Workaround "dynamic interface conversion not supported by configuration"

Question

I have an object oriented design as follows (Ada 2012). The question is not about the design itself, but its consequences with a particular runtime profile.

-- several packages ommitted here, ads/adb mixed together
type Interface_A is interface;
type Interface_A_Class_Access is access all Interface_A'Class;

type Interface_B is interface and Interface_A;
type Interface_B_Class_Access is access all Interface_B'Class;

type Interface_C is interface and Interface_B
type Interface_C_Class_Access is access all Interface_C'Class;

type B_Impl is abstract tagged ...;
type B_Impl_Access is access all B_Impl;
type C_Impl is new B_Impl and Interface_C ...;
type C_Impl_Access is access all C_Impl;

function Create_C return C_Impl_Access is begin
   return new C_Impl'(...);
end Create;

I have a factory to instantiate objects of Interface_A, Interface_B or Interface_C.

package body My_Factory is
   procedure Create_A return Interface_A_Class_Access is begin
      return Create_A_Impl; -- error: dynamic interface conversion not supported by configuration
   end Create_B;

   procedure Create_B return Interface_B_Class_Access is begin
      return Create_C_Impl; -- error: dynamic interface conversion not supported by configuration
   end Create_B;

   procedure Create_C return Interface_C_Class_Access is begin
      return Create_C_Impl; -- error: dynamic interface conversion not supported by configuration
   end Create_C;
end package My_Factory;

With my switches, I'm getting the following error for both factory create functions :

error: dynamic interface conversion not supported by configuration

Environment:

• GNAT 17.2
• ZFP MPC8641
• GPRBUILD Pro 18+

---

What I tried so far :

1. Change factory implementation with explicit cast or explicit temp variable allocation:

Sample:

package body My_Factory is
   ...
   procedure Create_B return Interface_B_Class_Access is begin
      return Interface_B_Class_Access(Create_C); -- error: dynamic interface conversion not supported by configuration
   end Create_B;

   procedure Create_C return Interface_C_Class_Access is
      tmp : Interface_C_Class_Access;
   begin
      tmp := Create_C; -- error: dynamic interface conversion not supported by configuration
      return tmp;
   end Create_C;
end package My_Factory;

Same problem.

2. Add explicit constructor methods (affectation of the 'new' into a class access variable)

Sample:

function Create_C return Interface_A_Class_Access is begin
   return new C_Impl'(...); -- error: dynamic interface conversion not supported by configuration
end Create;

function Create_C return Interface_B_Class_Access is
   tmp : Interface_B_Class_Access;
begin
   tmp := new C_Impl'(...); -- works fine
   return tmp;
end Create;

function Create_C return Interface_C_Class_Access is
   tmp : Interface_B_Class_Access;
begin
   tmp := new C_Impl'(...); -- works fine
   return tmp;
end Create;

This second option works fine.

3. With the standard profile the problem does not arise. I faced the issue while (naively) porting into a specific profile. As far as I get it, this is legit object oriented design, but some constructions are not handled in the same way.

---

My questions:

1. Is my second option acceptable ? Why does it work ?

2. Am I missing something ? I understand that this is somewhat related to dispatch tables management by compiler generated code, but I don't really get the deep mechanics/reasons.

Answer 1

The "not supported by configuration" message hints that this is a limitation of the runtime. And you are using a zero-footprint (ZFP) runtime, which has severe limitations when it comes to, for example, indefinite types (like class-wide types and unconstrained arrays).

The runtime documentation should provide more information about these restrictions/limitations.