[Bug c++/47940] New: can call a pure virtual from a constructor/destructor

[Bug c++/47940] New: can call a pure virtual from a constructor/destructor

[mlg7 at yandex dot ru]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47940

           Summary: can call a pure virtual from a constructor/destructor
           Product: gcc
           Version: 4.4.5
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: mlg7@yandex.ru


A colleague of mine have been examining a crash that
boils down to the following:
--hellobug.cpp--
class Base {
public:
    Base() { usefunc(); }
    virtual void func()=0;
    void usefunc() { func(); }
};

class Derived: public Base {
public:
    virtual void func() {}
};

int main() {
    Derived d;
    return 0;
}
--eof--

$ g++ -Wall -Wextra hellobug.cpp
{just nothing}
$ ./a.out 
pure virtual method called
terminate called without an active exception
Aborted

That is, "you asked for it, you got it" [1].
==
[1] from "Real Programmers Don't"
==

A variation:

==byebug.cpp==
#include <stdio.h>
class Base {
public:
    ~Base() { usefunc(); }
    virtual void func()=0;
    void usefunc() { func(); }
};

class Derived: public Base {
public:
    virtual void func() {}
};

int main() {
    Derived d;
    printf("life is good\n");
    return 0;
}
==eof==

$ g++ -Wall -Wextra byebug.cpp
{again nothing}
$ ./a.out 
life is good
pure virtual method called
terminate called without an active exception
Aborted


I have to note that in a big software project, 
the mistake is not so obvious.
But what can be done?


Functions that call pure virtual functions cannot
be called from constructors and destructors.
This may be discovered at compile-time, and the above
text makes a good error/warning message.



WHAT I GET: no warning
WHAT I WOULD EXPECT: a warning or an error message:
"Functions that call pure virtual functions cannot
be called from constructors and destructors"


$ gcc -v
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro
4.4.4-14ubuntu5' --with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.4 --enable-shared --enable-multiarch
--enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.4 --libdir=/usr/lib --enable-nls
--with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc
--disable-werror --with-arch-32=i686 --with-tune=generic
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 4.4.5 (Ubuntu/Linaro 4.4.4-14ubuntu5)

[Bug c++/47940] can call a pure virtual from a constructor/destructor

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47940

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> 2011-03-01 10:13:57 UTC ---
This is hard to detect really.  Basically what the function usefunc needs to
marked as calling a virtual function.  If the call was direct to func instead,
it would be better.

[Bug c++/47940] can call a pure virtual from a constructor/destructor

[mlg7 at yandex dot ru]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47940

--- Comment #2 from mlg <mlg7 at yandex dot ru> 2011-03-01 11:34:30 UTC ---
Yes it _is_ hard to detect. My colleague was suspecting
a build system bug/feature at first (hours for a full rebuild).
If it was a direct call, there would be a linker error.

>If the call was direct to func instead,
>it would be better.

Unfortunately or fortunately, refactoring a huge 
real world application to make bugs easier 
for the compiler to detect is not an option.

[Bug c++/47940] warn about calls to a pure virtual from a constructor/destructor

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47940

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
            Summary|can call a pure virtual     |warn about calls to a pure
                   |from a                      |virtual from a
                   |constructor/destructor      |constructor/destructor

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-03-01 12:19:40 UTC ---
(In reply to comment #2)
> If it was a direct call, there would be a linker error.

Not if the pure virtual has a definition, and not with all compilers.
G++ does warn about that case though.

None of the compilers I tested issued a diagnostic about indirectly making a
virtual call to a pure virtual function.

[Bug c++/47940] warn about calls to a pure virtual from a constructor/destructor

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47940

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-03-01 12:30:06 UTC ---
(In reply to comment #0)
> 
> Functions that call pure virtual functions cannot
> be called from constructors and destructors.
> This may be discovered at compile-time, and the above
> text makes a good error/warning message.

I'm not sure it makes a good diagnostic, consider:

struct abc {
  abc(bool nasal_demons) { if (nasal_demons) fly(); }
  void fly() { doFly(); }
  virtual void doFly() = 0;
};

Ideally the diagnostic for this would say "may call a pure virtual" for cases
where it can't be determined.

But then I don't really like the current diagnostic for direct calls either:

warning: abstract virtual 'virtual void abc::doFly()' called from constructor

I don't like the duplication of the word "virtual" and I don't like the term
"abstract virtual" - the class is abstract, the function is pure virtual.

[Bug c++/47940] warn about calls to a pure virtual from a constructor/destructor

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47940

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-03-01 12:32:19 UTC ---
(In reply to comment #4)
> "abstract virtual" - the class is abstract, the function is pure virtual.

I forgot I already changed that ;)
http://gcc.gnu.org/ml/gcc-cvs/2010-05/msg00437.html

[Bug c++/47940] warn about calls to a pure virtual from a constructor/destructor

[mlg7 at yandex dot ru]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47940

--- Comment #6 from mlg <mlg7 at yandex dot ru> 2011-03-01 17:23:41 UTC ---
(In reply to comment #4)
> (In reply to comment #0)
> > 
> > Functions that call pure virtual functions cannot
> > be called from constructors and destructors.
> > This may be discovered at compile-time, and the above
> > text makes a good error/warning message.
> 
> I'm not sure it makes a good diagnostic, consider:
> 
> struct abc {
>   abc(bool nasal_demons) { if (nasal_demons) fly(); }
>   void fly() { doFly(); }
>   virtual void doFly() = 0;
> };
>

That is the most dangerous situation: the crashing call is
buried [deep] in the control flow.
The deeper it is buried, the more valuable a warning/error
message would be.

Ok, "a pure virtual function may be called indirectly 
from a constructor/destructor"

> Ideally the diagnostic for this would say "may call a pure virtual" for cases
> where it can't be determined.

Probably you are right, a kind of "I know what I'm doing"
pragma would be useful as well.

I also want to note that the top-level function may be virtual:
[NOTE: usefunc() is virtual here]
==byebug2.cpp==
#include <stdio.h>
class Base {
public:
    ~Base() { usefunc(); }
    virtual void func()=0;
    virtual void usefunc() { func(); }
};

class Derived: public Base {
public:
    virtual void func() {}
};

int main() {
    Derived d;
    printf("life is good\n");
    return 0;
}
==eof==
$ g++ -Wall -Wextra byebug2.cpp
$ ./a.out 
life is good
pure virtual method called
terminate called without an active exception
Aborted

[Bug c++/47940] warn about calls to a pure virtual from a constructor/destructor

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47940

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |redi at gcc dot gnu.org

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 115435 has been marked as a duplicate of this bug. ***

[Bug c++/47940] warn about calls to a pure virtual indirectly from a constructor/destructor

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47940

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2024-06-11
             Blocks|                            |87403
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=1925,
                   |                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=6006
     Ever confirmed|0                           |1
            Summary|warn about calls to a pure  |warn about calls to a pure
                   |virtual from a              |virtual indirectly from a
                   |constructor/destructor      |constructor/destructor

--- Comment #8 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87403
[Bug 87403] [Meta-bug] Issues that suggest a new warning