Re: Concatenation and Characters

[Jeffrey Carter <jrcarter@acm.org>]

Programmer Dude wrote:
> Upthread Preben Randhol wrote this:
> 
> | with Ada.Text_IO; use Ada.Text_IO;
> | 
> | procedure uninit is
> |    A  : Integer;
> |    B  : Integer := 2;
> | begin
> | 
> |    if A > B then
> |       Put_Line ("A is greater than B");
> |    else
> |       Put_Line ("B is greater than A");
> |    end if;
> | end uninit;
> 
> Maybe my question is best stated thus: is there any correct implentation
> of Ada that would allow this code to run?

All correct implementations of Ada must compile and execute this 
program. Ada does not require default initialization of variables except 
for access variables, if we limit ourselves to elementary types. Ada 
does not require that compilers detect references to uninitialized 
variables. Ada does not require that references to uninitialized 
variables be detected at run time.

Some compilers will warn you of references to uninitialized variables, 
but that is not required.

In general, detecting all references to uninitialized variables during 
compilation, without ever incorrectly flagging a reference to an 
initialized variable, is impossible. Compilers that issue warnings 
usually only do so for "simple" cases.

Ada is intended for embedded, real-time systems. The overhead of 
requiring default initialization of all variables may not be acceptable 
for such systems. This was especially true in the early 1980s when Ada 
was first being developed.

Between the overhead of default initialization and the impossibility of 
detecting references to uninitialized variables, the language did not 
require such things. The revision process for the language has backwards 
compatibility as a major goal, so don't expect this to change.

However, you might be interested in pragma Normalize_Scalars in the 
Safety and Security Annex (Annex H). If your compiler supports this 
Annex and you can accept the overhead, this pragma insures that scalars 
are default initialized to some value. When possible, this value will be 
out of range for the subtype.

-- 
Jeff Carter
"Your mother was a hamster and your father smelt of elderberries."
Monty Python & the Holy Grail