How to cast a pointer to a base class with multiple inheritance

How to cast a pointer to a base class with multiple inheritance

[Matthieu VIAL]

Hello,

I would like to know if it is possible to cast a pointer the way the
compiler do for classes with multiple inheritance with archer. The this
pointer changes depending on the base class. So how to find the other
pointers with the top pointer ?

The following code show the problem:

--------------------------

#include <iostream>

using namespace std;

class A
{
public:
    A(int a);

    int m_intA;
};

class B 
{
public:
    B(int b);

    int m_intB;
};

class C : public A, B
{
public:
    C(int c);

    int m_intC;
};


A::A(int a)
{
    cout << "ctor A :" << hex << this << endl;
    m_intA = a;
}

B::B(int b)
{
    cout << "ctor B :" << hex << this << endl;
    m_intB = b;
}

C::C(int c) : A(c+1), B(c+2)
{
    cout << "ctor C :" << hex << this << endl;
    m_intC = c;
}


int main()
{
    C c(1);

    cout << "@C     :" << hex << &c << endl;
    cout << "@C as A:" << hex << (A*)&c << endl;
    cout << "@C as B:" << hex << (B*)&c << endl;

    return 0;
}

--------------------------

Output :
ctor A :0xbfd572d8
ctor B :0xbfd572dc
ctor C :0xbfd572d8
@C     :0xbfd572d8
@C as A:0xbfd572d8
@C as B:0xbfd572dc

--------------------------


Thanks,
Matthieu VIAL

Re: How to cast a pointer to a base class with multiple inheritance

[Tom Tromey]

>>>>> "Matthieu" == Matthieu VIAL <matthieu.vial@orange-ftgroup.com> writes:

Matthieu> I would like to know if it is possible to cast a pointer the
Matthieu> way the compiler do for classes with multiple inheritance with
Matthieu> archer. The this pointer changes depending on the base
Matthieu> class. So how to find the other pointers with the top pointer
Matthieu> ?

I think Value.cast should do the right thing here.  I haven't tried this
myself, though... did you try it?  It wasn't clear from your message.

If it doesn't work, please file a bug report in the gdb bugzilla using
'archer' as the version, and one of us will fix it.

Tom

Re: How to cast a pointer to a base class with multiple inheritance

[Matthieu VIAL]

> Matthieu> I would like to know if it is possible to cast a pointer the
> Matthieu> way the compiler do for classes with multiple inheritance with
> Matthieu> archer. The this pointer changes depending on the base
> Matthieu> class. So how to find the other pointers with the top pointer
> Matthieu> ?
> 
> Tom> I think Value.cast should do the right thing here.  I haven't tried this
> Tom> myself, though... did you try it?  It wasn't clear from your message.
> 

Thank for your answer.

Sorry Value.cast seems to work but my problem is actually a bit more
complex.

I have many classes which implement an interface and inheritate a base
class.

I need to compare pointers to the interface with pointers to the base
class. But these classes are not directly related so Value.cast is
useless. The solution is to up cast the base class pointer to the final
class and then down cast to the interface. But i need run time type
information for that. The typeid operator doesn't seem to be available
in archer.

Here is a new code sample

#include <iostream>

using namespace std;

class BaseClass
{
public:
    BaseClass(int a);

    int m_intA;
};

class Interface
{
public:
    virtual void Method() = 0;
};

class Class1 : public Interface, public BaseClass
{
public:
    Class1(int c);
    
    virtual void Method();

    int m_intC;
};

class Class2 : public Interface, public BaseClass
{
public:
    Class2(int c);

    virtual void Method();

    int m_intC;
};


BaseClass::BaseClass(int a)
{
    cout << "ctor BaseClass :" << hex << this << endl;
    m_intA = a;
}

