NetNews Usenet Archive 1992 #26/NN_1992_26.iso/spool/comp/lang/cplus/16229

Text File | 1992-11-14 | 3.0 KB | 89 lines

Newsgroups: comp.lang.c++
Path: sparky!uunet!snorkelwacker.mit.edu!ira.uka.de!ira.uka.de!slsvaat!josef!kanze
From: [e-mail] (James Kanze)
Subject: Re: how to initilize an array inside of a class??
In-Reply-To: [e-mail]'s message of 12 Nov 92 03:46:14 GMT
Message-ID: <[e-mail]>
Sender: [e-mail]
Organization: SEL
References: <[e-mail]>
Date: 13 Nov 92 19:34:46
Lines: 76

In article <[e-mail]> [e-mail] (Jeff
Cooper) writes:

|> I have a bit of a problem that I can't really see an elegant solution
|> for and I hope someone can point me in the right direction.  I have
|> a class that looks something like this:

|> class Y : public X
|> {
|>     public:
|>         int some_ints[3000];
|>         Y();
|>         ...
|> };

|> Now what I'd need to do is initilize the array 'some_ints' with some
|> preset values...normally one would do it like:

|> int some_ints[3000] = { 1, 6, 34, 3, ... ,0 };

|> But my compiler complains about that, saying that I can't initialize
|> class members there.  The only other way I can think of doing it is
|> to do something like:

|> Y::Y()
|> {
|>     some_ints[0] = 1;
|>     some_ints[1] = 6;
|>     ...
|>     some_ints[2999] = 0;
|> }

|> Is this my only other option??  If it is, it's not really all that bad
|> as this code is being generated by another program, so I won't have to
|> type in 3000 entries like that, but I was hoping for a more elegant
|> solution...Would there be a performance hit doing it that way, I kinda
|> think so as the first one is done at compile time, while the other is
|> done at run-time, correct??

|> Any Ideas?

First, note that there is a slight difference in declaring an array
outside of a class, and declaring one inside the class.  The array
declared (and initialized) outside the class defines a single instance
of a variable, so a single initialization is in order.  The array
declared inside the class does not define an instance of a variable;
the array will be instantiated once for every instance of the class.

So, the first question is: do you always want to initialize it in the
same manner.  And if so, do you ever change it later.

If the answer to the latter question is false, then maybe what you
want is a static class member; that is, a member common to all
instances of the class.  In this case, the solution is easy:

    class Y
    {
    public :
        static int someInts[ 3000 ] ;
    } ;

and in one (and only one) source file:

    int        Y::someInts[ 3000 ] = { 1 , 6 , ... , 0 } ;

If you really need for each instance of the class to contain its own
array member, which is always initialized with the same values, then
my own solution would be to declare a private static member with the
initial values, and use "memcpy" to copy it into the dynamic member.
This is, of course, basically just doing what you suggested in another
way, but I find it more readable.
--
James Kanze            GABI Software, Sarl.
email: [e-mail]    8 rue du Faisan
                67000 Strasbourg
                France