Call function by memory address in Pascal - Delphi

Question

Hi all.

I have a function in C++ that I call with its memory address with typedef, and I want to do the same thing in Delphi.

typedef void (*function_t)(char *format, ...);

function_t Function;

Function = (function_t)0x00477123;

And then, I call it with: Function("string", etc);

I tried to do this in Delphi, but got no results.

Is there any way to do this without using "asm" in Delphi?

If so, please give me a example code.

Thanks.

Answer 1

Yes, Delphi supports function pointers. Declare it like this:

type MyProcType = procedure(value: string);

Then declare a variable of type MyProcType and assign the address of your procedure to it, and you can call it the same way you would in C.

If you want a pointer to a method of an object instead of a standalone procedure or function, add "of object" to the end of the function pointer declaration.

Answer 2

program Project1;

type
  TFoo = procedure(S: String);

var
  F: TFoo;
begin
  F := TFoo($00477123);
  F('string');
end.

Of course, if you just run the above you'll get a runtime error 216 at address $00477123.

Answer 3

An idiomatic translation for this:

typedef void (*function_t)(char *format, ...);
function_t Function;
Function = (function_t)0x00477123;

Is this:

type
  TFunction = procedure(Format: PAnsiChar) cdecl varargs;
var
  Function: TFunction;
// ...
  Function := TFunction($00477123);

The 'cdecl varargs' is required to get the C calling convention (where the caller pops the stack) and the variadic argument support (which is only supported with the C calling convention). Varargs is only supported as a means for calling C; there is no built-in support in Delphi for implementing variadic parameter lists in the C style. Instead, there is a different mechanism, used by the Format procedure and friends:

function Format(const Fmt: string; const Args: array of const): string;

But you can find out more about that elsewhere.