Class1::Class1(int c) : BaseClass(c+1)
{
    cout << "ctor Class1 :" << hex << this << endl;
    m_intC = c;
}

void Class1::Method()
{
}

Class2::Class2(int c) : BaseClass(c+2)
{
    cout << "ctor Class2 :" << hex << this << endl;
    m_intC = c;
}

void Class2::Method()
{
}


int main()
{
    Class1 c1(1);
    Class2 c2(1);

    Interface* pIc1 = &c1;
    Interface* pIc2 = &c2;
    BaseClass* pBc1 = &c1;
    BaseClass* pBc2 = &c2;

    cout << "@C1 :" << hex << &c1 << endl;
    cout << "@C1 as Interface :" << hex << pIc1 << endl;
    cout << "@C1 as BaseClass :" << hex << pBc1 << endl;
    cout << "@C2 :" << hex << &c2 << endl;
    cout << "@C2 as Interface :" << hex << pIc2 << endl;
    cout << "@C2 as BaseClass :" << hex << pBc2 << endl;

    return 0;
}

Re: How to cast a pointer to a base class with multiple inheritance

[Tom Tromey]

>>>>> "Matthieu" == Matthieu VIAL <matthieu.vial@orange-ftgroup.com> writes:

Matthieu> Sorry Value.cast seems to work but my problem is actually a bit more
Matthieu> complex.

Ok.

Matthieu> I need to compare pointers to the interface with pointers to
Matthieu> the base class. But these classes are not directly related so
Matthieu> Value.cast is useless. The solution is to up cast the base
Matthieu> class pointer to the final class and then down cast to the
Matthieu> interface. But i need run time type information for that. The
Matthieu> typeid operator doesn't seem to be available in archer.

Nope.  The Python API just exposes things that already exist in gdb.
typeid is not implemented yet, there's a bug for it:

http://sourceware.org/bugzilla/show_bug.cgi?id=9065


However, gdb does have "set print object on"... if this prints the right
thing for you, I think we could hook up the internal code to find the
"real object" as a Value method.

Tom

Re: How to cast a pointer to a base class with multiple inheritance

[Richard Ward]

I have a patch I've been meaning to submit for a while, it adds
gdb.Value.rtti_type which returns the run time type of an object (as a
gdb.Type) using similar code to `set print object on'. The object can
then be cast using gdb.Value.cast.
I'm not at the right pc just now, but I'll try and mail it to this
list later today.

Just in case it helps in the mean time, it is possible to look at the
first 4 (8 on 64-bit) bytes of an object which has a vtable (which
should be a pointer to the vtable), and run `addr2line -Cfe
name-of-executable 0xBLAHBLAH' which should then say something like
`0xBLAHBLAH: vtable for class FOO'. I think you can also find the
vtable this way using nm or objdump, there may be a similar way to do
this within gdb, but I wouldn't know how. Sorry if these instructions
are rough/wrong, I'm at a non gnu/linux machine at the moment.

Richard Ward.


2009/8/3 Tom Tromey <tromey@redhat.com>:
>>>>>> "Matthieu" == Matthieu VIAL <matthieu.vial@orange-ftgroup.com> writes:
>
> Matthieu> Sorry Value.cast seems to work but my problem is actually a bit more
> Matthieu> complex.
>
> Ok.
>
> Matthieu> I need to compare pointers to the interface with pointers to
> Matthieu> the base class. But these classes are not directly related so
> Matthieu> Value.cast is useless. The solution is to up cast the base
> Matthieu> class pointer to the final class and then down cast to the
> Matthieu> interface. But i need run time type information for that. The
> Matthieu> typeid operator doesn't seem to be available in archer.
>
> Nope.  The Python API just exposes things that already exist in gdb.
> typeid is not implemented yet, there's a bug for it:
>
> http://sourceware.org/bugzilla/show_bug.cgi?id=9065
>
>
> However, gdb does have "set print object on"... if this prints the right
> thing for you, I think we could hook up the internal code to find the
> "real object" as a Value method.
>
> Tom
>

Re: How to cast a pointer to a base class with multiple inheritance

[Matthieu VIAL]

Your explanations sound interesting because for the moment I store rtti
from g++ in my code (with mangling problem).

