Multiple distinct objects: Difference between revisions

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This creates an array of distinct elements of type <code>T</code>. A type may specify a default value for its fields, so long as the values are compile-time constants. Similarly, an array can initialize its entries to multiple different values, also compile-time constants. Naturally, a program may initialize this data at run-time using a <code>FOR</code> loop.

Modula-3 offers reference and pointer types, so the mistaken way of initializing is quite easy to do for the careless.

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The example program below demonstrates each of these ~~three~~ methods, ~~and~~ so is a bit long.

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The example program below demonstrates each of these methods, including the mistaken way, so is a bit long.

<lang modula3>MODULE DistinctObjects EXPORTS Main;

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t2 := T { 4 };

t3 : T; (* t3's value will be default (2) *)

(* initialize a reference to T with value 100 *)

tr := NEW(REF T, value := 100);

(* initialize an array of records *)

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(* initialize an array of integers *)

b := ARRAY[1..Size] OF INTEGER { -9, 2, 6 };

(* initialize an array of references to a record -- NOT copied! *)

c := ARRAY[1..Size] OF REF T { tr, tr, tr };

BEGIN

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FOR i := 1 TO Size DO

IO.PutInt(a[i].value); IO.Put(" , ");

IO.PutInt(b[i]); IO.Put(" ; ");

IO.PutInt(b[i]); IO.Put(" , ");

IO.PutInt(c[i].value); IO.Put(" ; ");

END;

IO.PutChar('\n');

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(* re-initialize a's data to random integers *)

FOR i := 1 TO Size DO a[i].value := random.integer(-10, 10); END;

(* modify "one" element of c *)

c[1].value := 0;

(* display the data *)

FOR i := 1 TO Size DO

IO.PutInt(a[i].value); IO.Put(" , ");

IO.PutInt(b[i]); IO.Put(" ; ");

IO.PutInt(b[i]); IO.Put(" , ");

IO.PutInt(c[i].value); IO.Put(" ; ");

END;

IO.PutChar('\n');

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END DistinctObjects.</lang>

Each line interleaves the initial values of <code>a</code> and <code>b</code>. The first one has default values; the second replaces the values of <code>a</code> with random, "re-initialized" integers. Only <code>a[3]</code> starts with the default value for <code>T</code>; see the fifth number in the first line.

Each line interleaves the initial values of <code>a</code> and <code>b</code>. The first one has default values; the second replaces the values of <code>a</code> with random, "re-initialized" integers. Only <code>a[3]</code> starts with the default value for <code>T</code>; see the fifth number in the first line. On the other hand, the modification of "one" element of <code>c</code> actually modifies every element, precisely because it is a reference and not an object.

<pre>

3 , -9 ; 4 , 2 ; 2 , 6 ;

3 , -9 , 100 ; 4 , 2 , 100 ; 2 , 6 , 100 ;

3 , -9 ; -8 , 2 ; 7 , 6 ;

-1 , -9 , 0 ; -9 , 2 , 0 ; 8 , 6 , 0 ;

</pre>