Re: Ada and string literals

[Adam Beneschan <adam@irvine.com>]

On Wednesday, January 30, 2013 8:52:32 AM UTC-8, codeallergy wrote:
> Okay. assuming dat we use GNAT 2012
> 
> Literal1 : aliased constant String := "LITERAL"; 
> Literal2 : aliased constant String := "LITERAL"; 
> Literal3 : aliased constant String := "LITERAL"; 
> 
> how the above code is handled ?
> 
> compiler duplicate the string three times into the data section or
> the three constants will share the same string or
> one string into the data section but duplicated at the run time ?

I don't know how GNAT handles it; but in Ada, having the string objects share the same memory causes a problem if the program later compares the 'Access of two different objects.  Ada says they're supposed to be unequal, but I think that would be hard to do if the string literals shared the same memory.  (I can think of ways to do this by adding some additional data to the access values, but that could easily result in way more overhead than just duplicating the strings would take up.)


> another question: how free a object from a procedure ?
> 
> example:
> 
> procedure Free_Abc (Target : access String)
> is
> begin
>    GNAT.Strings.Free(Target); -- error
> end Free_Abc;
> 
> this code produce the error: actual for "X" must be a variable
> 
> why disallow that ?

Because the definition of Free is that you give it an access variable, it frees the storage, then it sets the variable to null.  Target is basically a read-only value in this procedure, so you can't set it to null.  You can get around it easily enough by declaring a Temp variable and then

   Temp := Target;
   GNAT.Strings.Free(Temp);

But then whoever called Free_Abc may still have a variable that points to a string that no longer exists.  So you have to be careful.  I don't really recommend having a procedure such as Free_Abc that frees storage and isn't able to set the access to null; better to define a *named* access type that is access-to-String and make it an IN OUT parameter to Free_Abc.

                             -- Adam