Back to General and Gameplay Programming

unnamed namespaces

TonyFish · 2004-02-21T16:57:59

Hello, I have been reading up on unnamed namespaces in the context of hiding classes as mentionned by the GOF Facade pattern description. I was hoping someone could shed some light on this. I would like the following classes where Facade has access to Alpha and Beta which are otherwise hidden. class Alpha { Beta *b; // and methods which call b methods }; class Beta { Alpha *a; // and methods which use a methods }; class Facade { // various manipulations of Alpha and Beta objects }; However I would like to be able to seperate these classes into seperate files (as in the case of say a compiler with a facade, the subsystem classes are numerous and large and keeping this in one file would be crazy). Is this at all feasible with unnamed namespaces as the GOF book describes? My major problem is the organisation of the code in headers and source files. On this front I'm totally lost! NB: Yes I know I could always do this with private constructors and friend classes but I would like to see a namespace version in action. Any help would be greatly appreciated. Many Thanks Anthony. [edited by - TonyFish on February 20, 2004 9:00:14 PM]

General and Gameplay Programming Programming

Started by TonyFish February 20, 2004 06:38 PM

12 comments, last by TonyFish 20 years, 2 months ago

Jingo

582

February 21, 2004 05:39 AM

The problem with unnamed namespaces is that the unique identifier for the namespace is different in each translation unit. This means it is practically impossible to split the implementation of the classes declared in the unnamed namespace across multiple translation unit, and will no doubt give you linker errors. e.g.

//unnamed.hpp...namespace{class A{ public: void somefunc();};}...//A.cpp#include "unnamed.hpp"void someuniqueidentifier::A::somefunc(){...}//Facade.cpp#include "unnamed.hpp"#include "Facade.hpp"void Facade::somefunc(){anotheruniqueidentifier::A instance;instance.somefunc();}

Since the unique identifier is different in each translation unit, you have ended up defining a class that belongs to a different namespace than you have used, and so you will get a linker error

.

Only way you could solve this is by defining the unnamed namespace objects inside the unnamed namespace.

[edited by - Jingo on February 21, 2004 6:49:12 AM]

TonyFish

126

Author

February 21, 2004 04:27 PM

Well I've solved it thus:

*all the relevant external headers*namespace Compiler{    class Facade;    namespace // _UNIQUE_    {        #define _HEADER_            #include "SubAlpha.hpp"            #include "SubBeta.hpp"        #undef	_HEADER_        #define _SOURCE_            #include "SubAlpha.hpp"            #include "SubBeta.hpp"        #undef	_SOURCE_    }    class Facade {…};}

Where SubAlpha.hpp will look like this

#ifdef _HEADER_class SubAlpha{    friend class Facade;    friend class SubBeta;public:    int Method(void);private:    SubAlpha();};#endif#ifdef _SOURCE_int SubAlpha::Method(void) {…}#endif

This works but if it is used to hide 99% of a large project results in a massive translation unit which will a. be slower to compile and more importantly b. require a re-compile of everything whenever anything is changed.

Of course if anyone can think of anything wrong with this or any way of improving it I'd love to hear.

Thanks

[edited by - TonyFish on February 21, 2004 5:42:27 PM]

<Fish>{

PlayGGY

122

February 21, 2004 04:56 PM

Since he uses pointers, objects aren''t actually created, so what he did wouldn''t cause an infinite loop of inclusions.

And the rockets' red glare, the bombs bursting in air,gave proof through the fight that our flag was still there.Oh say, does that star-spangled banner yet waveover the land of the free and the home of the brave?

TonyFish

126

Author

February 21, 2004 04:57 PM

Christ could people stop posting about the pointers and possible cyclic dependencies. It was just something to fill the skeleton example. Jeez

<Fish>{

unnamed namespaces

This topic is closed to new replies.

Popular Topics

Recommended Tutorials

unnamed namespaces

This topic is closed to new replies.

Popular Topics

Recommended Tutorials

Reticulating splines