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From: info-ada@ucbvax.UUCP
Newsgroups: net.lang.ada
Subject: Re: More fun with public domain Ada
Message-ID: <8601291840.AA27130@cod.ARPA>
Date: Wed, 29-Jan-86 13:40:45 EST
Article-I.D.: cod.8601291840.AA27130
Posted: Wed Jan 29 13:40:45 1986
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References: <8601210108.AA00678@ucbvax.berkeley.edu>
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Organization: Naval Ocean Systems Center, San Diego
List-Id: <comp.lang.ada>

In article <8601210108.AA00678@ucbvax.berkeley.edu> you write:
>
> . . .
>
>  One program had the following statement (without the comment!)
>
>    LNTH : NATURAL range 1..8 := 0;  --whoops, can't count
>
>  I worry when I see things such as that, but the interesting part came
>after I had compiled the program containing that declaration (I didn't pick
>up on it during a cursory scan of the source).  It compiled FINE, and it wasn't
>till I began to execute the program that I recieved a CONSTRAINT_ERROR (very
>helpful message, but standard) and a stack dump.  Chee, and I had even turned on
>the debugger... anyway, the stack dump pointed me to the statement after the
>offending line, and I could take it from there.
>
>  Why can't the compiler catch such problems during compile time??  I thought
>an Ada compiler should have caught such things.  Interestingly enough, there
>are multiple examples of a string initializer being one character too long
>in thsi same program, and the results are similar.  No compile problem, but
>a CONSTRAINT_ERROR and stack dump during elaboration.


According to one textbook I have:
	
	...there is a general principle that a declaration with
	initial value
	    A: T:=I;
	is essentially equivalent to
	    A: T;
	    ...
	    A:=I;
	in which the declaration is given without the initial value
	and is then followed by an assignment statement.

Also, in the Ada Reference Manual, section 3.2.1 you will find:

	The initialization of an object (the declareed object or one
	of its subcomponents) checks that the initial value blongs to
	the subtype of the object....  The exception CONSTRAINT_ERROR
	is raised if this check fails.

That is to say that Ada requires a run-time check, but not a compile
time check for CONSTRAINT.


				David Collom

(All opinions expressed are my own.)