VC++ 2005: private virtual functions

[Nemanja Trifunovic]

Something I don't get it:

Say we have:

ref class Base {
virtual void SomeVirtualFunc tion()
{Console::Write Line(L"Base");}
public:
void SomeAccessibleF unction()
{SomeVirtualFun ction();}
};

ref class Derived : public Base {
virtual void SomeVirtualFunc tion()
{Console::Write Line(L"Derived" );}
};

int main(array<Syst em::String ^> ^args)
{
Base^ handle = gcnew Derived();
handle->SomeAccessible Function();

return 0;
}

I would expect to get "Derived" as the result, right? However, I get
"Base", and strange warnings during compilation:

".\PrivateVirtu al.cpp(8) : warning C4486: 'Base::SomeVirt ualFunction' :
a private virtual method of a ref class or value class should be marked
'sealed'
..\PrivateVirtu al.cpp(16) : warning C4486:
'Derived::SomeV irtualFunction' : a private virtual method of a ref
class or value class should be marked 'sealed'
"

If I try explicit overriding:

..\PrivateVirtu al.cpp(17) : error C3671: 'Derived::SomeV irtualFunction'
: function does not override 'Base::SomeVirt ualFunction'
..\PrivateVirtu al.cpp(16) : warning C4486:
'Derived::SomeV irtualFunction' : a private virtual method of a ref
class or value class should be marked 'sealed'

What is going on here?

[Jochen Kalmbach [MVP]]

Hi Nemanja!

ref class Base {
virtual void SomeVirtualFunc tion()
{Console::Write Line(L"Base");}
public:
void SomeAccessibleF unction()
{SomeVirtualFun ction();}
};

ref class Derived : public Base {
virtual void SomeVirtualFunc tion()
{Console::Write Line(L"Derived" );}
};

int main(array<Syst em::String ^> ^args)
{
Base^ handle = gcnew Derived();
handle->SomeAccessible Function();

return 0;
}

I would expect to get "Derived" as the result, right? However, I get
"Base", and strange warnings during compilation:

".\PrivateVirtu al.cpp(8) : warning C4486: 'Base::SomeVirt ualFunction' :
a private virtual method of a ref class or value class should be marked
'sealed'

"private" virtual functions are useless! because nobody can overwrite it!

You need to declare your virtual functions at least as "protected" !

--
Greetings
Jochen

My blog about Win32 and .NET
http://blog.kalmbachnet.de/

[Nemanja Trifunovic]

> private" virtual functions are useless! because nobody can overwrite it!

Read this article: http://www.gotw.ca/publications/mill18.htm

[Bo Persson]

"Jochen Kalmbach [MVP]" <no************ ********@holzma .de> skrev i
meddelandet news:ef******** ******@tk2msftn gp13.phx.gbl...

Hi Nemanja!

ref class Base {
virtual void SomeVirtualFunc tion()
{Console::Write Line(L"Base");}
public:
void SomeAccessibleF unction()
{SomeVirtualFun ction();}
};

ref class Derived : public Base {
virtual void SomeVirtualFunc tion()
{Console::Write Line(L"Derived" );}
};

int main(array<Syst em::String ^> ^args)
{
Base^ handle = gcnew Derived();
handle->SomeAccessible Function();

return 0;
}

I would expect to get "Derived" as the result, right? However, I get
"Base", and strange warnings during compilation:

".\PrivateVirtu al.cpp(8) : warning C4486: 'Base::SomeVirt ualFunction'
:
a private virtual method of a ref class or value class should be
marked
'sealed'

"private" virtual functions are useless! because nobody can overwrite
it!

You need to declare your virtual functions at least as "protected" !

But note that this is so for managed code only. In real C++ it would
have worked!
Bo Persson

[Nemanja Trifunovic]

Even in "old" Managed C++ it works.

[Tamas Demjen]

It seems to be stupid from the C++ programmer's point of view, but then
..NET was not designed by C++ programmers.

Unfortunately in .NET virtual functions can't be private, they must be
protected. But even that modification won't solve your problem, you even
have to declare the function override to override a virtual function.
The correct code is

ref class Base {
protected:
virtual void SomeVirtualFunc tion()
{Console::Write Line(L"Base");}
public:
void SomeAccessibleF unction()
{SomeVirtualFun ction();}
};

ref class Derived : public Base {
protected:
virtual void SomeVirtualFunc tion()
{Console::Write Line(L"Derived" );}
};

I share your pain...

