Re: how to create a record

[Jeffrey Creem <jeff@thecreems.com>]

markww wrote:
> Hi,
> 
> How can I define a record or a class to this source file and create an
> instance of it?
> 
> with Ada.Text_IO;
> 
>     type struct_device is
>         record
>            major_number : Integer          := 0;
>            minor_number : Integer          := 0;
>         end record;
> 
> procedure Ada_LinkedList is
> begin
>    Ada.Text_IO.Put_Line("Hello, world!");
>    Ada.Text_IO.Put_Line("just terrible!");
> end Ada_LinkedList;
> 
> 
> I tried creating a record like in the above example, and compiling with
> gnatmake but get the error:
> 
> C:\>gnatmake C:\Ada_LinkedList.adb
> gcc -c -IC:\ -I- C:\ada_linkedlist.adb
> ada_linkedlist.adb:4:05: compilation unit expected
> gnatmake: "C:\ada_linkedlist.adb" compilation error
> 
> Thanks
> 

You can't declare anything in Ada in the scope where you declared it. 
Records need to be declared inside of procedures or inside of packages.

So, to do something close to what you appear to be asking for (define 
the record and create an instance of it) try this


  with Ada.Text_IO;
  procedure Ada_LinkedList is
   type struct_device is
          record
             major_number : Integer          := 0;
             minor_number : Integer          := 0;
          end record;

  My_Happy_Record : struct_device;

  begin
     Ada.Text_IO.Put_Line("Hello, world!");
     Ada.Text_IO.Put_Line("just spiffy!");
  end Ada_LinkedList;


A few thoughts though (most of them useless but think about them)

1) You are using two different conventions in your program above. Both 
CamelCase and the Traditional_Ada_Underscore method. Most Ada code tends 
to use _ since that is what the LRM packages do (e.g. text_io) but 
whatever you pick, try to stick with one style

2) Can't really guess where this is going but "struct_device" sounds 
pretty C like. There are no 'structs' in Ada. Again this is just a silly 
style thing and may not matter but it is probably best just live in the 
style of whatever language you use.

3) This is probably the most controvertial statement I'll make. Others 
will disagree...but.. I am not a big fan of the initialize everything in 
the record definition approach. It has its uses. It even makes a lot of 
sense in a lot of cases, but it does not always make sense in every 
case. There are two reasons I dont like it. a - As you start declaring 
local variables of these things in procedures you take the hit of the 
initialization in a lot of cases where you don't need it. yes the 
optimizer can sometimes find those cases....but in practice, they don't. 
b - While this safe practice of initialize everying feels good and 
prevents the read from unitialiazed memory issues, it creates a new kind 
of problem. You can still end up with code where  you expect that some 
path puts some meaningful data into some field but it does not. You feel 
good because it is initialized but in fact, it is still initialized with 
something that is not contextually meaningful. Now, if there is 
something that makes sense, then my all means do it. If not, don't. 
Initializing the fields prevents you from using various cool tools to 
detect problems like this in your code statically (polyspace) or 
dynamically (i.e. gnat with some of its cooler run time checks
(i.e.  http://tinyurl.com/ygrut9)

4) I don't know what it is about Ada but people love the word so much 
they feel the need to say it all the time. Ada_LinkList? Would you have 
written CLinkedList? ...Note this one is a really silly comment and 
probably mostly because it is late. It is always a bad sign when someone 
writes back a response 5x longer than the question when they really have 
only 2 lines worth of useful thoughts.