Re: gdb VS c++ template instances

Jim King <>
Mon, 7 Jun 2010 08:58:44 -0700 (PDT)
On Jun 7, 11:45 pm, Jim King <> wrote:

On Jun 7, 11:05 pm, Paul Bibbings <> wrote:

Jim King <> writes:

Sorry for my ambiguous words.

in xxxx.cpp there's code snippet:

template<class T>
class Temp
    Temp<T> *clone() const
        new Temp<t>(a, b);

    ... a;
    ... b;

In gdb, I type "ptype Temp<StdMap>", the message is:
class Temp<StdMap>
    ... a;
    ... b;

Then I interrupt on Temp<StdMap>::clone(), and type "ptype *this":
class Temp<StdMap>
    ... b;
    ... a;


See, the order of member variables are different. There must be
something wrong. Since they are all Temp<StdMap>, how can I tell whic=


is which?

As far as I understand it a conforming implementation gives a guarantee=


to the ordering of non-static data members that are contained within th=


same access-specifier (though not necessarily that they be contiguous.
The wording is:

   "Nonstatic data members of a (non-union) class declared without =


   intervening access-specifier are allocated so that later members=


   higher addresses within a class object. The order of allocation =


   nonstatic data members separated by an access-specifier is
   unspecified (11.1). Implementation alignment requirements might =


   two adjacent members not to be allocated immediately after each
   other; so might requirements for space for managing virtual func=


   (10.3) and virtual base classes (10.1)."

Having said that, the issue that you are facing seems, more likely, to
have something to do with how gdb is /reporting/ the class structure in
the two instances rather than with what is actually going on behind the
scenes. Perhaps you could post a complete, minimal code example that
can be compiled to illustrate what you are seeing.


Paul Bibbings

I'm sorry that it can't be extracted. Actually, the code is definitely
correct. In one branch, it works well, another branch, it fails. I
don't know the difference of the two branches (that are huge module
with more than 10 million lines), they are almost the same. In one
sentence, if I extract the code, it absolutely will work.

I don't expect member "a" is always ahead "b". I just hope the order
is fixed on runtime. It's odd that the two instances of template which
has the same parameter are not the same. There are some unconfirmed
reasons (probably wrong):
1. I have a cpp which use old template class which b comes first, it
has been deleted while the obj remains.
2. Some unexpected compile parameter are passed to g++ when compiling
the cpp.
3. Two header files contains the different definitions of the template

Any ideas?

Jim King

Thank you Paul. The real code is a little bit more complex. Template
class is inherited from a non-template interface class.

class interface
    virtual ~interface(void) {}
    virtual interface *clone(void) const = 0;

template<class T>
class Temp : public interface

And yes, the template class is distributed by the unique header file.
I searched the whole hard disk, no other definition.

Again, code is definitely correct. It works in the main branch.

I really hope "ptype" could print the source file name in which the
symbol is defined, just like "info types". If it is true, only ptype
*this could find the culprit.

Jim King

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