Tom

Nemanja Trifunovic wrote:

Something I don't get it:

Say we have:

ref class Base {
virtual void SomeVirtualFunc tion()
{Console::Write Line(L"Base");}
public:
void SomeAccessibleF unction()
{SomeVirtualFun ction();}
};

ref class Derived : public Base {
virtual void SomeVirtualFunc tion()
{Console::Write Line(L"Derived" );}
};

int main(array<Syst em::String ^> ^args)
{
Base^ handle = gcnew Derived();
handle->SomeAccessible Function();

return 0;
}

I would expect to get "Derived" as the result, right? However, I get
"Base", and strange warnings during compilation:

".\PrivateVirtu al.cpp(8) : warning C4486: 'Base::SomeVirt ualFunction' :
a private virtual method of a ref class or value class should be marked
'sealed'
.\PrivateVirtua l.cpp(16) : warning C4486:
'Derived::SomeV irtualFunction' : a private virtual method of a ref
class or value class should be marked 'sealed'
"

If I try explicit overriding:

.\PrivateVirtua l.cpp(17) : error C3671: 'Derived::SomeV irtualFunction'
: function does not override 'Base::SomeVirt ualFunction'
.\PrivateVirtua l.cpp(16) : warning C4486:
'Derived::SomeV irtualFunction' : a private virtual method of a ref
class or value class should be marked 'sealed'

What is going on here?

[Tamas Demjen]

Tamas Demjen wrote:

ref class Derived : public Base {
protected:
virtual void SomeVirtualFunc tion()
{Console::Write Line(L"Derived" );}
};

Sorry I copied the wrong code:

ref class Derived : public Base {
protected:
virtual void SomeVirtualFunc tion() override
{Console::Write Line(L"Derived" );}
};

Note the override keyword, it indicates that the function is not
replacing Base::SomeVirtu alFunction, but is overriding it.

Tom

[Nemanja Trifunovic]

Hi Tom.

Unfortunatel y in .NET virtual functions can't be private, they must be protected
But I am pretty sure CLR allows private virtual functions. It works
fine with old MC++.
But even that modification won't solve your problem, you even have to declare the function override to override a virtual function.

Yep, just noticed this in the C++/CLI spec. Actually, I pretty much
like this "controlled virtuality". Just have no idea why to disable
private virtual functions. Any clues?

[Tamas Demjen]

Nemanja Trifunovic wrote:

Yep, just noticed this in the C++/CLI spec. Actually, I pretty much
like this "controlled virtuality". Just have no idea why to disable
private virtual functions. Any clues?

The reason I don't like this override keyword is that because if you
miss it, the function won't work like a virtual function anymore, and
there's not even a compiler warning. No override keyword == non-virtual
function, even if the virtual keyword is there. It's very frustrating
when you have a virtual function and it's not polymorphic.

If I make the virtual function private, I get the following .NET runtime
exception:

<quote>
An unhandled exception of type 'System.TypeLoa dException' occurred in
Virtual.exe

Additional information: Method 'SomeVirtualFun ction' on type 'Derived'
from assembly 'Virtual, Version=1.0.199 9.26811, Culture=neutral ,
PublicKeyToken= null' is overriding a method that is not visible from
that assembly.
</quote>

This is not a language feature, it's a .NET runtime exception, which
clearly states that overriding a private virtual function is not
supported by the runtime library.

I don't know if it's intentional, but this is happening with VC++ 2005
Beta2.

Tom

[Nemanja Trifunovic]

You are right - it does throw. However, with VC++ 2003, the following
code works just fine:

__gc class Base {
virtual void SomeVirtualFunc tion()
{Console::Write Line(S"Base");}
public:
void SomeAccessibleF unction()
{SomeVirtualFun ction();}
};
__gc class Derived : public Base {
virtual void SomeVirtualFunc tion()
{Console::Write Line(L"Derived" );}
};
int _tmain()
{
Base* handle = new Derived();
handle->SomeAccessible Function();
return 0;
}

Did they introduce this "feature" in CLR 2.0? That would be very
strange, indeed.