using a class inside a class.

[phil.pellouchoud@gmail.com]

I did some searching online and i couldn't find anything in reference
to this.

I am using MinGW, gcc 4.3 and am having the following compilation
issue:

class CFoo
{
public:
...
private:
std::list<CFoom _children;
};

The complaint from the compiler looks like this:

c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
boost_concept_c heck.h: In instantiation of
'__gnu_cxx::_SG IAssignableConc ept<CFoo>':
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
stl_list.h:420: instantiated from 'std::list<CFoo ,
std::allocator< CFoo'
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
boost_concept_c heck.h:216: error:
'__gnu_cxx::_SG IAssignableConc ept<_Tp>::__a' has incomplete type

So, basically, I think what it's saying is that it doesn't know how to
build the class because CFoo hasn't been defined by the time it's
trying to create it (i.e. incomplete type). I tried adding
"std::list<clas s CFoo>" as well, but that didn't work either. This
compiles on visual studio with some warnings.

I can make this work by making it "std::list<CFoo *>", but I'm
wondering if there's a way to get this to work as defined.

thanks for listening.

-phil

[Jim Langston]

Re: using a class inside a class.

phil.pellouchou d@gmail.com wrote:

I did some searching online and i couldn't find anything in reference
to this.
>
I am using MinGW, gcc 4.3 and am having the following compilation
issue:
>
class CFoo
{
public:
...
private:
std::list<CFoom _children;
};
>
The complaint from the compiler looks like this:
>
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
boost_concept_c heck.h: In instantiation of
'__gnu_cxx::_SG IAssignableConc ept<CFoo>':
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
stl_list.h:420: instantiated from 'std::list<CFoo ,
std::allocator< CFoo'
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
boost_concept_c heck.h:216: error:
'__gnu_cxx::_SG IAssignableConc ept<_Tp>::__a' has incomplete type
>
So, basically, I think what it's saying is that it doesn't know how to
build the class because CFoo hasn't been defined by the time it's
trying to create it (i.e. incomplete type). I tried adding
"std::list<clas s CFoo>" as well, but that didn't work either. This
compiles on visual studio with some warnings.
>
I can make this work by making it "std::list<CFoo *>", but I'm
wondering if there's a way to get this to work as defined.
>
thanks for listening.

All I can tell you is I've done this exact same thing in the past
successfully in both Microsoft Visual C++ .net 2003 and DevC++. Here is
something that compiles sucessfully in Microsoft Visual C++ .net 2003:

#include <list>

class CFoo
{
private:
std::list<CFoom _children;
};

int main()
{
CFoo Foo;
}

No warnings, no errors.

I don't know why your version of gcc is complaining, I don't think it
should, AFAIK this is perfectly legal, although I haven't read the standard
on it.

--
Jim Langston
tazmaster@rocke tmail.com

[phil.pellouchoud@gmail.com]

Re: using a class inside a class.

Here's exactly what i get when I try to compile that code:

C:\>type foo.cpp
#include <list>

class CFoo
{
private:
std::list<CFoom _children;

};

int main()
{
CFoo Foo;

}
C:\>gcc -v
Using built-in specs.
Target: i386-pc-mingw32
Configured with: ../gcc-4.3.0/configure --prefix=/mingw --disable-nls
--enable-languages=c,c++ ,fortran,objc,o bj-c++,ada,
java --disable-win32-registry --disable-sjlj-exceptions --enable-
libstdcxx-debug --enable-version-specific-runtime-libs
--with-gmp=/mingw/src/gcc/gmp-mpfr-root --with-mpfr=/mingw/src/gcc/gmp-
mpfr-root --enable-libgcj --enable-concept-checks
--enable-shared
Thread model: win32
gcc version 4.3.0 (GCC)

