undefined error message for static variable during link phase

[jk]

struggling with C++ I run into an error which I do not understand.

see the attached files

the static variable foo1 is being declared undefined during link phase.

the various messages are in File undefErrMsg.txt

I have an example from "goto Objekt Orientierung" by Herold/Klar/Klar 
(Addison Wesley) which exactly allows this kind of use of a "static" 
variable
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kampmann@ibk-node14:/home/kampmann/tests/cpp->make undefind
g++     undefind.cc   -o undefind
kampmann@ibk-node14:/home/kampmann/tests/cpp->undefind
foo1 = 1
kampmann@ibk-node14:/home/kampmann/tests/cpp->cp undefind.cc undefStatic.cc
kampmann@ibk-node14:/home/kampmann/tests/cpp->make undefStatic
g++     undefStatic.cc   -o undefStatic
/tmp/cc2aNvzh.o(.text+0x5): In function `CFoo::CFoo[not-in-charge]()':
: undefined reference to `CFoo::foo1'
/tmp/cc2aNvzh.o(.text+0x15): In function `CFoo::CFoo[in-charge]()':
: undefined reference to `CFoo::foo1'
/tmp/cc2aNvzh.o(.text+0x51): In function `main':
: undefined reference to `CFoo::foo1'
collect2: ld returned 1 exit status
make: *** [undefStatic] Error 1
kampmann@ibk-node14:/home/kampmann/tests/cpp->make undefStatic
g++     undefStatic.cc   -o undefStatic
/tmp/cc0unNKD.o(.text+0x37): In function `CFoo::loadFoo(int)':
: undefined reference to `CFoo::foo1'
/tmp/cc0unNKD.o(.text+0x6f): In function `main':
: undefined reference to `CFoo::foo1'
collect2: ld returned 1 exit status
make: *** [undefStatic] Error 1
kampmann@ibk-node14:/home/kampmann/tests/cpp->cp undefStatic.cc undefInline.cc
kampmann@ibk-node14:/home/kampmann/tests/cpp->make undefInline
g++     undefInline.cc   -o undefInline
/tmp/ccMSAGs6.o(.text+0x7): In function `CFoo::loadFoo(int)':
: undefined reference to `CFoo::foo1'
/tmp/ccMSAGs6.o(.text+0x3f): In function `main':
: undefined reference to `CFoo::foo1'
collect2: ld returned 1 exit status
make: *** [undefInline] Error 1

// file: undefInline.cc
//

#include <stdio.h>

class CFoo{

public:
    static int  foo1 ;
    void loadFoo( int ) ;

    CFoo() {
        loadFoo( 1 ) ;
    }

    ~CFoo() ;
} ;


void CFoo::loadFoo( int aa ) {
    foo1 = aa ;
}

CFoo::~CFoo(){}

main(){
    CFoo foo ;

    printf( "foo1 = %d\n", foo.foo1 ) ;

}

#include <stdio.h>

class CFoo{

public:
    static int  foo1 ;

    CFoo() ;
    ~CFoo() ;
    void loadFoo( int ) ;
} ;

CFoo::CFoo(){
    loadFoo( 1 ) ;
}

void CFoo::loadFoo( int aa ) {
    foo1 = aa ;
}

CFoo::~CFoo(){}

main(){
    CFoo foo ;

    printf( "foo1 = %d\n", foo.foo1 ) ;

}

#include <stdio.h>

class CFoo{

public:
    int  foo1 ;

    CFoo() ;
    ~CFoo() ;
} ;

CFoo::CFoo(){
    foo1 = 1 ;
}

CFoo::~CFoo(){}

main(){
    CFoo foo ;

    printf( "foo1 = %d\n", foo.foo1 ) ;

}