Newbie problem: unconstrained subtype not allowed

Newbie problem: unconstrained subtype not allowed

[Frank Gerlach]

Hello,
I am new to Ada and struggling to get TCP working....
GNAT says "unconstrained subtype not allowed (need initialization)"
when I compile the following piece of code:

   feldEA : Ada.Streams.Stream_Element_Array;

How do I "initialize" this data type ?

Thanks for helping an Ada newbie !

Re: Newbie problem: unconstrained subtype not allowed

[Eric Jacoboni]

frankgerlach@gmail.com (Frank Gerlach) writes:

>    feldEA : Ada.Streams.Stream_Element_Array;
>
> How do I "initialize" this data type ?
>
> Thanks for helping an Ada newbie !

Type Stream_Element_Array is an unconstrained type:

type Stream_Element_Array is
      array (Stream_Element_Offset range <>) of aliased Stream_Element;

So, you have to constraint it to create a object of this type:

A_Stream_Element_Array : Stream_Element_Array(1..100);

for exemple (a better solution is to use a constrained subtype, but
you've got the idea).

-- 
�ric Jacoboni, n� il y a 1417398423 secondes

Re: Newbie problem: unconstrained subtype not allowed

[Stephen Leake]

frankgerlach@gmail.com (Frank Gerlach) writes:

> Hello,
> I am new to Ada and struggling to get TCP working....

You should probably start with something simpler ...

> 
> GNAT says "unconstrained subtype not allowed (need initialization)"
> when I compile the following piece of code:
>
>    feldEA : Ada.Streams.Stream_Element_Array;
>
> How do I "initialize" this data type ?

This is a mildly confusing message.

Stream_Element_Array is an unconstrained type; it can hold 1 byte, or
20 bytes, whatever you need. 

But, each _object_ of that type must have a definite size. This is the
same as the Ada String type.

So the declaration needs to be something like:

  feldEA : Ada.Streams.Stream_Element_Array (1 .. Max_Buffer_Size);

where Max_Buffer_Size is appropriate for your application.

The "need initialization" is refering to another way for objects of an
unconstrained type to get a definite size; you can provide an initial
value:

   A_String : String := "Hello";

   feldEA : Ada.Streams.Stream_Element_Array := 
     (1 .. Max_Buffer_Size => 0);

In this case, the object size is the same as the initial value size.

-- 
-- Stephe