Object-oriented Software in Ada 95

Dynamic memory allocation 211
In a program it is usual to deal with the contents or rvalue of an object. The access value of an object is its
lvalue. For example, after the following fragment of code has been executed:
declare
type P_Integer is access all Integer;
type P_P_Integer is access all P_Integer;
P_P_People : P_P_Integer;
P_People : aliased P_Integer;
People : aliased Integer;
begin
People := 42;
P_People := People'Access;
P_P_People := P_People'Access;
end;
the following expressions will deliver the contents of the object people:
Expression
People
Diagram
People
24
P_People.all P_People People
24
P_P_People.all.all P_P_People P_People People
24
In a similar way the following statements will assign 42 to the object people.
Statement
Explanation
People := 42;
Straight-forward assignment.
P_People.all := 42;
Single level of indirection.
P_P_People.all.all := 42; Double level of indirection.
15.1.3 Read only access
Access to an object via an access value may be restricted to read only by replacing all with constant in the
declaration of the access type. For example, in the following fragment of code, only read access is allowed to
People when using the access value held in P_People.
declare
type P_Integer is access constant Integer;
People : aliased Integer;
P_People : P_Integer;
begin
People := 24;
P_People := People'Access; --Access value for people
Put("The number of people is : "); Put( P_People.all );
New_Line;
end;
© M A Smith - May not be reproduced without permission