I am experimenting a bit (out of academic interest, no tangible use-case) with protected interfaces and have come-up with the following code:
procedure Protected_Map is
type Key is new Character range 'A' .. 'K';
type Value is range -10 .. 10;
package Maps is
type Map is protected interface;
procedure Insert(M: out Map; K: in Key; V: in Value) is abstract;
procedure Find(M: in out Map; K: in Key; V: out Value) is abstract;
end Maps;
use Maps;
-- protected type implementation
protected type A_Map is new Map with
overriding
procedure Insert(K: in Key; V: in Value);
not overriding
function Get(K: in Key) return Value;
end A_Map;
protected body A_Map is
procedure Insert(K: in Key; V: in Value) is null; -- dummy
function Get(K: in Key) return Value is (0); -- dummy
end A_Map;
overriding
procedure Find(M: in out A_Map; K: in Key; V: out Value) is
begin
V:= M.Get(K);
end Find;
begin
null;
end Protected_Map;
The idea is to implement a task-safe Map by declaring a protected interface type with the associated procedures of inserting and finding values by some key. A protected type is then declared in order to implement the interface which only partly implements it (i.e. Insert(..)) and introduces a Get(..) function to retrieve a value by some key. The second required procedure (Find(..)) is implemented in an independent full-blown way.
The reason for this is so that writers continue to access the map in a mutually exclusive way whilst readers are able to access it concurrently.
However, compilation is failing for Find(..) with the compiler complaining that no selector "Get" for type derived from A_Map. Why can it not see Get which is clearly in the protected type declaration? Is it a scoping issue i.e. do the protected type declaration and the independent implementation need to be in the same package? (or some other unit?)
The problem is your compiler has a bug with the error message. The actual error you are making is trying to declare an overriding operation of your A_Map type after specifying a body for the protected type. Ada wants you to declare all your overrides before the type "freezes" and making a body for the protected type freezes it. If you add in a declaration for your Find procedure before the body of the protected type, then all is good. It's a bad error message.
See example
with Ada.Text_IO; use Ada.Text_IO;
procedure jdoodle is
procedure Protected_Map is
type Key is new Character range 'A' .. 'K';
type Value is range -10 .. 10;
package Maps is
type Map is protected interface;
procedure Insert(M: out Map; K: in Key; V: in Value) is abstract;
procedure Find(M: in out Map; K: in Key; V: out Value) is abstract;
end Maps;
use Maps;
-- protected type implementation
protected type A_Map is new Map with
overriding
procedure Insert(K: in Key; V: in Value);
not overriding
function Get(K: in Key) return Value;
end A_Map;
-- Notice the declaration I added here before the body code.
overriding
procedure Find(M: in out A_Map; K: in Key; V: out Value);
protected body A_Map is
procedure Insert(K: in Key; V: in Value) is null; -- dummy
function Get(K: in Key) return Value is (0); -- dummy
end A_Map;
procedure Find(M: in out A_Map; K: in Key; V: out Value) is
begin
V:= M.Get(K);
end Find;
begin
null;
end Protected_Map;
begin
null;
end jdoodle;