Ada String Concatenation

Question

I have a function that returns a string for a particular item, and I need to call that function numerous times and combine those strings into one. The combined string is bounded. I've made sure to fill it when space characters when it initializes but I keep getting "length check failed" errors. Is there something basic I'm doing wrong here?

FOR I IN 1..Collection.Size LOOP  
    Combined_String :=  combined_string & Tostring(Collection.Book(I));  
END LOOP;

Answer 1

Unbounded_String is probably the easiest way to go:

with Ada.Strings.Unbounded;
use Ada.Strings.unbounded;

   ...

Temp_Unbounded_String : Unbounded_String;  -- Is empty by default.

   ...

for I in 1 .. Collection.Size loop
   Append(Temp_Unbounded_String, ToString(Collection.Book(I));
end loop;

If you then need to have the result placed in your fixed length standard string:

declare
   Temp_String : constant String := To_String(Temp_Unbounded_String);
begin
   -- Beware! If the length of the Temp_String is greater than that of the
   -- fixed-length string, a Constraint_Error will be raised.  Some verification
   -- of source and target string lengths must be performed!
   Combined_String(Temp_String'Range) := Temp_String;
end;

Alternatively, you can use the Ada.Strings.Fixed Move() procedure to bring the Unbounded_String into the target fixed-length string:

Ada.Strings.Fixed.Move(To_String(Temp_Unbounded_String), Combined_String);

In this case, if the source string is "too long", by default a Length_Error exception is raised. There are other parameters to Move() that can modify the behavior in that situation, see the provided link on Move for more detail.

Answer 2

In order to assign Combined_String, you must assign the full correct length at once. You can't "build up" a string and assign it that way in Ada.

Without seeing the rest of your code, I think Ada.Strings.Unbounded is probably what you should be using.