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NULL in functions

Started by luxitan Jan 6, 2003 at 4:28 PM 4 replies 450+ views

Original Post

Jan 6, 2003 at 4:28 PM

how can i allow myself to put NULL instead of the predefined value to the function ie:

  
D3DMATERIAL9 CNothing::MakeMaterial(D3DCOLORVALUE diff, D3DCOLORVALUE amb, D3DCOLORVALUE emi, D3DCOLORVALUE spec, float power)
{
D3DCOLORVALUE black = MakeColorA(0.0f, 0.0f, 0.0f);
D3DMATERIAL9 temp = 
{
  diff==NULL?black:diff, 
  amb==NULL?black:amb, 
  spec==NULL?black:spec, 
  emi==NULL?black:emi, 
  power
}; 
return temp;
}

for example if diff is NULL i put the default value in if it isn't i put the value. thx [edited by - luxitan on January 6, 2003 5:34:07 PM]

CWizard

Jan 6, 2003 at 4:34 PM

I cannot easily see your question (i'm a bit drunk

), but if yyou replaced your == with a single =, I think it would do what you want.

EDIT: hehe, I was aparently too drunk... I cannot see any problem with your function. Is there a problem, and if so, what?

[edited by - CWizard on January 6, 2003 5:36:08 PM]

luxitan

Jan 6, 2003 at 4:45 PM

the problem is :

MakeMaterial(MakeColorA(0.1f,0.1f,1.0f),NULL,NULL,NULL);

gives me an error saying that const int cannot be converted to color, ie doesnt accept my NULL

i think the only option is to use pointers instead of the value

[edited by - luxitan on January 6, 2003 6:18:53 PM]

Cedric

Jan 6, 2003 at 6:44 PM

Why don''t you create a dummy color?

const D3DCOLORVALUE NULLCOLOR = MakeColorA(0.666f,0.666f,0.666f);

It''s very bad style, but I don''t think there''s a good way to do what you want in C++. You could always pass a structure, and only fill the required members.

Cédric

sardak

Jan 7, 2003 at 5:10 AM

Are the D3DCOLORVALUE parameters pointers or actual values? If they''re pointers, NULL should work fine in that, but if they''re actual values, then there''d be a parameter size conflict I would think.

SabreMan

Jan 7, 2003 at 5:38 AM

quote:
Original post by luxitan
MakeMaterial(MakeColorA(0.1f,0.1f,1.0f),NULL,NULL,NULL);

gives me an error saying that const int cannot be converted to color, ie doesnt accept my NULL

Yes, and that's an accurate description of the problem. Presumably MakeColorA is a macro or function returning some sort of "Color" object, no? In that case, the infinite wisdom of C++ dictates that you cannot pass null where an object is expected. You have a few choices:

a) pass the parameters by pointer, so that you can check for absence of a parameter.

b) work around the C++ type system by use of the "Null Object Idiom". Basically, the Null Object Idiom involves identifying a set of values for a Color object which are to be interpreted as giving null identity. You might do this by using a combination of values which will never be viable for normal usage. For a Color object, I suspect there is no such set of values, so you might consider modifying the Color class to have an "is_null" flag, and add a constructor that accepts "Color(NULL)".

c) provide an overload of MakeMaterial which omits the extraneous values, possibly delegating to a common function which contains the shared implementation between the extant and new functions.

[edited by - SabreMan on January 7, 2003 6:39:51 AM]

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