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mangled_us@yahoo.com · Guest

I have googled but was not able to find the actual reason why we cannot
initialise non-static constant members in the class definition ? ISTM
that the definition will always be available where the const value
would be used so what was the actual problem with permitting this dak ?

A short example of what I mean is as follows :

class a_class
{
const int c_int = 33;
};

Why should that be illegal except for static members ?

Thanks,

David

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Allan W · Guest

mangled... (AT) yahoo (DOT) com wrote:

Thomas Maeder · Guest

[email]mangled_us (AT) yahoo (DOT) com[/email] writes:

AnonMail2005@gmail.com · Guest

Non-static class members are initialized in the constructor(s) that's
why. For non-statics, each instance of a class will have a different
instance of the data member contained within it. And that data member
can be initialized however you choose in the constructor(s).

If it is truly a class level constant and you don't need to have each
class contain the data member you can choose a static member. Or you
can choose an enum.

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Improving · Guest

This is guesswork but it seems to make sense to me.

Suppose you created three objeacts from same class, each of which has a
const member that as usual is initialised in the constructor, well
theres nothing to say that all those const members have to be equal to
each other. it could be a const id field or something, then you wouldnt
want two objects with same id would you?
Now if we had initialised this const member as you wanted, we would
lose the ability to have the behaviour above and still keep the member
const.
Whereas a static is shared amongst all objects of the class so that is
the behaviour that we just described above.
I suppose if you think about it for a bit it makes perfect sense.

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Abhishek Pamecha · Guest

There can be 2 cases what the const-ness of a value can mean:

class-wide const : You should use static as that saves storage space
and is not dependent on instantiation of an object.

object-wide const: If you give it a thought, it would be really
immaterial whether you can declare a member a const or not. Probably
what you are looking for is a variable that, once initialized, is not
accidently changed in the internal implementation of the class as well.

so now you would say, why make it illegal to declare this way, when it
does not make a difference?

It has to do with problems in linking and the fact the "definition" of
a variable is separate from "declaration." As you may be knowing, when
we assign something to a variable we are "defining" it and also
allocating memory to store that value. So compilation ensures that one
variable should never have two different memory allocations. Or else
how would we know which one to change?

So, There can be only one storage allocation for each variable. which
translates to "there can be only one definition for a variable".

Let us compare and see what are const static members and how are they
stored.

Const Static members are declared in a class(.h file) and
defined(assigned a value) EXACTLY at one place (presumably in a .cpp
file). So any usage of that static const member will resolve to only
one symbol definition and hence there will be no "duplicate symbols
found" during compliation. A Happy world!

Where as with const member variables the only place you can declare and
"define" them is in a *.h file. (inside the braces enclosing class
declaration or else i run out of ideas where will you initialize it.
You dont have an object for it and you have to respect the scope
restriction). But alas, this header file is included in so may .cpp
files. so the same const class member will be declared and defined in
all those .cpp files.

Since there would arise multiple definitions for the same
identifier(const class member) the resolution of that symbol during
compile time will not be unique and compilation would fail. A confusing
world.

That is a reason, a compiler will not let you declare a const class
member.

Anyways, declaring static const members is a bad practice as it may
introduce mysterious errors. ( imagine using an uninitialized global
var! )

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werasm · Guest

[email]mangled_us (AT) yahoo (DOT) com[/email] wrote:

Improving · Guest

Abhishek Pamecha · Guest

is it impossible to imagine? :->

We can atleast _imagine_ the impossibility and some rogue compilers
have the daring to _realize_ it !!

I agree with you but I was just giving an analogy of how bad it could
be.
and Sankara said that no analogy is perfect. :-)

Please check
Bjarne Stroustrup and Margaret Ellis, The Annotated C++ Reference
Manual, Addison-Wesley, 1990, pp. 20-21.
(reference used from Bruce Eckel chapter 10 subtopic: Static
initialization dependency

where it discourages static initializations. So I think it would apply
to static const initializations too.

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Francis Glassborow · Guest

In article <1129229427.614151.298670 (AT) g44g2000cwa (DOT) googlegroups.com>,
Improving <russ (AT) haydon449 (DOT) plus.com> writes

tpochep@mail.ru · Guest

Christoph Schulz · Guest

Hello!

Francis Glassborow wrote:

Martin Bonner · Guest

Francis Glassborow wrote:

James Kanze · Guest

Martin Bonner wrote: