指向指针（ Pointer to pointer）

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小牛编辑

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2023-12-01

指向指针的指针是多个间接或指针链的形式。通常，指针包含变量的地址。当我们定义指向指针的指针时，第一个指针包含第二个指针的地址，它指向包含实际值的位置，如下所示。

必须声明一个指向指针的指针的变量。例如，

type
   iptr = ^integer;
   pointerptr = ^ iptr;

以下示例将说明概念并显示地址 -

program exPointertoPointers;
type
   iptr = ^integer;
   pointerptr = ^ iptr;
var
   num: integer;
   ptr: iptr;
   pptr: pointerptr;
   x, y : ^word;
begin
   num := 3000;
   (* take the address of var *)
   ptr := @num;
   (* take the address of ptr using address of operator @ *)
   pptr := @ptr;
   (* let us see the value and the adresses *)
   x:= addr(ptr);
   y := addr(pptr);
   writeln('Value of num = ', num );
   writeln('Value available at ptr^ = ', ptr^ );
   writeln('Value available at pptr^^ = ', pptr^^);
   writeln('Address at ptr = ', x^); 
   writeln('Address at pptr = ', y^);
end.

编译并执行上述代码时，会产生以下结果 -

Value of num = 3000
Value available at ptr^ = 3000
Value available at pptr^^ = 3000
Address at ptr = 45664
Address at pptr = 45680