Template static member initialization

[ALiX]

Hi!

My template class has a const static data member, which is used by the
constructor. It seems that the program does not produce the correct
result. Either it is the fault of the compiler (gcc 4.1) or I need to
learn more about initialization of static members... The code below
captures the problem and is as simple as I could make it:

#include <iostream>
#include <limits>

template<typename Tstruct A {
const static unsigned NONE;
unsigned x;
A() { x = NONE; }
};

template<typename Tconst unsigned A<T>::NONE =
numeric_limits<unsigned>::max();
A<doublea;

int main(void) {
cout << a.x << "\n";
cout << A<double>::NONE << "\n";
return 0;
}

The output of the program is:
0
4294967295

Mustn't NONE be initialized before a is constructed?

Cheers,
/ALiX

[Victor Bazarov]

ALiX wrote:

My template class has a const static data member, which is used by the
constructor. It seems that the program does not produce the correct
result. Either it is the fault of the compiler (gcc 4.1) or I need to
learn more about initialization of static members... The code below
captures the problem and is as simple as I could make it:

#include <iostream>
#include <limits>

template<typename Tstruct A {
const static unsigned NONE;
unsigned x;
A() { x = NONE; }
};

template<typename Tconst unsigned A<T>::NONE =
numeric_limits<unsigned>::max();

std::numeric_limits

A<doublea;

int main(void) {

Drop the 'void' -- bad style

cout << a.x << "\n";
cout << A<double>::NONE << "\n";
return 0;
}

The output of the program is:
0
4294967295

Mustn't NONE be initialized before a is constructed?

It's not *instantiated* unless it's "used". Perhaps referring to
it in the c-tor does not get it instantiated (and therefore does
not get it initialised), but I would consider it a compiler bug.

Try initialising it right in the class:

template<typename Tstruct A {
static unsigned const NONE
= std::numeric_limits<unsigned>::max();
unsigned x;
A() : x(NONE) {}
};

V
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[ALiX]

On Jul 26, 4:50 pm, "Victor Bazarov" <[e-mail]:

Try initialising it right in the class:

template<typename Tstruct A {
static unsigned const NONE
= std::numeric_limits<unsigned>::max();
unsigned x;
A() : x(NONE) {}
};

Then I get an error:
error: 'std::numeric_limits<unsigned int>::max()' cannot appear in a
constant-expression
error: a function call cannot appear in a constant-expression

Making NONE non-const does not help either as non-const static members
cannot be initialized in the class declaration.

/ALiX

[ALiX]

This seems to work. However, it gives me another headache since in the
real application I'm writing, NONE is actually public (I should have
made it public in the example code... sorry!). I use it like
std::string::npos to return 'end-values'. I'd like a solution that
allows me to keep NONE as a public static member of the class A.

Cheers,
ALiX

[Victor Bazarov]

ALiX wrote:

This seems to work. However, it gives me another headache since in the
real application I'm writing, NONE is actually public (I should have
made it public in the example code... sorry!). I use it like
std::string::npos to return 'end-values'. I'd like a solution that
allows me to keep NONE as a public static member of the class A.

Don't initialise it with 'max()', initialise it with -1.

V
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[ALiX]

On Jul 26, 5:36 pm, "Victor Bazarov" <[e-mail]:

Don't initialise it with 'max()', initialise it with -1.

Do I still need to define NONE outside the class declaration? Are we
always guaranteed that ((unsigned)-1) ==
numeric_limts<unsigned>::max()?

/ALiX

[Victor Bazarov]

ALiX wrote:

On Jul 26, 5:36 pm, "Victor Bazarov" <[e-mail]:

>Don't initialise it with 'max()', initialise it with -1.

Do I still need to define NONE outside the class declaration?

Not unless you take its address anywhere.

Are we
always guaranteed that ((unsigned)-1) ==
numeric_limts<unsigned>::max()?

Yep.

V
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[ALiX]

Thanks! Your help is much appreciated.

Cheers,
/ALiX

[James Kanze]

On Jul 26, 5:52 pm, ALiX <[e-mail]:

On Jul 26, 5:36 pm, "Victor Bazarov" <[e-mail]:

Don't initialise it with 'max()', initialise it with -1.

Do I still need to define NONE outside the class declaration? Are we
always guaranteed that ((unsigned)-1) ==
numeric_limts<unsigned>::max()?

Yes and yes. You can also use UINT_MAX, which is guaranteed to
be an integral constant expression.

--
James Kanze (GABI Software) email:ja*********@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[Michael DOUBEZ]

ALiX a écrit :

On Jul 26, 5:36 pm, "Victor Bazarov" <[e-mail]:

>Don't initialise it with 'max()', initialise it with -1.

Do I still need to define NONE outside the class declaration?

If it clarifies the purpose of A, you may alias it in your class:

template<typename T>
struct A
{
static const unsigned int NONE=A_traits<T>::NONE;
//...
};
Michael

[Pete Becker]

On 2007-07-27 04:11:03 -0400, James Kanze <ja*********@gmail.comsaid:

That's only legal if the initializer is a constant expression.
A constant expression cannot (at present, anyway) contain a
function call, even if the function is inline, and only returns
a constant value.

To elaborate a bit on the "at present, anyway": at last week's
standards meeting two proposals concerning constant expressions were
voted into the next standard. One permits calling appropriately
decorated functions as part of a constant expression, and the other
adds those decorations to various functions in the standard library,
including the members of the numeric_limits explicit instantiations. So
std::numeric_limits<unsigned>::max() will be acceptable as a static
initializer in C++0x.

--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)

[Victor Bazarov]

X-No-Archive: yes
James Kanze wrote:

On Jul 26, 8:50 pm, "Victor Bazarov" <[e-mail]:

>Victor Bazarov wrote:

>>ALiX wrote:

[...]

>Curiously enough, it might just be QoI issue. I built your code with
VC++ 2005 SP1. In Debug mode it gave me your output, and in Release
mode it gave the correct output... Go figure.

What part of "undefined" don't you understand? [..]

Please refrain from using this tone with me. Thank you.

V
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