Re: [Q] Returning Strings From A Function

[Jeff Carter <carter@spam.innocon.com>]

Guess I ought to put my 2 (cents|pence|centimes|etc :) in.

The poster requested information on how to handle the return value from
a function that returns type String in Ada 83. Given

function F return String;

the simplest is

I. Use it to initialize a constant

S : constant String := F;

This is fine if he only needs to store the value. If he also needs to
modify the value, things get a little more complicated. The simple
answer is

II. Do I. and copy the value to a variable

Cs : constant String := F;
S : String (Cs'range) := Cs;

This is often acceptable. Another way is

III. Obtain a variable-length string handling package, that provides for
conversions to and from String

with VSH;
...
S : VSH.V_String := VSH.Convert (F);

This is sometimes overkill. Intermediate complexity is

IV. Write a small function to store a length and value

type V_String is record
   Length : Natural := 0;
   Value  : String (1 .. Max);
end record;

function Convert (S : String) return V_String is
   Result : V_String;
begin -- Convert
   Result.Length := S'Length;
   Result.Value (1 .. S'Length) := S;

   return Result;
end Convert;
...
S : V_String := Convert (F);

Convert raises Constraint_Error if S won't fit in a V_String. It is also
possible for Convert to silently truncate.

A common objection to II. is that it "wastes space." Since the strings
found in real applications are usually short, this is not a real
concern. The poster has since indicated that he's interested in strings
<= 12 characters long, so I. or II. should work for him.

A common objection to III. is that it's overkill and links in a bunch of
excess stuff the program doesn't need. This is also often not a real
concern. There are also objections based on NIH or the cost of writing
or buying such a package. A good variable-length string package is an
essential part of the standard library for an Ada-83 project. Any
compentent software engineer can develop one in a few days. PragmAda
Software Engineering will sell you one for $25.

IV. attempts to overcome the objections to II. and III. This may
complicate the software with no true benefit.

In Ada (95), one can write

S : String := F;

and variable-length string packages are part of the language-defined
standard library.
-- 
Jeff Carter  ( carter @ innocon . com )

"Now go away, or I shall taunt you a second time." Monty Python & the
Holy Grail