how to design multiple inheritance classes?

[nm_114]

I have a class named CWinFileIn and one named CWinFileOut, and I wanted to derived another one CWinFileInOut from the other two, but the compiler gives errors about functions and data members being ambiguous (both CWinFileIn and CWinFileOut have m_hFile etc.). So what's the right way of designing these classes? - thanks

[Christian Graus]

Don't give them both variables of the same name, I would have thought. The other possibility is containment, write a class that has instances of both classes, or derives from one and has an instance of the other. Christian Graus - Microsoft MVP - C++

[Tim Smith]

You can place the common elements in a virtual base class. Tim Smith I'm going to patent thought. I have yet to see any prior art.

[nm_114]

Christian Graus wrote: Don't give them both variables of the same name but I want CWinFileInOut to inherit CWinFileIn's read functions and CWinFileOut's write functions, using the same file handle and other private data. but i guess it's not possible.

[Christian Graus]

nm_114 wrote: but I want CWinFileInOut to inherit CWinFileIn's read functions and CWinFileOut's write functions, using the same file handle and other private data. but i guess it's not possible. Then derive from a common base class that contains those variables, and/or derive from multiple classes that define interfaces. Christian Graus - Microsoft MVP - C++

[nm_114]

Christian Graus wrote: Then derive from a common base class that contains those variables I made a struct WFIOInfo that has the handle info etc. and CWinFileIn and CWinFileOut both derive from it, so if CWinFileInOut derive from both CWinFileIn and CWinFileOut which both have their own instances of the base class i still get the same ambiguous errors. Or did I not understand what you were suggesting? Christian Graus wrote: and/or derive from multiple classes that define interfaces I don't know what that means. do you know of any examples i could look at?

[Christian Graus]

struct WFIOInfo { ... } class WinFileInInterface // Note - NOT derived from WFIOInfo, defines an interface for what CWinFIleIn has that WFIOInfo does not { virtual void myMethod; //etc } class WinFileOutInterface { virtual void myMethod; //etc } class CWinFileIn : WFIOInfo, WinFileInInterface { } class CWinFileOut : WFIOInfo, WinFileOutInterface { } class CWinFileInOut : WFINInfo, WinFileInInterface, WinFileOutInterface { } So, CWinFileInOut is not derived from either derived class, but from all three base classes that combine to make the class you're after. Christian Graus - Microsoft MVP - C++

[sunit5]

go for virtual base class

[nm_114]

ok, i think i *almost* have it, but its giving me the following errors:

Linking...
winiotest.obj : error LNK2001: unresolved external symbol "public: virtual int __thiscall WinFileOutInterface::DoWriteFile(void const *,unsigned long)" (?DoWriteFile@WinFileOutInterface@@UAEHPBXK@Z)
winiotest.obj : error LNK2001: unresolved external symbol "public: virtual int __thiscall WinFileInInterface::DoReadFile(void *,unsigned long)" (?DoReadFile@WinFileInInterface@@UAEHPAXK@Z)
winiotest.obj : error LNK2001: unresolved external symbol "public: virtual int __thiscall WinFileInInterface::Open(char const *)" (?Open@WinFileInInterface@@UAEHPBD@Z)
winiotest.obj : error LNK2001: unresolved external symbol "public: virtual int __thiscall WinFileOutInterface::Open(char const *)" (?Open@WinFileOutInterface@@UAEHPBD@Z)
Debug/winiotest.exe : fatal error LNK1120: 4 unresolved externals

here's my code so far:

struct WFIOInfo
{
	HANDLE m_hFile;
	DWORD m_dwBytes;
};

class WinFileInInterface
{
public:
	virtual BOOL Open(LPCTSTR lpszFileName);
	virtual BOOL DoReadFile(LPVOID lpBuffer, DWORD nNumberOfBytesToRead);
};

class WinFileOutInterface
{
public:
	virtual BOOL Open(LPCTSTR lpszFileName);
	virtual BOOL DoWriteFile(LPCVOID lpBuffer, DWORD nNumberOfBytesToWrite);
};

class CWinFileIn : WFIOInfo, WinFileInInterface
{
public:
	virtual BOOL Open(LPCTSTR lpszFileName)
	{
		return TRUE;
	}
	virtual BOOL DoReadFile(LPVOID lpBuffer, DWORD nNumberOfBytesToRead)
	{
		return ::ReadFile(m_hFile, lpBuffer, nNumberOfBytesToRead, &m_dwBytes, NULL);
	}
};

class CWinFileOut : WFIOInfo, WinFileOutInterface
{
public:
	virtual BOOL Open(LPCTSTR lpszFileName)
	{
		return TRUE;
	}
	virtual BOOL DoWriteFile(LPCVOID lpBuffer, DWORD nNumberOfBytesToWrite)
	{
		return ::WriteFile(m_hFile, lpBuffer, nNumberOfBytesToWrite, &m_dwBytes, NULL);
	}
};

class CWinFileInOut : WFIOInfo, WinFileInInterface, WinFileOutInterface
{
public:
	BOOL Open(LPCTSTR lpszFileName)
	{
		return TRUE;
	}
};

any idea what i'm doing wrong?

[GDavy]

an interface should be purely abstract so put a =0 behind function definitions. class WinFileInInterface { public: virtual BOOL Open(LPCTSTR lpszFileName) = 0; virtual BOOL DoReadFile(LPVOID lpBuffer, DWORD nNumberOfBytesToRead) = 0; }; class WinFileOutInterface { public: virtual BOOL Open(LPCTSTR lpszFileName) = 0; virtual BOOL DoWriteFile(LPCVOID lpBuffer, DWORD nNumberOfBytesToWrite) = 0; };

[vikramlinux]

Why dont u use classname as specifier? like ObjectInstanceName.Classname::variablename?

