const object declaration

[subramanian100in@yahoo.com, India]

Consider the following program:

#include <iostream>

using namespace std;

class X
{
public:
int value() const { return val; }
int val;
};

int main()
{
const X obj;

return 0;
}

When this program is compiled with g++ as
g++ -std=c++98 -pedantic -Wall -Wextra x.cpp

the following compilation error is generated for the line const X obj;

error: uninitialized const `obj'

However VC++ 2005 Express Edition gives only the following warning(no
error):

warning: 'obj' : 'const' automatic data initialized with compiler
generated default constructor produces unreliable results

Which is the correct behaviour as per the standard ?

Kindly clarify.

Also please let me know how to set the compilation flags in VC++ 2005
Express Edition IDE similar to the command line g++ compilation flags
as mentioned above.

Thanks
V.Subramanian

[benben]

Re: const object declaration

Which is the correct behaviour as per the standard ?

I don't know. But my bet is if you define an empty constructor your
problem should go away.

>
Kindly clarify.
>
Also please let me know how to set the compilation flags in VC++ 2005
Express Edition IDE similar to the command line g++ compilation flags
as mentioned above.

That, im afraid, is better asked in the gcc group.

Ben

[Victor Bazarov]

Re: const object declaration

benben wrote:

>[..]
>Also please let me know how to set the compilation flags in VC++ 2005
>Express Edition IDE similar to the command line g++ compilation flags
>as mentioned above.

>
That, im afraid, is better asked in the gcc group.

Or in the VC++ newsgroup.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[Abhishek Padmanabh]

Re: const object declaration

On Dec 3, 7:54 pm, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:

benben wrote:

[..]
Also please let me know how to set the compilation flags in VC++ 2005
Express Edition IDE similar to the command line g++ compilation flags
as mentioned above.

>

That, im afraid, is better asked in the gcc group.

>
Or in the VC++ newsgroup.
>
V

I asked this once on clc++m and had not got a convincing answer. It is
mandated by the standard that a user-defined constructor must be
provided. It is needed even though there are no members in the class.
It just is as it is. I am not aware of the rationale behind this
though.

In case the class has a POD type, it would remain uninitialized
(indeterminate value) when default constructor (by the compiler
generated default constructor). In case of non-POD members, they would
have been default initialized. An indeterminate POD type's value is
rather useless but for non-POD types, it might make sense. I don't
know about the rationale but may be it is based on something along
these lines, to avoid complexity. I don't know for sure. May be some
guru will enlighten us! :)

[Victor Bazarov]

Re: const object declaration

Abhishek Padmanabh wrote:

On Dec 3, 7:54 pm, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:

>benben wrote:

>>>[..]
>>>Also please let me know how to set the compilation flags in VC++
>>>2005 Express Edition IDE similar to the command line g++
>>>compilatio n flags as mentioned above.

>>

>>That, im afraid, is better asked in the gcc group.

>>
>Or in the VC++ newsgroup.
>>
>V

>
I asked this once on clc++m and had not got a convincing answer. [..]

As you may have noticed, I removed all mention of the C++ problem but
left in the "compilatio n flags" request. That (and only that) is what
I am suggesting asking about in the VC++ newsgroup.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[Abhishek Padmanabh]

Re: const object declaration

On Dec 3, 8:30 pm, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:

Abhishek Padmanabh wrote:

On Dec 3, 7:54 pm, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:

benben wrote:
>>[..]
>>Also please let me know how to set the compilation flags in VC++
>>2005 Express Edition IDE similar to the command line g++
>>compilation flags as mentioned above.

>

>That, im afraid, is better asked in the gcc group.

>

Or in the VC++ newsgroup.

>

I asked this once on clc++m and had not got a convincing answer. [..]

>
As you may have noticed, I removed all mention of the C++ problem but
left in the "compilatio n flags" request. That (and only that) is what
I am suggesting asking about in the VC++ newsgroup.
>

Sorry about that! I wrongly quoted your response. My reply was to the
original post. I wouldn't have said : "May be some
guru will enlighten us! :)" to you! :)

[Victor Bazarov]

Re: const object declaration

Abhishek Padmanabh wrote:

[..] I wouldn't have said : "May be some
guru will enlighten us! :)" to you! :)

Hey, why not? I'll gladly take a guru's advice. After all it's not
that often gurus grace us with their presence here...

