C / C++ / MFC

11-Dec-01 8:23

I am new to C, and I have some questions about data types that I would really like answered.

What is the difference between using "const" and not using it, e.g. "const char" versus "char"?

What is the difference between "const" and "static"?

What is the difference between using "const" and "#define"?

C^hris L^osinger11-Dec-01 8:41

11-Dec-01 8:41

'const' tells the compiler that you won't change the value. if you try to, the compiler will complain.

in C, static, when applied to variables means the item is only visible in the current module (source file) and that the value will persist between function calls.

when applied to functions, static means the function is only visible within the current module. in C++, static takes on the job of declaring member functions that can be used outside of an instance of the class ('extern' would've been a better word for this, IMO).

#define is a preprocessor thing, const is a compiler thing. #define is an alias for some text which the preprocessor will swap out before the compiler gets to it. const is just a way to make a variable 'constant' (an oxymoron).

-c

Stu Pedful11-Dec-01 9:36

11-Dec-01 9:36

Thanks for the reply. I have a couple of followup questions though to help me clarify things.

Chris Losinger wrote:
in C, static, when applied to variables means the item is only visible in the current module (source file) and that the value will persist between function calls.

Isn't this normally true anyway (at least the first part) unless "extern" is used?

Chris Losinger wrote:
#define is a preprocessor thing, const is a compiler thing. #define is an alias for some text which the preprocessor will swap out before the compiler gets to it. const is just a way to make a variable 'constant' (an oxymoron).

So, when would one use something like

#define CPUSTRING "Intel - x86"

versus

const char CPUSTRING[] = "Intel - x86";

? CPUSTRING is a const char in either case, right?

moliate11-Dec-01 21:13

moliate

11-Dec-01 21:13

Stu Pedful wrote:
CPUSTRING is a const char in either case, right?

CPUSTRING is not a const char, just a bunch of text. if you have a small program:

#define CPUSTRING "Intel - x96"
#define SQUARE(x) x*x

main()
{
 CallTextFunction(CPUSTRING);
 CallNumberFunction(SQUARE(3+4));
}

The precompiler will change your program into this before compiling:

main()
{
 CallTextFunction("Intel - x96");
 CallNumberFunction(3+4*3+4); // :omg: 
}

Note that defines just replaces text, it is not typechecked, and might give some odd results as SQUARE did above. If you want a working version of SQUARE, it could be defined as:

#define SQUARE(x) ((x)*(x))

/moliate

Thanks!

Stu Pedful12-Dec-01 2:26

12-Dec-01 2:26

Ah, I undertand. Thanks for the terrific example. Smile | :)

Alvaro Mendez12-Dec-01 8:48

Alvaro Mendez

12-Dec-01 8:48

Stu Pedful wrote:
Isn't this normally true anyway (at least the first part) unless "extern" is used?

No.

Say you declare a variable or function at file scope inside A.cpp:

int nTest = 0;
 
void foo()
{
}

Then you go to file B.cpp and do the same thing:

int nTest = 0;
 
void foo()
{
}

When you go link these two files to create the final executable, the linker will complain that you've multiply defined the same variable and function. Thus, the variable and function in A.cpp are treated as "global" by the linker. When it then sees them also defined in B.cpp, it errors out.

This is different than going to B.cpp and declaring the variable and function extern.

extern int nTest;
 
extern void foo();

In that case, the linker uses the ones defined globally in A.cpp.

To finish my point, by making the variable and function "static", the linker ties them to the files themselves and does not treat them as globals.

static int nTest = 0;
 
static void foo()
{
}

This allows them to be redefined in multiple files without duplicity.

Regards,
Alvaro

Extern Question

Stu Pedful14-Dec-01 15:19

14-Dec-01 15:19

Thanks for the reply. I would like you to clarify a couple of points though.

Alvaro Mendez wrote:

extern int nTest; extern void foo();

In that case, the linker uses the ones defined globally in A.cpp.

To finish my point, by making the variable and function "static", the linker ties them to the files themselves and does not treat them as globals.

If all variables and functions that are not declared as "static" are automatically treated as global, then what is the purpose of "extern"? I was under the impression that extern was necessary for the value of a variable or function to persist across multiple source files. Is this true or no? And, how and when is "extern" used? If I wanted a variable or function to persist across multiple source files, would I just declare and define it in one of the source files and then put "extern <datatype> <variable name="">" in one of the common header files included at the top of each source files? Or is "extern" unnecessary in that case?

Re: Extern Question

Alvaro Mendez16-Dec-01 11:36

Alvaro Mendez

16-Dec-01 11:36

Remember that everything in C must be declared. If you want to use a variable defined in another file, you need to declare it in yours as extern. That's what extern is for: to tell the current file that you're using a variable defined somewhere else. If you don't use the extern keyword, the compiler thinks that you're defining it in that file.

