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const problem...

Started by mrmrcoleman Jan 20, 2005 at 12:05 PM 3 replies 450+ views

Original Post

Jan 20, 2005 at 12:05 PM

I am having trouble with a method which accepts a reference to a const RECT. It looks like this:


void SetPrintRect(const RECT& RPrint) { DoSetPrintRect(RPrint); }

I am trying to use this function but I am not sure how to do it! I tried this but it doesn't work because the RECT is not const.


RECT CustomRect.
CustomRect.top = 10;
etc etc

SetPrintRect(&CustomRect);

Then I tried this:


const RECT CustomRect;
// assignments here
SetPrintRect(&CustomRect);

But obviously this won't work because I can't assign to a const RECT, so what do I do? I know this obviously seems like a stupid question, but I don't really use const at all in my programming (before you start to flame, just stop). Thanks for any help. Mark Coleman

BigBadBob

Jan 20, 2005 at 12:11 PM

your problem is the function accepts a reference, you are passing a pointer value (the address of the RECT).

when a function accepts a reference, the calling code is no different than pass by value.

RECT rect;
// stuff here
SetPrintRect(rect);

ontheheap

Jan 20, 2005 at 12:12 PM

AFAIK, you don't have to pass a const object to a function expecting a const object. It's just a "promise" saying "I won't change your data." It's safe to pass a non-const as a const argument because it's already safe to change the data. In this case you're just saying you won't. The other way around, however (passing a const object to a function requiring a non-const object won't work). I get the feeling that this isn't what you're looking for though...

ajas95

Jan 20, 2005 at 12:16 PM

You don't use CONST ?!?! You Idiot! Of all the programming practices.....
Ok, I'm just kidding.

The problem is probably that DoSetPrintRect() doesn't take a const parameter. 'const' attached to an argument to the function just promises that the function won't change it, it's not a requirement on the input passed to it. The side-effect of that is that the function taking the const argument can't then pass it to some other function as a non-const argument (in this case DoSetPrintRect).

Something like:

void DoSetPrintRect(const RECT& RPrint) { /* blahbitty blah blah. */ }void SetPrintRect(const RECT& RPrint) { DoSetPrintRect(RPrint); }RECT CustomRect;CustomRect.top = 10;SetPrintRect(CustomRect); // note you don't need the ampersand here.

should work.

mrmrcoleman

Author

mrmrcoleman

Jan 20, 2005 at 12:33 PM

Thanks everyone for your help, ho hum! Should have consulted the good old C++ Programming Language but I have been in and out of it all day and I just wanted AN EASY ANSWER, which I got!!

Thanks again, and thanks for the lack of flames!

Kind regards.

Mark Coleman

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