h:780:18: note: shadowed declaration is here
  780 |     QWidgetData *data;


Sorry, but this seems really silly. Clang doesn't warn in this case.