What does 'cdecl = nil' (placed after a function declaration) mean?

Question

Check this demo source from the excellent Detour library:

implementation

{$R *.dfm}

var
  TrampolineGetMemory: function(Size: NativeInt): Pointer;
cdecl = nil;

Please look at the cdecl = nil; statement. What does it mean in this context?

Note - I already know that cdecl stands for a calling convention.

@DavidHeffernan I think that particular initialization, in some programs, can actually gain you about ten million years of perf increase. — J..., Apr 19 '16 at 16:07

score 7 · Accepted Answer · answered Apr 19 '16 at 13:58

7

This is just another way to initialize the variable. For example:

program Project1;

{$APPTYPE CONSOLE}

var
  i : integer = 5;
begin
  WriteLn(i);
  ReadLn;
end.

it may be clearer if it was written on one line as

var
  TrampolineGetMemory: function(Size: NativeInt): Pointer; cdecl = nil;

Or maybe even better if a type was defined :

type
  TTrampolineGetMemory = function(Size: NativeInt): Pointer; cdecl;

//... 
 var
   TrampolineGetMemory: TTrampolineGetMemory = nil;

answered Apr 19 '16 at 13:58

J...

Note that it completely redundant as global variables are initialised to `0/nil/false` anyway - see [Are delphi variables initialized with a value by default?](http://stackoverflow.com/questions/132725/are-delphi-variables-initialized-with-a-value-by-default). This is like initialising an event handler to nil or a boolean field to false in a constructor - completely pointless – Gerry Coll Apr 19 '16 at 21:19
@GerryColl Indeed it is. Sometimes coding standards will dictate this sort of thing, though (I guess?). Could be explicit for clarity, or possibly just a reflex carried over from locals. It's not necessary, but I'm not sure I'd go so far as to call it wrong. – J... Apr 19 '16 at 22:44
Seeing initialisation to 0 in a constructor is a code smell to me - it implies that the original coder either didn't understand Delphi, or couldn't be bothered to find out what the default is. – Gerry Coll Apr 19 '16 at 23:34
4

@Gerry or wanted the reader to be in no doubt and not have to think – David Heffernan Apr 20 '16 at 04:52

score 7 · Answer 2 · answered Apr 19 '16 at 13:59

7

TrampolineGetMemory is a procedural variable initialized to nil.

It's easier to see if rewritten like

type
  TTrampolineGetMemory = function(Size: NativeInt): Pointer; cdecl;
var
  TrampolineGetMemory: TTrampolineGetMemory = nil;

answered Apr 19 '16 at 13:59

Uli Gerhardt

2 Answers2