Re: Newbie question : types , representation

[Brian Rogoff <bpr@shell5.ba.best.com>]

On Tue, 24 Aug 1999, Jos De Laender wrote:
> What I do know that in C, I can do all of my operations (shifting,
> anding ..) on chars.

In Ada (we always mean Ada 95 here when we say Ada, a situation unlike the 
VHDL world ;-) the package Interfaces contains the Shift_Left,
Shift_Right, and other operations you're looking for, as well as types
like Unsigned_8, Unsigned_16, etc. These are probably what you're looking
for. I rather wish there were operators (<<, >>) for this, as in C, but
it is not a problem, just my syntactic preference. I won't even start 
talking about <<= :-)

> It's ugly in the sense that I'm mixing really semantics of chars and
> bytes and integers and whatever. But on compiler output I can expect a
> fairly optimal implementation.

I guess I don't find it (C) that ugly here, but my tastes are my own. C is
a very low level language compared to Ada, so it is to be expected that it 
would be this way. C declaration syntax is ugly IMO.

> In ADA , I think (but this was part of the question) , I have to go
> through some tables and through some conversion functions. Although it
> is not necessarily so , there is at least the increased risk of less
> optimal implementation, because the compiler has to find out those
> things. I didn't say this is not possible, I asked how one could write
> it without having this risk.

If you know that characters are the same size as your "byte" type, be it
an Interfaces.Unsigned_8 or a System.Storage_Element then it should be OK 
to use Unchecked_Conversion if you're worried about the overhead of a
conversion function, but I really can't imagine any overhead even from a 
safe type conversion if you turn checking off, which you really must do if 
you want to compare with C. I don't understand why you think there is some 
table lookup involved.

Part of the problem with C and Ada comparisons is that every C operation is  
unchecked, whereas in Ada everything is checked by default. You have to
know enough about your Ada compiler to turn off those checks when doing
the performance comparisons.

-- Brian