Re: Fixed point constants issue

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

On 09/13/2010 12:05 PM, Niklas Holsti wrote:
>
> The declaration of TEN_FEET_1 is a named-number declaration, so the
> expression is evaluated using the "root" multiplication operator "*"
> (Left : root_integer; Right : root_real) return root_real, which is done
> at compile-time and exactly (with unlimited precision).

Right.

>  > I would expect the compiler to evaluate the expression
>  > "HEIGHT * FEET_PER_METER" [for TEN_FEET_2] _before_
>  > converting it to the appropriate fixed point type.
>
> The declaration of TEN_FEET_2 is an object declaration with type
> Altitude, therefore the right-hand-side expression is expected to be of
> type Altitude and so the "*" will be resolved to the Altitude
> multiplication operator "*" (Left, Right : Altitude) return Altitude,
> with result zero.

More likely,

function "*" (Left : in Integer; Right : in Altitude) return Altitude;

[ARM 4.5.5(14)]. Feet_Per_Meter will be converted to Altitude before calling 
this function, resulting in a value of 0.0.

Note that this changed between Ada 83 and Ada 95. In Ada 83, the expression is 
evaluated exactly at compile time using universal_real, just as for the named 
number. In Ada 95, this was changed to use the "*" function defined for Altitude.

If the target compiler gives 3.0 as the value, then perhaps it is an Ada-83 
compiler.

There is a further wrinkle with the Ada 95 (and current Ada) rule. If "*" has 
not been overridden, this is a static expression and is evaluated at compile 
time. If "*" has been overridden, then this is not a static expression and it 
will be evaluated at run time.

-- 
Jeff Carter
"Mr. President, we must not allow a mine-shaft gap!"
Dr. Strangelove
33

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