friend class across namespaces & different .H files - c++

Question

I'm trying to make the following compile under VS 2008 SP1 C++ project, but the friend class statement doesn't seem to have any effect. (See error message in the last code snippet.)
What am I doing wrong with the friend definition?
// EncryptionTypes.h file
#pragma once
//#include "Encryption.h" //adding this line doesn't help
using namespace crypto;
struct FILE_DATA_CACHE{
FILE_DATA_CACHE()
{
};
~FILE_DATA_CACHE()
{
}
friend class CEncryption;
private:
bool _isIndexFileUsed()
{
return bResult;
}
};
then:
// Encryption.h
#pragma once
#include "EncryptionTypes.h"
namespace crypto
{
class CEncryption
{
public:
CEncryption(void);
~CEncryption(void);
private:
BOOL _openFile();
private:
FILE_DATA_CACHE gFData;
};
};
and lastly:
// Encryption.cpp
#include "StdAfx.h"
#include "Encryption.h"
namespace crypto
{
CEncryption::CEncryption(void)
{
}
CEncryption::~CEncryption(void)
{
}
void CEncryption::_openFile()
{
//The line below generates this error:
//1>.\Encryption.cpp(176) : error C2248: 'FILE_DATA_CACHE::_isIndexFileUsed' : cannot access private member declared in class 'FILE_DATA_CACHE'
//1> c:\users\blah-blah\EncryptionTypes.h(621) : see declaration of 'FILE_DATA_CACHE::_isIndexFileUsed'
//1> c:\users\blah-blah\EncryptionTypes.h(544) : see declaration of 'FILE_DATA_CACHE'
gFData._isIndexFileUsed();
}
};

Answer 1

You have a circular dependency problem.
Encryption.h needs FILE_DATA_CACHE, which is defined in EncryptionTypes.h.
EncryptionType.h needs CEncryption, which is defined in Encryption.h.
Fortunately, you can get by with using a forward declaration of CEncryption in EncryptionType.h.
Modify EncryptionType.h to:
// EncryptionTypes.h file
#pragma once
// Can't #include Encryption.h. That will lead to circular
// #includes.
namespace crypto
{
// Forward declaration of crypto::CEncryption
class CEncryption;
}
struct FILE_DATA_CACHE{
FILE_DATA_CACHE()
{
};
~FILE_DATA_CACHE()
{
}
friend class crypto::CEncryption;
private:
bool _isIndexFileUsed()
{
return bResult;
}
};