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[Bo Persson]

Re: const object declaration

Abhishek Padmanabh wrote:
:: On Dec 3, 7:54 pm, "Victor Bazarov" <v.Abaza...@com Acast.net>
:: wrote:
::: benben wrote:
::::: [..]
::::: Also please let me know how to set the compilation flags in
::::: VC++ 2005 Express Edition IDE similar to the command line g++
::::: compilation flags as mentioned above.
:::
:::: That, im afraid, is better asked in the gcc group.
:::
::: Or in the VC++ newsgroup.
:::
::: V
::
:: I asked this once on clc++m and had not got a convincing answer.
:: It is mandated by the standard that a user-defined constructor
:: must be provided. It is needed even though there are no members in
:: the class. It just is as it is. I am not aware of the rationale
:: behind this though.
::
:: In case the class has a POD type, it would remain uninitialized
:: (indeterminate value) when default constructor (by the compiler
:: generated default constructor). In case of non-POD members, they
:: would have been default initialized. An indeterminate POD type's
:: value is rather useless but for non-POD types, it might make
:: sense. I don't know about the rationale but may be it is based on
:: something along these lines, to avoid complexity. I don't know for
:: sure. May be some guru will enlighten us! :)

Since no guru appeared, I will try to answer this. :-)

The reason for PODs to be different, is that they behave like they do
in C. Had C++ added initialization to types that C does not, it would
have

1) changed old behaviour of existing code
and
2) lost all benchmarks to C code.


Bo Persson

[James Kanze]

Re: const object declaration

On Dec 3, 4:05 pm, Abhishek Padmanabh <abhishek.padma n...@gmail.com>
wrote:

I asked this once on clc++m and had not got a convincing
answer. It is mandated by the standard that a user-defined
constructor must be provided.

No it isn't. It is mandated that there be some non-trivial
initialization. In his case, something like:

X const obj = { 1 } ;

would be legal, for example.

It is needed even though there are no members in the class.
It just is as it is. I am not aware of the rationale behind
this though.

The requirement in the case of no data members is probably just
the result of an oversight. Since it's trivial to add a no-op
default constructor, however, it's no big problem.

In case the class has a POD type, it would remain
uninitialized (indeterminate value) when default constructor
(by the compiler generated default constructor).

In the case we're considering, the class is a POD.

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
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[subramanian100in@yahoo.com, India]

Re: const object declaration

The following program DOES NOT COMPILE as mentioned earlier in the
beginning of the thread

#include <iostream>

using namespace std;

class X
{
public:
int val;
};

int main()
{
const X obj;

return 0;

}

However the following program COMPILES fine.

#include <iostream>

using namespace std;

class Test
{
public:
Test() { cout << "Test() default ctor" << endl; }
};

class X
{
public:
Test obj;
int val;
};

int main()
{
const X obj;

return 0;
}


I am unable to understand why this program compiles well and the
previous program does not compile.

Kindly explain.

Thanks
V.Subramanian

[James Kanze]

Re: const object declaration

On Dec 5, 7:15 am, "subramanian10. ..@yahoo.com, India"
<subramanian10. ..@yahoo.comwro te:

The following program DOES NOT COMPILE as mentioned earlier in the
beginning of the thread

#include <iostream>

using namespace std;

class X
{
public:
int val;
};
>
int main()
{
const X obj;
>
return 0;
}

However the following program COMPILES fine.

#include <iostream>

using namespace std;

class Test
{
public:
Test() { cout << "Test() default ctor" << endl; }
};
>
class X
{
public:
Test obj;
int val;
};

int main()
{
const X obj;
>
return 0;
}

I am unable to understand why this program compiles well and the
previous program does not compile.

I was about to say: because in the first example, the default
constructor is trivial (so an initialization is required),
whereas in the second it is not. On re-reading the standard,
however... in §8.5/9, it says "If no initializer is specified
for an object, and the object is of (possibly cv-qualified)
non-POD class type (or array thereof), the object shall be
default-initialized; if the object is of const-qualified type,
the underlying class type shall have a user-declared default
constructor." According to this, since X does not have a
user-declared default constructor, it shouldn't compile. But it
does, with all three of the compilers at my disposition (g++,
Sun CC and VC++).

I'll admit that this somewhat surprises me. I had always
thought (and apparently, the compilers agree with me) that the
rule was that the class must have a non-trivial default
constructor. Since the (compiler generated) default constructor
for X in the second example is non-trivial, the code would be
OK. Given that three compilers also disagree with what I've
just quoted in the standard, I wonder, too, if there isn't
something else that I've missed.

Thinking about it, I think what the rule should be is: an
initializer is required if the class has no user defined
default constructor, and has one or more sub-objects which have
trivial default constructors. That would allow const instances
without initializers for things like:

struct A
{
virtual void f() ;
} ;

or

struct B
{
std::vector< int v ;
} ;

. In both cases, all fields are sufficiently initialized (and
it was to allow such things that I thought the rule involved
trivial vs. non-trivial default constructor). And would cause
your second example to fail to compile, since X::val has a
trivial default constructor. (It doesn't cover all cases, of
course. For example:

struct A
{
virtual void f() ; // means default ctor non-trivial.
int i ;
} ;

struct B
{
A a ;
} ;

B const b ;

would still create a const object in which A::i wasn't
initialized.)

--
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