C:\>gcc -o foo foo.cpp
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
boost_concept_c heck.h: In instantiation of '__gnu_cxx::_S
GIAssignableCon cept<CFoo>':
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
stl_list.h:420: instantiated from 'std::list<CFoo , std:
:allocator<CFoo '
foo.cpp:6: instantiated from here
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
boost_concept_c heck.h:216: error: '__gnu_cxx::_SG IAssigna
bleConcept<_Tp> ::__a' has incomplete type
foo.cpp:4: error: forward declaration of 'class CFoo'
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
boost_concept_c heck.h: In member function 'void __gnu_cxx
::_SGIAssignabl eConcept<_Tp>:: __constraints() [with _Tp = CFoo]':
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
stl_list.h:420: instantiated from 'std::list<CFoo , std:
:allocator<CFoo '
foo.cpp:6: instantiated from here
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
boost_concept_c heck.h:208: error: using invalid field '__
gnu_cxx::_SGIAs signableConcept <_Tp>::__a'
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
boost_concept_c heck.h:209: error: using invalid field '__
gnu_cxx::_SGIAs signableConcept <_Tp>::__a'
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
boost_concept_c heck.h:209: error: using invalid field '__
gnu_cxx::_SGIAs signableConcept <_Tp>::__a'
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
stl_list.h:420: instantiated from 'std::list<CFoo , std:
:allocator<CFoo '
foo.cpp:6: instantiated from here
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
boost_concept_c heck.h:210: error: using invalid field '__
gnu_cxx::_SGIAs signableConcept <_Tp>::__a'
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
boost_concept_c heck.h: In member function 'void __gnu_cxx
::_SGIAssignabl eConcept<_Tp>:: __const_constra ints(const _Tp&) [with
_Tp = CFoo]':
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
boost_concept_c heck.h:210: instantiated from 'void __gn
u_cxx::_SGIAssi gnableConcept<_ Tp>::__constrai nts() [with _Tp = CFoo]'
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
stl_list.h:420: instantiated from 'std::list<CFoo , std:
:allocator<CFoo '
foo.cpp:6: instantiated from here
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
boost_concept_c heck.h:214: error: using invalid field '__
gnu_cxx::_SGIAs signableConcept <_Tp>::__a'

C:\>

[Ian Collins]

Re: using a class inside a class.

phil.pellouchou d@gmail.com wrote:

Here's exactly what i get when I try to compile that code:
>
C:\>type foo.cpp
#include <list>
>
class CFoo
{
private:
std::list<CFoom _children;
>
};
>
int main()
{
CFoo Foo;
>
}
>
C:\>gcc -o foo foo.cpp
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
boost_concept_c heck.h: In instantiation of '__gnu_cxx::_S
GIAssignableCon cept<CFoo>':

<snip>

Looks like your compiler isn't configured correctly, try a gcc group.

--
Ian Collins.

[kwikius]

Re: using a class inside a class.

On Apr 28, 7:33 pm, "Jim Langston" <tazmas...@rock etmail.comwrote :

phil.pellouch.. .@gmail.com wrote:

I did some searching online and i couldn't find anything in reference
to this.

>

I am using MinGW, gcc 4.3 and am having the following compilation
issue:

>

class CFoo
{
public:
   ...
private:
   std::list<CFoom _children;
};

>

The complaint from the compiler looks like this:

>

c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
boost_concept_c heck.h: In instantiation of
'__gnu_cxx::_SG IAssignableConc ept<CFoo>':
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
stl_list.h:420:   instantiated from 'std::list<CFoo ,
std::allocator< CFoo'
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
boost_concept_c heck.h:216: error:
'__gnu_cxx::_SG IAssignableConc ept<_Tp>::__a' has incomplete type

>

So, basically, I think what it's saying is that it doesn't know how to
build the class because CFoo hasn't been defined by the time it's
trying to create it (i.e. incomplete type).  I tried adding
"std::list<clas s CFoo>" as well, but that didn't work either.  This
compiles on visual studio with some warnings.

>

I can make this work by making it "std::list<CFoo *>", but I'm
wondering if there's a way to get this to work as defined.

>

thanks for listening.

>
All I can tell you is I've done this exact same thing in the past
successfully in both Microsoft Visual C++ .net 2003 and DevC++.  Here is
something that compiles sucessfully in Microsoft Visual C++ .net 2003:
>
#include <list>
>
class CFoo
{
private:
    std::list<CFoom _children;
>
};
>
int main()
{
    CFoo Foo;
>
}
>
No warnings, no errors.
>
I don't know why your version of gcc is complaining, I don't think it
should, AFAIK this is perfectly legal, although I haven't read the standard
on it.

