Re: Substrings as argument to procedures/functions

[rieachus@comcast.net]

On Wednesday, April 13, 2016 at 5:29:29 PM UTC-4, Randy Brukardt wrote:

> Probably a better example is to remember that the range of Positive is 
> implementation-defined, and that the language only requires the upper bound 
> to be at least 32767. So if you need strings that have potentially more 
> characters than that (to read an entire text file, for instance), and you 
> want the code to be unconditionally portable (to steal someone else's line), 
> you need to declare a type yourself (here, assuming that a million 
> characters are enough)...

Is it time to "fix" this?  Or are there still Ada compilers around that use 16-bit String indexes by default? I remember when (in the early days of Ada83) the question of whether String should use 16 or 32 bit indexes was a major implementation decision.  Today, compilers that support hardware indexes shorter than 32-bits are probably mapping to some hardware defined offset field in instructions. Using such indexes when the subtype allows is obviously an optimization worth supporting.  But in Ada defining "subtype Short_String is String range (1..32767);" or whatever allows using such a hardware type if available.  But there is no need to make Short_String a type.

Yes, programs may have records with character string fields limited such that the index can fit into a 16-bit field in the record.  But I can't imagine doing that without an explicit layout for the record or at least a size for the index field. Back to the original topic here, you want/need sliding to work such that

    type Rec is
      Name_Length : Integer range 0..32_767; 
      Name: String;
    end;
    for Rec use ...
    ...
    Name_Length := Param'Length;
    Name(1..Name_Length) := Param;

 works as expected.