Then I use the bitpos attribute of gdb.Field to find the appropriate
class in the inheritance tree.

I wrote the following code to have dynamic upcast feature not available
with gdb.Value.cast
May be this kind of feature could be useful in archer ?


--CODE--

def find_class_offset(init, src, dst):
    if str(src) == str(dst):
        return [True, init]
    
    for field in src.fields():
        if field.is_base_class:
            result = find_class_offset(init - field.bitpos / 8,
field.type, dst)
            if result[0]:
                return result
    
    return [False, init]
    

def dynamic_upcast(cast_type, value):
    assert(value.type.code == gdb.TYPE_CODE_PTR)
    
    if (cast_type.code == gdb.TYPE_CODE_PTR) or (cast_type.code ==
gdb.TYPE_CODE_REF):
        src = cast_type.target()
    else:
        src = cast_type
    dst = value.type.target()
    
    result = find_class_offset(0, src, dst)
    
    if not result[0]:
        raise Exception('Invalid dynamic cast')
    else:
        addr = int(str(value), 16)
        addr += result[1]
        eval_str = '(\'%s\'*)0x%X' % (src.tag, addr)
        return gdb.parse_and_eval(eval_str)

-- CODE --


Matthieu



Le mardi 04 août 2009 à 09:41 +0100, Richard Ward a écrit :
> I have a patch I've been meaning to submit for a while, it adds
> gdb.Value.rtti_type which returns the run time type of an object (as a
> gdb.Type) using similar code to `set print object on'. The object can
> then be cast using gdb.Value.cast.
> I'm not at the right pc just now, but I'll try and mail it to this
> list later today.
> 
> Just in case it helps in the mean time, it is possible to look at the
> first 4 (8 on 64-bit) bytes of an object which has a vtable (which
> should be a pointer to the vtable), and run `addr2line -Cfe
> name-of-executable 0xBLAHBLAH' which should then say something like
> `0xBLAHBLAH: vtable for class FOO'. I think you can also find the
> vtable this way using nm or objdump, there may be a similar way to do
> this within gdb, but I wouldn't know how. Sorry if these instructions
> are rough/wrong, I'm at a non gnu/linux machine at the moment.
> 
> Richard Ward.
> 
> 
> 2009/8/3 Tom Tromey <tromey@redhat.com>:
> >>>>>> "Matthieu" == Matthieu VIAL <matthieu.vial@orange-ftgroup.com> writes:
> >
> > Matthieu> Sorry Value.cast seems to work but my problem is actually a bit more
> > Matthieu> complex.
> >
> > Ok.
> >
> > Matthieu> I need to compare pointers to the interface with pointers to
> > Matthieu> the base class. But these classes are not directly related so
> > Matthieu> Value.cast is useless. The solution is to up cast the base
> > Matthieu> class pointer to the final class and then down cast to the
> > Matthieu> interface. But i need run time type information for that. The
> > Matthieu> typeid operator doesn't seem to be available in archer.
> >
> > Nope.  The Python API just exposes things that already exist in gdb.
> > typeid is not implemented yet, there's a bug for it:
> >
> > http://sourceware.org/bugzilla/show_bug.cgi?id=9065
> >
> >
> > However, gdb does have "set print object on"... if this prints the right
> > thing for you, I think we could hook up the internal code to find the
> > "real object" as a Value method.
> >
> > Tom
> >

Re: How to cast a pointer to a base class with multiple inheritance

[Tom Tromey]

>>>>> "Richard" == Richard Ward <richard.j.ward1@googlemail.com> writes:

Richard> I have a patch I've been meaning to submit for a while, it adds
Richard> gdb.Value.rtti_type which returns the run time type of an object (as a
Richard> gdb.Type) using similar code to `set print object on'. The object can
Richard> then be cast using gdb.Value.cast.

Sounds useful.  I'm not sure about the method name, but this is exactly
what I was picturing.

Tom