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Solved: Confused about friend operators, namespaces, and private members


From: Robert Heller
Subject: Solved: Confused about friend operators, namespaces, and private members...
Date: Wed, 18 Nov 2009 13:46:24 -0600

At Tue, 17 Nov 2009 15:56:08 -0600 Robert Heller <address@hidden> wrote:

> 
> I am using gcc 3.4.6 under CentOS 4.8 (RHEL 4.8):
> Reading specs from /usr/lib/gcc/i386-redhat-linux/3.4.6/specs
> Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
> --infodir=/usr/share/info --enable-shared --enable-threads=posix
> --disable-checking --with-system-zlib --enable-__cxa_atexit
> --disable-libunwind-exceptions --enable-java-awt=gtk
> --host=i386-redhat-linux
> Thread model: posix
> gcc version 3.4.6 20060404 (Red Hat 3.4.6-11)
> 
> And I am getting errors like this:
> 
> ./../../C++/ParserClassesBoost/TrackGraph.cc: In function 
> `Parsers::TrackGraph::Transform2D* operator*(const 
> Parsers::TrackGraph::Transform2D&, const Parsers::TrackGraph::Transform2D&)':
> ./../../C++/ParserClassesBoost/TrackGraph.h:747: error: `float 
> Parsers::TrackGraph::Transform2D::matrix[3][3]' is private
> ./../../C++/ParserClassesBoost/TrackGraph.cc:675: error: within this context
> ./../../C++/ParserClassesBoost/TrackGraph.h:747: error: `float 
> Parsers::TrackGraph::Transform2D::matrix[3][3]' is private
> ./../../C++/ParserClassesBoost/TrackGraph.cc:675: error: within this context
> ./../../C++/ParserClassesBoost/TrackGraph.h:747: error: `float 
> Parsers::TrackGraph::Transform2D::matrix[3][3]' is private
> ./../../C++/ParserClassesBoost/TrackGraph.cc:676: error: within this context
> make[1]: *** [libMRRParserClassesBoost_la-TrackGraph.lo] Error 1
> make[1]: Leaving directory 
> `/home/heller/Deepwoods/ModelRRSystem/Linux/C++/ParserClassesBoost'
> make: *** [all] Error 2
> 
> Here is the relevant fragment of TrackGraph.cc:
> 
> using namespace Parsers;
> 
> TrackGraph::Transform2D* operator * (const TrackGraph::Transform2D& t1, const 
> TrackGraph::Transform2D& t2)
> {
>   TrackGraph::Transform2D *new_t = new TrackGraph::Transform2D;
> 
>   for(int i=0; i < 3; i++)
>     for(int j=0; j < 3; j++) {
>       float sum = 0.0;
>       for(int k=0; k < 3; k++) 
>       sum += t1.matrix[j][k] * t2.matrix[k][i];
>       new_t->matrix[j][i] = sum;
>     }
> 
>   return new_t;
> }
> 
> And of TrackGraph.h:
> 
> namespace Parsers {
> 
> class TrackGraph {
> public:
>       class Transform2D {
>       private:
>               /**  Transform matrix.
>                 */
>               float  matrix[3][3];
>               /**  Fuzz factor.
>                 */
>               const static float FUZZ = .00001;
>       public:
>               /**  Matrix multiplication.
>                 */
>               friend Transform2D* operator * (const Transform2D& t1,const 
> Transform2D& t2);
>       };
> };
> 
> };
> 
> I don't understand these error messages.  They didn't happen before I
> added the namespace code.  I would *like* to embed my code in a
> namespace and not put random stuff into the global namespace, but
> either gcc 3.4.6 does not handle namespaces well when dealing with
> friend operators or I am doing something wrong, but I don't know what
> that could be.  For various reasons I don't want to install a newer
> version of gcc (I want to stay binary compatible with the rest of my
> system). 
> 

OK, I figured out that I needed to qualify the friend operators with the
namespace -- appearently things are ambigious otherwise.  The error
message is a little confusing (to me).


-- 
Robert Heller             -- 978-544-6933
Deepwoods Software        -- Download the Model Railroad System
http://www.deepsoft.com/  -- Binaries for Linux and MS-Windows
address@hidden       -- http://www.deepsoft.com/ModelRailroadSystem/
                                                                                
 


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