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extern const

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sethjackson:
Why do we need extern const?

IE:


--- Code: ---#ifndef APPGLOBALS_H
#define APPGLOBALS_H

#include <wx/string.h>

extern const wxString APP_VENDOR;
extern const wxString APP_NAME;
extern const wxString APP_VERSION;
extern const wxString APP_ACTUAL_VERSION_VERB;
extern const wxString APP_ACTUAL_VERSION;
extern const wxString APP_URL;
extern const wxString APP_CONTACT_EMAIL;
extern const wxString APP_PLATFORM;
extern const wxString APP_WXANSI_UNICODE;
extern const wxString APP_BUILD_TIMESTAMP;

#endif // APPGLOBALS_H

--- End code ---

Then in appglobals.cpp set the wxString's to something? Why is this needed? Why can't you set the strings to something in the header? Sorry if this is the most stupid question ever, but I'm not understanding.  :(

I read this on MSDN...

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vccelng/htm/basic_17.asp

thomas:
Because it does not work otherwise, we tried ;)

sethjackson:

--- Quote from: thomas on January 07, 2006, 01:08:18 am ---Because it does not work otherwise, we tried ;)

--- End quote ---

Oh wonder why? Do you know?

yop:
AFAIK you can't initialize non integral data (or POD) in a header. This applies to staitc const members of a class but I don't think that there would be a different initialization mechanism for const (non static) non-POD variables. The difference is only that if they are class members they have to be declared static to avoid multiple copies between instances of the same class, if they are not members then the const identifier is enough but the same restrictions apply. I don't have Meyer's book around but there is a topic about initialization of such variables (where he talks about not using #define and generaly preprocessor macros) that answers this topic specificaly.

Ceniza:

--- Quote from: sethjackson ---Then in appglobals.cpp set the wxString's to something? Why is this needed?
--- End quote ---

Declarations in header files and definitions in source files? :)

Those macros are just to protect against re-declaring them for the same Translation Unit, but if more than one Translation Unit includes the same header file and instead of declarations it finds definitions you'll get problems linking.

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