NetNews Usenet Archive 1992 #30/NN_1992_30.iso/spool/comp/lang/cplus/17816

Text File | 1992-12-11 | 3.6 KB | 114 lines

Newsgroups: comp.lang.c++
Path: sparky!uunet!cs.utexas.edu!sdd.hp.com!spool.mu.edu!agate!boulder!happy.colorado.edu!srheintze
From: [e-mail]
Subject: Re: Class template problem
Message-ID: <[e-mail]>
Lines: 100
Sender: [e-mail] (The Daily Planet)
Nntp-Posting-Host: happy.uccs.edu
Organization: University of Colorado, Boulder
References: <[e-mail]>
Date: 11 Dec 92 11:03:57 MDT
Lines: 100

In article <[e-mail]>, [e-mail] (Stein Kulseth FBA) writes:
> I am new to C++ and is currently testing out some of its features.
> Maybe someone could help me with the following problems regarding
> class templates.
>
> I am using the GNU g++ compilater v.2.3.2 on a SUN SPARC.
>
> Consider the following small program defining an Array class template:
>
> ___
>
> #include <stdlib.h>
> #include <iostream.h>
>
> template <class T>
> class Array {
>     private:
>         int sz;
>         T* element;
> public:
>         Array(int size) : sz(size) { element = new T[sz]; };
>         ~Array() { delete [] element; };
>         T& operator [] (int i) { return element[i]; };
> } ;
>
>
> int main()
> {
> int i;
>     Array<int> a(2);
>     a[0] = 1;
>     a[1] = 3;
>
>     Array< char * > c(2);
>     c[0] = "Hello";
>     c[1] = "World";
>
>     for (i=0;i<2;i++) {
>         cout << a[i] << "\n";            // OK, prints integers
>         cout << c[i] << "\n";            // Prints pointer values (0x....) ???
>         cout << (char *) c[i] << "\n";    // Prints strings
>     }
> }
>
> ---
> Main question:
>     Why isn't c[i] recognised as a char * ???
>     After all c *is* declared as Array< char * >, and the return
>     value of [] is a reference to T, ie a reference to a char *.

     When I used a VMS port of G++ I had a similar problem with printing char
values.  That version of of G++ did not seem to understand that  char was a
distinct type (it is not a distinct type in C) so, like C it prompted char
constants to an int and printed the ASCII value instead of the character
itself.  This looks like a similar problem.  I do not have either problem on
Borland C++ or MicrosoftC++.

>
> Secondary question:
>     If the template argument is itself a class, and this class'
>     constructor needs argument, how would this be specified?
>
>     Ex. I want to implement a 4x3 2D array of int using an Array
>     of an Array of int:
>
>         Array< Array<int> >
>
>     However here I run into problems as I cannot specify the
>     innermost array dimension. How would I go about this?
>
> Post or reply by email as you see fit.
> Thank you.

  This is an ugly problem.  Twice I have posted queries on this questions and
twice I have received no replies.  I'm beginning to wonder if there is
something wrong with this program I use to post to the news groups. I'll tell
you what I know about it.

  See page 61 of the ARM:  "No initializers can be specified for arrays"
when using "new" to allocate arrays.  Normally we use "[]" (like char* x = new
char[strlen(s)+1]) to allocate an array and "()" (like String* y = new
String("initialvalue");) to specify an initial value. But for reasons that are
beyond me, Bjarne and Margaret decided that "()" and "[]" were going to be
mutually exclusive (can someone explain this to me?).

 So what is the wordaround?   Margaret and Bjarne suggest using global values.
I think this is an absurd suggestion.

 Another suggestion is to pass two arguments to your parameterized class like
this:

    Array< Array<int, 23>, 45> arra;

This works, but there is a type compatibility problem here!  How do I pass such
an object to a function as an argument when the function argument must
accomodate all two diminsonal arrays of ints?

                Sieg