Re: Static member in a class

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

9 May 2007 00:12:05 -0700

Message-ID:

<1178694725.134877.68260@p77g2000hsh.googlegroups.com>

On May 9, 5:27 am, Phil Hobbs <p...@SpamMeSenseless.pergamos.net>
wrote:

Victor Bazarov wrote:

stonny wrote:

I read the following sentence from a c++ website, but can not
understand why. can anyone help me with it?

"
An important detail to keep in mind when debugging or implementing a
program using a static class member is that you cannot initialize the
static class member inside of the class. In fact, if you decide to put
your code in a header file, you cannot even initialize the static
variable inside of the header file; do it in a .cpp file instead.
"

The Standard requires that every static data member is defined at the
namespace level. In order to comply with the One Definition rule, you
are more likely to succeed if you place the definition of the static
data member in a .cpp file (instead of a header which can be included
in more than one translation unit). Initialisation accompanies the
definition. That's why you should initialise static data members in
a .cpp file (and only in one .cpp file).

This is such a pain, I agree. Some years ago, I started doing this:

// foo.h

class Phew [
    static int barre;
    ...
}

#ifdef COMPILING_MAIN
    int Phew::barre;
#endif

// end foo.h

and insisting that all modules containing main() define COMPILING_MAIN.

And what happens if the module containing main() doesn't include
your header?

I'll admit that I don't see where it is such a big thing. I'll
just define the variable in whatever file I define the
constructor in.

That way, all the static members get defined exactly once, and their
definitions are right there with the class declaration.

But why do you want the definitions right there with the class
definition. You don't require that for functions; why is it a
problem to define static variables in a separate source file,
but not functions.

--
James Kanze (GABI Software) email:james.kanze@gmail.com
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