Note that the extern keyword is only necessary for global variables. Functions don't really need it since the compiler can tell that you're not defining them by the fact it's a prototype with no body.

So, to sum it up, the extern keyword is necessary for a variable (not just its value) to be made visible across multile files.

Regards,
Alvaro

MMC applications

rk200011-Dec-01 8:04

rk2000

11-Dec-01 8:04

What's the easiest way to build an application with the look and feel of an MMC? I don't need any snap-ins. I need the look and feel plus the treeview on the left hand side and listview on the right hand side.

Re: MMC applications

Michael Dunn11-Dec-01 8:20

Michael Dunn

11-Dec-01 8:20

Use the MFC AppWizard and choose the Explorer-like UI. Smile | :)

--Mike--
http://home.inreach.com/mdunn/
Help! Help! I'm being repressed!!
Heart | [heart]

your

with

and

Sonork - 100.10414 AcidHelm

Help :D

RobJones11-Dec-01 6:55

RobJones

11-Dec-01 6:55

I have created a project a while back.. anyway, I selected not to have active x objects in my project.. now a few weeks later I decided that I need to put an active x object into my dialog project.. but when I put the object into the project the dialog box will no longer appear when its launched.. any ideas?

If I remove the object the dialog box will appear.. its a Microsoft web browser active x object.

Thanks

Rob

Re: Help :D

Carlos Antollini11-Dec-01 7:11

How to delete a character in CRichEditView?

11-Dec-01 7:11

Or The activex is not registered or you forgot to add AfxOleInit() in the InitInstance of your project....

Best Regards

Carlos Antollini.
Sonork ID 100.10529 cantollini

davilism11-Dec-01 6:42

davilism

11-Dec-01 6:42

Hi, veterans, how can I delete a character in CRichEditView?

Re: How to delete a character in CRichEditView?

Carlos Antollini11-Dec-01 7:17

Re: How to delete a character in CRichEditView?

11-Dec-01 7:17

First you need to use CRichEditCtrl::SetSel and select the text or character that you need to delete, then CRichEditCtrl::ReplaceSel replazing your selection for "" and ready Green Alien | [Alien]

Best Regards!!

Carlos Antollini.
Sonork ID 100.10529 cantollini

davilism12-Dec-01 6:21

davilism

12-Dec-01 6:21

Thanks, Carl, this is great.

Changing dialog size

C^hris L^osinger11-Dec-01 6:32

11-Dec-01 6:32

i have a dialog that looks like this:

+-------------+
| +---------+ |
| |    A    | |
| +---------+ |
|             |
| +---------+ |
| |    B    | |
| +---------+ |
+-------------+

A and B are static frames that may hold other controls.

i want to be able to dynamically resize it so that it is either just big enough to hold the "A" rectangle or big enough to hold both A and B. to keep it easy, the width never changes.

so, what's the secret to getting the height correct? i'm doing this:

CRect dispRect;
m_dispBox.GetWindowRect(dispRect);
ScreenToClient(dispRect);
dispRect.NormalizeRect();

CRect normalRect;
m_normalFrame.GetWindowRect(normalRect);
ScreenToClient(normalRect);
normalRect.NormalizeRect();

int w = normalRect.left + normalRect.Width() + GetSystemMetrics(SM_CXDLGFRAME) * 2;
int h = normalRect.top + normalRect.Height() + GetSystemMetrics(SM_CYDLGFRAME) * 2;
	
if (bShow) 
{
      h += dispRect.Height();
      h += (dispRect.top - normalRect.bottom); // space between them...
}

SetWindowPos(&wndTop, 0,0, w, h, SWP_NOMOVE | SWP_NOZORDER);

but the dialog is just a little bit too small to hold the bottom frame. - the bottom frame's bottom is clipped.

-c

Carlos Antollini11-Dec-01 6:49

11-Dec-01 6:49

I prefer to use GetWindowPlacement and SetWindowPlacement when I work with resize of controls...

Best Regards

Carlos Antollini.
Sonork ID 100.10529 cantollini

C^hris L^osinger11-Dec-01 7:43

11-Dec-01 7:43

but how do you get the dimensions correct?

-c

Carlos Antollini11-Dec-01 8:29

11-Dec-01 8:29

With GetClientRect() you get the dimension of the client rect.
I prefer to use GetWindowPlacemente because it has the rcNormalPosition member in the WINDOWPLACEMENT struct, when I change the size of each control I use SetWindowPlacement..... Try this

Carlos Antollini.
Sonork ID 100.10529 cantollini

C^hris L^osinger11-Dec-01 8:32

11-Dec-01 8:32

i think i found it... if i add the height of the caption bar to the rect height, everything fits nicely.

-c