Re: conversions between fixed-point types

["Jeffrey R. Carter" <spam.jrcarter.not@spam.acm.org>]

Dirk Herrmann wrote:
> 
> In particular I am disturbed by the fact that the following two lines 
> produce different results with GNAT, as has been confirmed for GNAT 
> 3.15p (thanks, tmoran):
>       FIO.Put(Float(FpB(Float(-1.5)))); TIO.Put(" ");      --> -1.20000E+00
>       FIO.Put(Float(MakeB(-1.5))); TIO.Put(" ");           --> -1.60000E+00
> In the first line, the conversion to FpB is done from a Float value, 
> which is a direct cast from -1.5.  In the second line, the conversion to 
> FpB is also done from a Float value, but in this case -1.5 is passed as 
> a Float argument to MakeB.  What I don't understand is, why it should 
> make a difference whether I cast -1.5 to Float and then convert it to 
> FpB compared to the situation where -1.5 is converted to a Float 
> argument, which is then converted to FpB.

You are complicating matters with your functions and your conversions to and 
from Float. Since IIRC your target H/W doesn't support floating-point, you will 
be able to see what happens and formulate your questions better if you deal only 
with the fixed-point types and direct type conversions between them:

A : Fpa := -1.5;
B : Fpb := Fpb (A);
...
Ada.Text_IO.Put_Line (Fpb'Image (B) );

What you have called a "direct cast" is a type conversion. Also, it is never 
necessary to convert a real numeric literal to a real type. Your 1st line could 
be written as

FIO.Put (Float (Fpb (Float'(-1.5) ) ) );

With the apostrophe, this is a "qualified expression" and tells the compiler 
that the expression in the parentheses is of type Float. However, I suspect that

FIO.Put (Float (Fpb'(-1.5) ) );

is even better.

-- 
Jeff Carter
"It's all right, Taggart. Just a man and a horse being hung out there."
Blazing Saddles
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