Re: Compile time executed functions

[Jean-Marc Bourguet <jbourguet@free.fr>]

Stephen Leake wrote:
> 
> Jean-Marc Bourguet <jbourguet@free.fr> writes:
> 
> > > : First feature, things like
> > > :    enum { foo = x+y*z; }
> > > : are valid as long as x, y and z value are known at compile time and have
> > > : to be evaluated at compile time.
> > >
> > > Isn't this usually resolvable as a constant by most Ada compilers? At least
> > > the ones I know will usually do so.
> >
> > It is quite difficult to come with the equivalent in Ada.  One is
> >    foo : constant = x+y*z;
> > is indeed resolvable by most Ada compiler.
> 
> Actually, since C and C++ define an enum to be just a named int,

I don't know for C but for C++ an enum cary more type information that
you seem to think.  There is an implicit conversion from enum to int but
not from int to an enum nor between enum.  The following does not
compile.

enum E1 {a = 1, b = 2, c = 4};
enum E2 {d, e};

void f(E1 x);

void g() {
  f(d);
}

because a E2 is passed where an E1 is expected.

> this
> is the _exact_ equivalent in Ada.

I don't agree.

> Perhaps you meant to say that
[use of representation clause] 
> is not legal Ada. However, GNAT 3.14a disagrees with you.

No.  Using representation clause to try to emulate C++ enum value is
doomed.  The enum values are far more visible to the program.  I think
the more complete way to represent a C++ enum in Ada is use an integer
type or an modular type with the correct range and give typed constant. 
Obviously the best way depend on the usage and may not be the more
complete way...

- Jean-Marc