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From: "aeiken at motortech dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/106924] New: Nested class: virtual function returns wrong
 pointer of covariant type
Date: Tue, 13 Sep 2022 10:07:51 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106924

            Bug ID: 106924
           Summary: Nested class: virtual function returns wrong pointer
                    of covariant type
           Product: gcc
           Version: 10.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: aeiken at motortech dot de
  Target Milestone: ---

Created attachment 53570
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D53570&action=3Dedit
test.ii file generated with g++ -save-temps test.cpp

The following was found with gcc-arm-none-eabi-10.3-2021.07, tests with oth=
er
versions for linux (Ubuntu 20.04.3 LTS) did also not work (e.g. g++ 9.4.0 on
Ubuntu).

Take a look at the example code in test.cpp:
A class (ABFabNested) is nested inside a class (class DerivedAB with multip=
le
inheritances (class BaseA and BaseB)).
Because of the class layout, the adresses of DerivedAB and BaseB are differ=
ent.
The nested class itself is derived from a Base class (FabForB) which has a
virtual function(getPtr), which return a pointer to class BaseA.
The nested class overrides this virtual function, but returns a pointer to
class DerivedAB. This is allowed, because DerivedAB and BaseB are covariant=
s.

If you have an instance of a pointer to FabForB, which points to an instanc=
e of
ABFabNested, getPtr returns the address of DerivedAB, not BaseA.
If you implement the same class (here ABFab) outside of DerivedAB (not nest=
ed)
and repeat the step before, getPtr returns the address of BaseA, which is w=
hat
I expected.

test.cpp was compiled with the 9.4.0 version of the g++ without any paramet=
ers:

g++ a.out

Output of a.out:
TEST fab:        addr e89f6030 base e89f6040
TEST             addr e89f6040=20
TEST fab nested: addr e89f6010 base e89f6020
TEST nested      addr e89f6010

g++ --version
g++ (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

The test was repeated with other g++ versions (also newer version) with the
same result.


test.cpp:
#include <stdio.h>
  class BaseA
  {
  public:
    virtual BaseA* getPtr(){return this;}
    virtual ~BaseA(){}
  };
  class BaseB
  {
  public:
    int i =3D 0;
    virtual int geti(){return i;};
    virtual ~BaseB(){}
  };
  class FabForB
  {
  public:
    virtual BaseA* getPtr()=3D0;
    virtual ~FabForB(){}
  };
  class DerivedAB: public BaseB, public BaseA
  {
  public:
    virtual DerivedAB* getPtr(){printf("TEST D this %x\n", this);return thi=
s;}
    virtual int geti(){return 5;};

    class ABFabNested: public FabForB
    {
    public:
      virtual DerivedAB* getPtr()
      {
        static DerivedAB staticD;
        BaseA* basePtr =3D &staticD;
        printf("TEST fab nested: addr %x base %x\n", &staticD, basePtr);
        return &staticD;
      }
      virtual int geti(){return 5;};
    };
  };
  class ABFab: public FabForB
  {
  public:
    virtual DerivedAB* getPtr()
    {
      static DerivedAB staticD;
      BaseA* basePtr =3D &staticD;
      printf("TEST fab:        addr %x base %x\n", &staticD, basePtr);
      return &staticD;
    }
    virtual int geti(){return 5;};
  };
int main()
{
  ABFab fab;
  DerivedAB::ABFabNested fabNested;

  FabForB* fabPtr =3D &fab;
  FabForB* fabNestedPtr =3D &fabNested;
  printf("TEST             addr %x \n", fabPtr->getPtr());
  printf("TEST nested      addr %x \n", fabNestedPtr->getPtr());=20=20
  return 0;
}=