[Bob Stanneveld]

Hello, As some other user already suggested, you should use virtual base classes (if you want to share among the different types). See Here[^] for more information on virtual inheritance. Another way to go (IMHO the better way) is to encapsulate some details of your base classes using private inheritance instead of public. Also you need to override some other functions to resolve the ambiguities:

struct WFIOInfo
{ HANDLE m_hFile;
DWORD m_dwBytes;
};

class WinFileInInterface
{
public:
virtual BOOL Open(LPCTSTR lpszFileName);
virtual BOOL DoReadFile(LPVOID lpBuffer, DWORD nNumberOfBytesToRead);
};

class WinFileOutInterface
{
public:
virtual BOOL Open(LPCTSTR lpszFileName);
virtual BOOL DoWriteFile(LPCVOID lpBuffer, DWORD nNumberOfBytesToWrite);
};

class CWinFileIn : private WFIOInfo, public WinFileInInterface
{
public:
virtual BOOL Open(LPCTSTR lpszFileName)
{return TRUE;}
virtual BOOL DoReadFile(LPVOID lpBuffer, DWORD nNumberOfBytesToRead)
{return ::ReadFile(m_hFile, lpBuffer, nNumberOfBytesToRead, &m_dwBytes, NULL);}
};

class CWinFileOut : private WFIOInfo, WinFileOutInterface
{
public:
virtual BOOL Open(LPCTSTR lpszFileName) {return TRUE;}
virtual BOOL DoWriteFile(LPCVOID lpBuffer, DWORD nNumberOfBytesToWrite)
{return ::WriteFile(m_hFile, lpBuffer, nNumberOfBytesToWrite, &m_dwBytes, NULL);}
};

class CWinFileInOut : private: WFIOInfo,
public: WinFileInInterface,
public: WinFileOutInterface
{
public:
BOOL Open(LPCTSTR lpszFileName)
{ return TRUE; }
};

Using the private inheritance, you make the class only visible and accessible to the class that derives from it. Classes which are derived from that class, do not know about the previous privatly inherited classes. This way, you can have multiple names inside different classes. Hope this helps :-D Behind every great black man...             ... is the police. - Conspiracy brother Blog[^]

[nm_114]

When I tried that I got the following errors:

Compiling...
winiotest.cpp
C:\C++\Projects\[Tests]\winiotest\winiotest.cpp(29) : error C2259: 'CWinFileInOut' : cannot instantiate abstract class due to following members:
c:\c++\projects\[tests]\winiotest\cwinfileio.h(109) : see declaration of 'CWinFileInOut'
C:\C++\Projects\[Tests]\winiotest\winiotest.cpp(29) : warning C4259: 'int __thiscall WinFileInInterface::DoReadFile(void *,unsigned long)' : pure virtual function was not defined
c:\c++\projects\[tests]\winiotest\cwinfileio.h(73) : see declaration of 'DoReadFile'
C:\C++\Projects\[Tests]\winiotest\winiotest.cpp(29) : warning C4259: 'int __thiscall WinFileOutInterface::DoWriteFile(const void *,unsigned long)' : pure virtual function was not defined
c:\c++\projects\[tests]\winiotest\cwinfileio.h(80) : see declaration of 'DoWriteFile'
C:\C++\Projects\[Tests]\winiotest\winiotest.cpp(29) : error C2259: 'CWinFileInOut' : cannot instantiate abstract class due to following members:
c:\c++\projects\[tests]\winiotest\cwinfileio.h(109) : see declaration of 'CWinFileInOut'
C:\C++\Projects\[Tests]\winiotest\winiotest.cpp(29) : warning C4259: 'int __thiscall WinFileInInterface::DoReadFile(void *,unsigned long)' : pure virtual function was not defined
c:\c++\projects\[tests]\winiotest\cwinfileio.h(73) : see declaration of 'DoReadFile'
C:\C++\Projects\[Tests]\winiotest\winiotest.cpp(29) : warning C4259: 'int __thiscall WinFileOutInterface::DoWriteFile(const void *,unsigned long)' : pure virtual function was not defined
c:\c++\projects\[tests]\winiotest\cwinfileio.h(80) : see declaration of 'DoWriteFile'

[nm_114]

After looking at this closer it turns out I don't understand how this works. If CWinFileInOut derives just from interfaces, doesn't that mean I have copy the implementations of the interfaces of CWinFileIn and CWinFileOut to CWinFileInOut, which makes inheritance pointless :confused:

[Christian Graus]

There's no such thing as interfaces in C++. These are base classes which define the interfaces. Yes, perhaps this is not what you want, but a virtual base class will have the same problem. Christian Graus - Microsoft MVP - C++

[nm_114]

"base classes which define the interfaces" is what i meant when saying interfaces, sorry for confusing the terms. so, um, sorry if i'm being dense but is what i was originally asking even possible? in the example with the virtual base classes, the derived classes CWinFileIn and CWinFileOut have to implement the virtual functions, and the same virtual functions have to be implemented again in CWinFileInOut, which kinda defeats the purpose of inheritance.

[Christian Graus]

Yes, that's definately the problem, I doubt what you want to do is possible. You can make a class that is all three class types, but you need to copy a bit of code between the classes to do it. Christian Graus - Microsoft MVP - C++