[GNAT] This restriction ensures at compile time that the code generated by the
compiler involves no dispatching calls. The use of this restriction allows the
safe use of record extensions, classwide membership tests and other classwide
features not involving implicit dispatching. This restriction ensures that
the code contains no indirect calls through a dispatching mechanism. Note that
this includes internally-generated calls created by the compiler, for example
in the implementation of class-wide objects assignments. The
membership test is allowed in the presence of this restriction, because its
implementation requires no dispatching.
This restriction is comparable to the official Ada restriction
No_Dispatch except that it is a bit less restrictive in that it allows
all classwide constructs that do not imply dispatching.
The following example indicates constructs that violate this restriction.
package Pkg is
type T is tagged record
Data : Natural;
end record;
procedure P (X : T);
type DT is new T with record
More_Data : Natural;
end record;
procedure Q (X : DT);
end Pkg;
with Pkg; use Pkg;
procedure Example is
procedure Test (O : T'Class) is
N : Natural := O'Size;-- Error: Dispatching call
C : T'Class := O; -- Error: implicit Dispatching Call
begin
if O in DT'Class then -- OK : Membership test
Q (DT (O)); -- OK : Type conversion plus direct call
else
P (O); -- Error: Dispatching call
end if;
end Test;
Obj : DT;
begin
P (Obj); -- OK : Direct call
P (T (Obj)); -- OK : Type conversion plus direct call
P (T'Class (Obj)); -- Error: Dispatching call
Test (Obj); -- OK : Type conversion
if Obj in T'Class then -- OK : Membership test
null;
end if;
end Example;