IMO gcc is correct. You could have a list of pointers or refs to CFoo
but not recursive types. One workaround *may* be to use a typedef
alias for CFoo. That may fool gcc, but I think its technically correct
to call this an error.

regards
Andy Little

[Ian Collins]

Re: using a class inside a class.

kwikius wrote:

On Apr 28, 7:33 pm, "Jim Langston" <tazmas...@rock etmail.comwrote :
>

>All I can tell you is I've done this exact same thing in the past
>successfully in both Microsoft Visual C++ .net 2003 and DevC++. Here is
>something that compiles sucessfully in Microsoft Visual C++ .net 2003:
>>
>#include <list>
>>
>class CFoo
>{
>private:
> std::list<CFoom _children;
>>
>};
>>
>int main()
>{
> CFoo Foo;
>>
>}
>>
>No warnings, no errors.
>>
>I don't know why your version of gcc is complaining, I don't think it
>should, AFAIK this is perfectly legal, although I haven't read the standard
>on it.

>
IMO gcc is correct. You could have a list of pointers or refs to CFoo
but not recursive types. One workaround *may* be to use a typedef
alias for CFoo. That may fool gcc, but I think its technically correct
to call this an error.
>

Why? The class declares m_children to be a list of CFoo. It does not
attempt to use any members of std::list that require a complete type.

In effect, it's just like writing

#include <list>

class X;

class CFoo
{
private:
std::list<Xm_ch ildren;
};

class X {};

int main()
{
CFoo Foo;
}

The complete class definition is not required until std::list's default
constructor is instantiated in main().

--
Ian Collins.

[kwikius]

Re: using a class inside a class.


"Ian Collins" <ian-news@hotmail.co mwrote in message
news:67mv5uF2p7 o2gU7@mid.indiv idual.net...

kwikius wrote:

>On Apr 28, 7:33 pm, "Jim Langston" <tazmas...@rock etmail.comwrote :
>>

>>All I can tell you is I've done this exact same thing in the past
>>successfull y in both Microsoft Visual C++ .net 2003 and DevC++. Here is
>>something that compiles sucessfully in Microsoft Visual C++ .net 2003:
>>>
>>#include <list>
>>>
>>class CFoo
>>{
>>private:
>> std::list<CFoom _children;
>>>
>>};
>>>
>>int main()
>>{
>> CFoo Foo;
>>>
>>}
>>>
>>No warnings, no errors.
>>>
>>I don't know why your version of gcc is complaining, I don't think it
>>should, AFAIK this is perfectly legal, although I haven't read the
>>standard
>>on it.

>>
> IMO gcc is correct. You could have a list of pointers or refs to CFoo
>but not recursive types. One workaround *may* be to use a typedef
>alias for CFoo. That may fool gcc, but I think its technically correct
>to call this an error.
>>

Why? The class declares m_children to be a list of CFoo. It does not
attempt to use any members of std::list that require a complete type.

Ah as to Why ... I Dunno. Hopefully someone of a more technical nature will
help out as to why. From the error messages gcc is using C++0x style Concept
checking.

Various things to try to diagnose the problem. Try some other dummy template
class than std::list.
If that succeeds then std::list is concept checking and wont allow itself to
be instantiated without a complete type or some such

There is presumably an option to turn it off . ..

regards
Andy Little

[Ian Collins]

Re: using a class inside a class.

kwikius wrote:

"Ian Collins" <ian-news@hotmail.co mwrote:

>>>

>Why? The class declares m_children to be a list of CFoo. It does not
>attempt to use any members of std::list that require a complete type.

>
Ah as to Why ... I Dunno. Hopefully someone of a more technical nature will
help out as to why. From the error messages gcc is using C++0x style Concept
checking.
>

It does, doesn't it? I thought that was a little odd, I didn't think
gcc had built in boost libraries.

--
Ian Collins.

[kwikius]

Re: using a class inside a class.


"Ian Collins" <ian-news@hotmail.co mwrote in message
news:67n1j4F2p7 o2gU9@mid.indiv idual.net...

kwikius wrote:

>"Ian Collins" <ian-news@hotmail.co mwrote:

>>>>
>>Why? The class declares m_children to be a list of CFoo. It does not
>>attempt to use any members of std::list that require a complete type.

>>
>Ah as to Why ... I Dunno. Hopefully someone of a more technical nature
>will
>help out as to why. From the error messages gcc is using C++0x style
>Concept
>checking.
>>

It does, doesn't it? I thought that was a little odd, I didn't think
gcc had built in boost libraries.

