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static variable - unresolved external symbol

Posted on 2009-04-14
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Last Modified: 2013-11-20
Hi Experts,

I've got a class where I'm creating a static variable which is a pointer.  In the implementation file, in the cpp file, I reference this pointer and I get the unresolved external symbol...  What am I doing wrong?  I'm trying to implement a singleton ...   The code below is what I believe is just the essentials.....

Thanks,
Mike
///  this is the .h file.....
#include "afxdb.h"
class CMyClass
{
public:
           CMyClass();
          static CDatabase * m_pDatabase;   ////  if I comment out this line, no more error.......
}
 
///// cpp file:
CMyClass::CMyClass() {}
 
BOOL CMyClass::Connect()
{
        m_pDatabase = new CDatabase();
        return TRUE;
}

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Question by:thready
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Accepted Solution

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mrjoltcola earned 500 total points
ID: 24143722
A static member variable is essentially a global variable with class scope. So declaring one is not the same as defining one.

So add this to your .cpp file, it will define the single memory variable that is a static member variable.



///// cpp file:
 
// define static members
static CDatabase * CMyClass::m_pDatabase;
 
 
CMyClass::CMyClass() {}
 
BOOL CMyClass::Connect()
{
        m_pDatabase = new CDatabase();
        return TRUE;
}

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Expert Comment

by:mrjoltcola
ID: 24143751
All that said, this is a throwback to C compatibility. Newer languages implement it in the compiler, but C++ is compatible with C linkers which expect the variable to exist in one compilation unit.

Maybe we see this ugly feature go away in C++0x
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Author Closing Comment

by:thready
ID: 31570213
I keep coming back to this solution - I know about it - then I forget it......  Then I do the same thing again...  This time I hope I remember it....  It's from my java background.....  works in java - makes me hate c++....  But you gotta hate java for how slow it is....  Thanks for the response!
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Author Comment

by:thready
ID: 24143928
Hi mrjoltcola - i`m getting this error - I did as you said - I got this error - I then removed the declaration I made in the .h - I still get this error......  How come?

C2761: 'CMyDatabase *CMyClass::m_pDatabase' : member function redeclaration not allowed
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Expert Comment

by:mrjoltcola
ID: 24144000
Something changed, it thinks you are declaring a member function. Post your code again so I can see.
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Author Comment

by:thready
ID: 24144020
It`s just that it thinks that m_pDatabase is a member function...  because of the :: operator perhaps?
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Author Comment

by:thready
ID: 24144061
I apologize - this is the error I am getting:

error C2720: 'CMyClass::m_pDatabase' : 'static ' storage-class specifier illegal on members
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Author Comment

by:thready
ID: 24144104
//  .h file:
class CDatabase;

class CMyClass
{
public:
     CMyClass();
     static CDatabase * m_pDatabase;
}

// .cpp file:
#include "afxdb.h"
static CDatabase * CMyClass::m_pDatabase;

///  with no other reference to m_pDatabase.....  (error c2720)
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Expert Comment

by:mrjoltcola
ID: 24144112
You don't use the "static" keyword in the .cpp file, only in the .h file inside the class declaration. Another C throwback.
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Author Comment

by:thready
ID: 24144125
Compiles - thanks again mrjoltcola - and I like the name - sounds heavily caffeinated...
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Expert Comment

by:mrjoltcola
ID: 24144196
Only sometimes!
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