CODEWITHSUNDEEP
pointersabstract-classabstractada
David DeMar
David DeMar

Reputation: 2660

Implementing an abstract function with access types in Ada

I have a package called Statements with an abstract type called Statement and an abstract function called execute(). In another package I have a type CompoundStatement which is a type Statement and it implements the execute() function.

I have a function called createStatement(). It's purpose is to evaluate a token of type Unbounded_String and determine what keyword it contains. Then based on this keyword it will generate an access type based on this keyword.

So far so good.

But what I can't figure out how to do is call the correct execute method. I only have one keyword coded in right now because it's not working yet.

Sorry if my description sounds convoluted.

package Statements is

   type Statement is abstract tagged private;
   type Statement_Access is access all Statement'Class;

   function execute(skip: in Boolean; T: in TokenHandler; S: in Statement) return Integer is abstract;

private
   type Statement is abstract tagged
      record
         tokens: Vector;
      end record;

end Statements;

   procedure createStatement(T : in TokenHandler; stmt: out Statement_Access) is
      currenttoken : Unbounded_String;
      C            : CompoundStatement;

   begin
      currenttoken := To_Unbounded_String(TokenHandlers.getCurrentToken(T));
      if currenttoken = "begin" then
         createCompoundStatement(T, C);
         stmt := new CompoundStatement;
         stmt.all := Statement'Class(C);
      end if;
   end createStatement;

   procedure createCompoundStatement(T : in TokenHandler; C: out CompoundStatement) is
   begin
      C.tokens := T.tokens;
   end createCompoundStatement;

   function execute(skip: in Boolean; T: in TokenHandler; C: in CompoundStatement) return Integer is
      TK: TokenHandler := T;
      stmt: Statement_Access;
      tokensexecuted: Integer;
      currenttoken : Unbounded_String;
   begin
      TokenHandlers.match("begin", TK);
      currenttoken := To_Unbounded_String(TokenHandlers.getCurrentToken(TK));
      while(currenttoken /= "end") loop
         Put(To_String(currenttoken));
         createStatement(T, stmt);
         tokensexecuted := execute(skip, TK, stmt);  //ERROR OCCURS HERE
         TokenHandlers.moveAhead(tokensexecuted, TK);
         currenttoken := To_Unbounded_String(TokenHandlers.getCurrentToken(TK));
      end loop;
      TokenHandlers.match("end", TK);
      return TokenHandlers.resetTokens(TK);      
   end execute;

I get this error when I compile:

statements-statementhandlers.adb:35:28: no candidate interpretations match the actuals:
statements-statementhandlers.adb:35:46: expected type "CompoundStatement" defined at statements-statementhandlers.ads:14
statements-statementhandlers.adb:35:46: found type "Statement_Access" defined at statements.ads:6
statements-statementhandlers.adb:35:46:   ==> in call to "execute" at statements-statementhandlers.ads:10
statements-statementhandlers.adb:35:46:   ==> in call to "execute" at statements.ads:8

Upvotes: 2

Views: 922

Answers (1)

Simon Wright
Simon Wright

Reputation: 25491

The third parameter to execute is expected to be a (child of) Statement, but what you’ve given it is a pointer to a (child of) Statement. You probably want

tokensexecuted := execute(skip, TK, stmt.all);

As a matter of style, by the way, it’s usually best to make the dispatching parameter the first; you could then (in Ada 2005) say

tokensexecuted := stmt.execute(skip, TK);

Upvotes: 3

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