Ada Access Types

This document contains a loose number of notes about access types in Ada. This is a complex topic with quite some pitfalls

Implicit Dereference

This is a feature that was introduced in Ada 2012 and is described In this document It allows to define reference types which can be passed around and used to access the underlying target. Normally, when using an access type, you need to use the .all attribute to de-reference it [1] but implicit de-reference allows you to use references without using .all.

Consider the type declaration:
1type LF_Ref (Element : not null access Real) is null record with
2  Implicit_Dereference => Element;

This defines an access type LF_Ref to access variables of type Real (just another name for a Long_Float) which implicitly dereferences Element. Now, this is really a record type with Element as its single member and discriminant and whenever you use a variable of this type to access a Real value, it will automatically dereference and allows read/write access to its target.

Example of using such a type:
1type AuxAngleClass is tagged record
2  xx : aliased Real := 0.0;
3  yy : aliased Real := 0.0;
4end record;
5
6AA : AuxAngleClass := (xx => 0.0, yy => 0.0);
7R  : LF_Ref := Aux_Angle.xx;

This will give you an access type to AA.xx but you do not have to use .all in order to access the target value. You can do something like R := 12.3; and

Tags: ada

—