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From: andy billimore <billimad@dcs.gla.NOSPAM.ac.uk>
Subject: problems using classwide types
Date: 1997/06/25
Message-ID: <33B10EF3.72B4@dcs.gla.NOSPAM.ac.uk>#1/1
X-Deja-AN: 252459304
Organization: Dept. Computing Science, University of Glasgow
Newsgroups: comp.lang.ada
Date: 1997-06-25T00:00:00+00:00
List-Id: <comp.lang.ada>


I'm looking for some help with classwide types.

I have defined a parent type -
  type Transaction_Type is tagged private;

And a classwide access type to objects of Transaction_Type'Class -
  type Transaction_Ptr_Type is access all Transaction_Type'Class;

I have then defined a child class, Transactions.Checks -
  type Check_Type is new Transaction_Type with private;

The main program looks like this -

with Transactions; use Transactions;
with Transactions.Checks; use Transactions.Checks;

procedure Main is
   L_Check : Check_Type;
   TP : Transaction_Ptr_Type;
begin
   TP := new Check_Type;
   TP := L_Check; -- Error (see below)
end Main;

Compiling this produces the following output - 

main.adb:24:10: expected type "Transaction_Ptr_Type" 
	        defined at transactions.ads:5

main.adb:24:10: found private type "Check_Type" 
		defined at transactions-checks.ads:2

gnatmake: "main.adb" compilation error

So I can allocate enough memory to hold an object of Check_Type, but I
can't assign anything to it which I don't understand.


Andrew