Ahha yes its not C++0x Concepts, its old style concept checking . Theres
bound to be an option to turn that off.
Probbably a macro somewhere

Its all legit AFAIK.
After all std::list may want to for example instantiate some private
template type of allocate<sizeof (T)>
when its constructed for which it would obviously need the complete type.

regards
Andy Little

[Ian Collins]

Re: using a class inside a class.

kwikius wrote:

"Ian Collins" <ian-news@hotmail.co mwrote in message
news:67n1j4F2p7 o2gU9@mid.indiv idual.net...

>kwikius wrote:

>>"Ian Collins" <ian-news@hotmail.co mwrote:
>>>Why? The class declares m_children to be a list of CFoo. It does not
>>>attempt to use any members of std::list that require a complete type.
>>Ah as to Why ... I Dunno. Hopefully someone of a more technical nature
>>will
>>help out as to why. From the error messages gcc is using C++0x style
>>Concept
>>checking.
>>>

>It does, doesn't it? I thought that was a little odd, I didn't think
>gcc had built in boost libraries.

>
Ahha yes its not C++0x Concepts, its old style concept checking . Theres
bound to be an option to turn that off.
Probbably a macro somewhere
>
Its all legit AFAIK.
After all std::list may want to for example instantiate some private
template type of allocate<sizeof (T)>
when its constructed for which it would obviously need the complete type.
>

True, but that only happens when a CFoo is created and fully declared.

--
Ian Collins.

[Jim Langston]

Re: using a class inside a class.

Ian Collins wrote:

kwikius wrote:

>"Ian Collins" <ian-news@hotmail.co mwrote:

>>>>
>>Why? The class declares m_children to be a list of CFoo. It does
>>not attempt to use any members of std::list that require a complete
>>type.

>>
>Ah as to Why ... I Dunno. Hopefully someone of a more technical
>nature will help out as to why. From the error messages gcc is using
>C++0x style Concept checking.
>>

It does, doesn't it? I thought that was a little odd, I didn't think
gcc had built in boost libraries.

That had thrown me off also, the boost error messages on something that has
nothing to do with boost. I wonder if it's some compiler switch?

--
Jim Langston
tazmaster@rocke tmail.com

[kwikius]

Re: using a class inside a class.


"Jim Langston" <tazmaster@rock etmail.comwrote in message
news:9SsRj.2310 $fn.1310@newsfe 05.lga...

Ian Collins wrote:

>kwikius wrote:

>>"Ian Collins" <ian-news@hotmail.co mwrote:
>>>>>
>>>Why? The class declares m_children to be a list of CFoo. It does
>>>not attempt to use any members of std::list that require a complete
>>>type.
>>>
>>Ah as to Why ... I Dunno. Hopefully someone of a more technical
>>nature will help out as to why. From the error messages gcc is using
>>C++0x style Concept checking.
>>>

>It does, doesn't it? I thought that was a little odd, I didn't think
>gcc had built in boost libraries.

>
That had thrown me off also, the boost error messages on something that
has nothing to do with boost. I wonder if it's some compiler switch?

FWIW following gives something similar. The trick is to make some
dependency, which must be going on in the O.P.

#include <list>

template<typena me T>
struct dummy : T{ };

struct CFoo {
dummy<CFoomy_li st;
};

int main()
{
CFoo foo;
}

[James Kanze]

Re: using a class inside a class.

On Apr 28, 8:33 pm, "Jim Langston" <tazmas...@rock etmail.comwrote :

phil.pellouch.. .@gmail.com wrote:

I did some searching online and i couldn't find anything in reference
to this.

I am using MinGW, gcc 4.3 and am having the following compilation
issue:

class CFoo
{
public:
...
private:
std::list<CFoom _children;
};

The complaint from the compiler looks like this:

c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
boost_concept_c heck.h: In instantiation of
'__gnu_cxx::_SG IAssignableConc ept<CFoo>':
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
stl_list.h:420: instantiated from 'std::list<CFoo ,
std::allocator< CFoo'
c:\mingw\bin\../lib/gcc/i386-pc-mingw32/4.3.0/include/c++/bits/
boost_concept_c heck.h:216: error:
'__gnu_cxx::_SG IAssignableConc ept<_Tp>::__a' has incomplete type

So, basically, I think what it's saying is that it doesn't know how to
build the class because CFoo hasn't been defined by the time it's
trying to create it (i.e. incomplete type). I tried adding
"std::list<clas s CFoo>" as well, but that didn't work either. This
compiles on visual studio with some warnings.

