How to recreate a function retrieving the highest element of an object?

Question

My plan is to make a function that retrieves the highest element of an object, so to speak, the upper range of an array.In other words,I'm trying to get the the code of the function High().

What I have tried so far:

 function High2(var X):integer;
 begin
   Result:=Pbyte(Cardinal(@X)-1)^-1;
 end;

The function above should read the value(length) before the position of the first element in the object(array/string) and return it decreased by 1.However It doesn't retrieve correct results neither on static nor dynamic array type.

How do I recreate the High() function in Pascal?

Answer 1

Not sure why you'd want to do that when Delphi already has a built-in High() compiler magic function, but OK, here goes.

Static arrays: Can't be done. No size information is stored at runtime since the size is known to the compiler and can't change. High() just drops the necessary number into the code as a constant.

Dynamic arrays: The compiler translates High to a call to DynArrayHigh in the System unit, which returns DynArrayLength - 1. DynArrayLength steps back 4 bytes from the start of the array (you're only stepping back 1) and returns the length as an integer instead of a byte.

Hope this is helpful. Why aren't you just using High, BTW?

Answer 2

The High() (and Low()) functions are referred to as "standard functions" which means they're intrinsic to the compiler. Like Write and Writeln, they don't really exist as normal Pascal function declarations. The compiler ensures that they are in the "System" unit scope merely as a convenience and to allow function of the same name within other scopes. This also allows you to qualify them by using System.High() to explicitly reference the System unit version. Since they are intrinsic, the compiler will automatically generate the proper code sequence for the type being considered. This also means that trying to duplicate the full functionality of them is nigh impossible. Just stick with the intrinsic standard functions.