Re: status of Ada STL?

[Fraser Wilson <newsfraser@blancolioni.org>]

18k11tm001@sneakemail.com (Russ) writes:

> By the way, it's too bad that Ada wasted "/=" on "not equals. It would
> be very useful for dividing a matrix by a scalar. For example, the
> construct
> 
> A := A / 2.0;
> 
> is very inefficient because a temporary matrix must be created to
> store A/2, then an assignment must be used to put the result back in
> A. On the other hand,
> the construct
> 
> A /= 2.0
> 
> is not only much cleaner looking, it is much more efficient because
> you can skip the temporary matrix and the assignment. And please don't
> argue with me here, because you will only be showing your ignorance.

C++ seems to tie itself in knots over the issue of temporaries, which
is why constructs such as the above have to be used.  Is it because of
the class/constructor/destructor thing?  I don't know, but it's always
seemed odd that programmers of this language get so caught up in it.

In any case, Ada has a different type model, and, even in the case of
matrices, the expression "A := A / 2.0" can, in many situations, be
optimised by the compiler to avoid temporaries.  I'm not saying they
do, but they can, and it's not particularly difficult as optimisations
go.

Look, I'll write some pseudo code: if the assignment expression looks
like "X := X <op> Y", X is of type T, T is big, and T, <op> are
declared in a pure package, put the address of X on the stack and call
the non-temporary version of <op>.  When compiling operators of a big
type in a pure package, create a non-temporary version of each
suitable operator, and have a standard version which calls that.

Alternatively, you could just pragma Inline these things, which will
also eliminate temporaries (I expect).

I'd much rather write my assignments in a natural and readable manner
("A becomes A divided by 2") than worry about arcane compiler
temporary issues.

> Good C++ programmers know all about this.

Ada programmers don't have to!

Fraser.