I can make this work by making it "std::list<CFoo *>", but I'm
wondering if there's a way to get this to work as defined.

Nothing standards conformant.

All I can tell you is I've done this exact same thing in the past
successfully in both Microsoft Visual C++ .net 2003 and DevC++. Here is
something that compiles sucessfully in Microsoft Visual C++ .net 2003:

#include <list>

class CFoo
{
private:
std::list<CFoom _children;
};

int main()
{
CFoo Foo;
}

No warnings, no errors.

That's what's so nice about undefined behavior. The fact that
it works doesn't prove anything.

I don't know why your version of gcc is complaining, I don't
think it should, AFAIK this is perfectly legal, although I
haven't read the standard on it.

§17.4.3.6/2:

In particular, the effects are undefined in the
following cases:

[...]

-- if an incomplete type (3.9) is used as a template
argument when instantiating a template component.

A class is only a complete type after the closing braces, so
your code has undefined behavior. (The standard could have
allowed incomplete types for certain specific cases, but it
didn't.)

The current working draft still has basically the same words
(with an "unless specifically allowed" exception, for e.g.
shared_ptr). I believe, however, that it is the intent to
require an error here once concepts have been added to the
language.

G++ uses boost::concepts (or something derived from it, I'm
not sure) to generate errors in most of cases, rather than
letting the undefined behavior through.

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

[James Kanze]

Re: using a class inside a class.

On Apr 29, 12:47 am, "kwikius" <a...@servocomm .freeserve.co.u kwrote:

"Ian Collins" <ian-n...@hotmail.co mwrote in message

news:67n1j4F2p7 o2gU9@mid.indiv idual.net...

kwikius wrote:

"Ian Collins" <ian-n...@hotmail.co mwrote:

>Why? The class declares m_children to be a list of CFoo.
>It does not attempt to use any members of std::list that
>require a complete type.

Ah as to Why ... I Dunno. Hopefully someone of a more
technical nature will help out as to why. From the error
messages gcc is using C++0x style Concept checking.

It does, doesn't it? I thought that was a little odd, I
didn't think gcc had built in boost libraries.

Ahha yes its not C++0x Concepts, its old style concept
checking. Theres bound to be an option to turn that off.
Probbably a macro somewhere

It's off by default (but I always turn it on). To get it, you
need to define _GLIBCXX_CONCEP T_CHECKS, i.e. specify the option
-D_GLIBCXX_CONCE PT_CHECKS.

Its all legit AFAIK.
After all std::list may want to for example instantiate some private
template type of allocate<sizeof (T)>
when its constructed for which it would obviously need the complete type.

I don't think that there's any particular justification for this
particular class. The rule in C++-03 was general: only complete
types when instantiating a template, regardless. The committee
made no effort to second guess what implementors might or might
not want to do in specific cases.

As far as I know, there's also no movement to change this
particular rule (although some of the new classes, like
shared_ptr, are an explicit exception, and other rules have been
loosened---std::list will no longer require assignable, for
example).

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

[Ian Collins]

Re: using a class inside a class.

James Kanze wrote:

On Apr 28, 8:33 pm, "Jim Langston" <tazmas...@rock etmail.comwrote :

>All I can tell you is I've done this exact same thing in the past
>successfully in both Microsoft Visual C++ .net 2003 and DevC++. Here is
>something that compiles sucessfully in Microsoft Visual C++ .net 2003:

>

>#include <list>

>

>class CFoo
>{
>private:
> std::list<CFoom _children;
>};

>

>int main()
>{
> CFoo Foo;
>}

>

>No warnings, no errors.

>
That's what's so nice about undefined behavior. The fact that
it works doesn't prove anything.
>

>I don't know why your version of gcc is complaining, I don't
>think it should, AFAIK this is perfectly legal, although I
>haven't read the standard on it.

>
§17.4.3.6/2:
>
In particular, the effects are undefined in the
following cases:
>
[...]
>
-- if an incomplete type (3.9) is used as a template
argument when instantiating a template component.
>
A class is only a complete type after the closing braces, so
your code has undefined behavior. (The standard could have
allowed incomplete types for certain specific cases, but it
didn't.)
>

But isn't the template component being instantiated in main after the
class is complete?

--
Ian Collins.