ada - 我怎样才能增加一个访问类型，就好像它是一个 C 指针一样？

Question

如何在 C 中递增访问类型的地址，如指针？我是 Ada 的新手...

procedure main is
  type myarr is array(1..5)of integer; --creating array of 5 integer.
  myarr_var:aliased myarr:=(2,5,7,9,0); --setting 5 values
  type my_access is access all myarr; --creating access type (pointer)
  var:my_access:=myarr_var'access;  --holding address of 5 integer
begin;
  -- since var holds the address of myarr_var now i want to increment
  -- the address by adding 1 to print next value (5) as we do in c?

  put((var+1).all); --???
  -- i know this is bad but how to increment its base address of 
  -- var(pointer) so that it will point to next element 5?

end main; 

Answer 1

实例化Interfaces.C.Pointers在 Ada 中进行 C 风格的指针运算。

最好用例子来解释:

with Ada.Text_IO; use Ada.Text_IO;
with Interfaces.C.Pointers;

procedure Access_Pointer_Arithmetic is

   type Myarr_Indices is range 1 .. 5;
   type Myarr is array (Myarr_Indices range <>) of aliased Integer;

   Myarr_Terminator : constant Integer := 0;

   package Myarr_Pointer_Arithmetic is new Interfaces.C.Pointers
     (Myarr_Indices, Integer, Myarr, Myarr_Terminator);

   use Myarr_Pointer_Arithmetic;

   Myarr_Var : aliased Myarr := (2, 5, 7, 9, 0);

   Var : Myarr_Pointer_Arithmetic.Pointer :=
              Myarr_Var(Myarr_Var'First)'access;

begin
   Put_Line(Integer'Image(Var.all));
   Increment(Var);
   Put_Line(Integer'Image(Var.all));
   Increment(Var);
   Put_Line(Integer'Image(Var.all));
   Increment(Var);
   Put_Line(Integer'Image(Var.all));

   Var := Var - 2;
   Put_Line(Integer'Image(Var.all));
end Access_Pointer_Arithmetic;

运行它:

C:\Ada\Sandbox\access_pointer_arithmetic
 2
 5
 7
 9
 5

该包提供了 ptrdiff_t 的单次递增/递减、加法和减法，以及指定终止符和定长元素数组的检索。

(注意跑到数组的末尾... :-)