extern const for memory allocation?

donglee · 11-03-2008, 01:23 AM

I was reading Microsoft's Inside COM, and I couldn't quite understand one of the comments for a code snippet.

There are two files of interest.

File 1 : Iface.h

Code:

extern "C"
{
extern const IID IID_IX
}

File 2 : GUID.cpp

Code:

#include <objbase.h>

extern "C"
{
extern const IID IID_IX = {0x32bb8320, 0x41b, 0x11cf, ...};
// The extern is required to allocate memory for C++ constants
}

There are two more files associated with this example, one is a file that implements a COM component, and the other is a file that implements a COM client. They both #include "Iface.h", and GUID.cpp are linked to both the component and client when compiled.

Here, Iface.h is created to "declare" IID_IX variable, and GUIDS.cpp is created to "define" the variable. So I understand why extern key word is used for the variable in Iface.h, but I don't undersatnd why it is used in GUIDS.cpp. So I turned to the comment below the definition of the variable in the GUIDS.cpp file, but I still couldn't make much sense out of it. Particularly, I didn't know what it meant when it stated that extern is requried to allocate memory for C++ constants. I would appreciate for some clarifications. Thank you.

Codeplug · 11-03-2008, 09:38 AM

The comment is bogus.

In C++, a const definition at global scope has internal linkage by default - which means it's only available to the translation unit (source file) in which it's defined. Slapping an extern on it gives it external linkage.

So a better comment would have been:
// extern needed since const globals have internal linkage

gg

11-03-2008, 01:23 AM	#1
donglee Registered User Join Date: Jul 2008 Posts: 19	extern const for memory allocation? I was reading Microsoft's Inside COM, and I couldn't quite understand one of the comments for a code snippet. There are two files of interest. File 1 : Iface.h Code: extern "C" { extern const IID IID_IX } File 2 : GUID.cpp Code: #include <objbase.h> extern "C" { extern const IID IID_IX = {0x32bb8320, 0x41b, 0x11cf, ...}; // The extern is required to allocate memory for C++ constants } There are two more files associated with this example, one is a file that implements a COM component, and the other is a file that implements a COM client. They both #include "Iface.h", and GUID.cpp are linked to both the component and client when compiled. Here, Iface.h is created to "declare" IID_IX variable, and GUIDS.cpp is created to "define" the variable. So I understand why extern key word is used for the variable in Iface.h, but I don't undersatnd why it is used in GUIDS.cpp. So I turned to the comment below the definition of the variable in the GUIDS.cpp file, but I still couldn't make much sense out of it. Particularly, I didn't know what it meant when it stated that extern is requried to allocate memory for C++ constants. I would appreciate for some clarifications. Thank you. Last edited by donglee; 11-03-2008 at 01:27 AM.
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11-03-2008, 09:38 AM	#2
Codeplug Registered User Join Date: Mar 2003 Posts: 3,844	The comment is bogus. In C++, a const definition at global scope has internal linkage by default - which means it's only available to the translation unit (source file) in which it's defined. Slapping an extern on it gives it external linkage. So a better comment would have been: // extern needed since const globals have internal linkage gg
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