Function Pointers

Function Pointers

[Barry L. Dorough]

Is there any way to implement function pointers in Ada?  Below is a
sample C program using function pointers.  How can I do this in Ada?

                                                                Thanks
in advance,
                                                                Barry
Dorough


#include <iostream.h>
double Add( double x, double y );
double Subtract( double x, double y );
double Combine( double p, double q, double (*fcn)(double r, double s) );

int main()
{
    double w = 20.0;
    double x = 5.0;
    double y = Combine( w, x, Add );
    double z = Combine( w, x, Subtract );
    cout << w << "  " << x << "  " << y << "  " << z << endl;
    return 0;
}
double Add( double x, double y )
{
    return x+y;
}
double Subtract( double x, double y )
{
    return x-y;
}
double Combine( double p, double q, double (*fcn)(double r, double s) )
{
    double a = 10.0 + p;
    return fcn( a, q );
}

Re: Function Pointers

[Matthew Heaney]

In article <353257CC.211B84EE@phaseiv.com>, "Barry L. Dorough"
<bdorough@phaseiv.com> wrote:

>Is there any way to implement function pointers in Ada?  Below is a
>sample C program using function pointers.  How can I do this in Ada?

[snip]

>double Combine( double p, double q, double (*fcn)(double r, double s) )
>{
>    double a = 10.0 + p;
>    return fcn( a, q );
>}

Declare a named access type, as follows:

package P is

   type Func_Access is access function (R, S : Long_Float) return Long_Float;

   function Combine (P, Q : Long_Float; Func : Func_Access) return Long_Float;

end P;

Re: Function Pointers

[Jerry van Dijk]

Barry L. Dorough (bdorough@phaseiv.com) wrote:

: Is there any way to implement function pointers in Ada?

Yes, basically the same way as in C. However, being a stronger
typed language, Ada asks you to explicitly create a pointer type
for the functions you want to access through a pointer, so that the
compiler can check argument and return types.

: Below is a
: sample C program using function pointers.  How can I do this in Ada?

Actually, your program is C++, not C. Anyway:

-- As GNAT allows only one compilation unit in a file
--  we nest the functions within main;
-- Long_Float would be GNAT's equivalent to gcc's double;
-- We pretend that Main is a 'void' function. 

with Ada.Text_IO; use Ada.Text_IO;

procedure Main is

   type Func_Ptr is access function (X, Y : Integer) return Integer;
   -- Declare a pointer type, pointing to a function that takes
   -- two Integer variables as input and returns a Integer

   function Add (X, Y : Integer) return Integer is
   begin
      return X + Y;
   end Add;

   function Subtract (X, Y : Integer) return Integer is
   begin
      return X - Y;
   end Subtract;

   function Combine (P, Q : Integer; Func : Func_Ptr) return Integer is
      A : Integer := 10 + P;
   begin
      return Func (A, Q);
   end Combine;

   W : Integer := 20;
   X : Integer := 5;
   Y : Integer := Combine (W, X, Add'Access);
   Z : Integer := Combine (W, X, Subtract'Access);

begin
   Put_Line (W'Img & " " & X'Img & " " & Y'Img & " " & Z'Img);
end Main;
-- 
-- Jerry van Dijk  | email: jdijk@acm.org
-- Leiden, Holland | member Team-Ada

Re: Function Pointers

[Robert Dewar]

Jerry said

<<-- As GNAT allows only one compilation unit in a file
--  we nest the functions within main;
>>

Surely a bad reason for doing anything. It is bad practice (see for example
AQ&S) generally to put more than one compilation unit in a file anyway.

But the issue of whether to write a single compilation unit or many
compilation units is surely a design issue that should not be affected
by low level representation issues (i.e. how compilation units are 
represented in files, whether you want to use gnatchop if you are using
GNAT etc.)

Re: Function Pointers

[Jerry van Dijk]

Robert Dewar (dewar@merv.cs.nyu.edu) wrote:

: Jerry said

: <<-- As GNAT allows only one compilation unit in a file
: --  we nest the functions within main;
: >>

: Surely a bad reason for doing anything. It is bad practice (see for example
: AQ&S) generally to put more than one compilation unit in a file anyway.

Agreed. I was just being lazy.

-- 
-- Jerry van Dijk  | email: jdijk@acm.org
-- Leiden, Holland | member Team-Ada