c++/cli define new explicit implementation of a sealed method

Marius Bancila

Let's say I have this code:

interface class IFoo
{
public:
   void foo();
};

ref class FooBase : public IFoo
{
public:
   virtual void foo() sealed = IFoo::foo
   {
   }
};

I need to define a new explicit foo() in a derived class, that overrides the sealed method in the base class. How do I do that? I tried a lot of things and none compiled.

ref class FooDerived : public FooBase
{
public:
   virtual void foo() 
   {
   }
};

results in error C4485: 'FooDerived::foo' : matches base ref class method 'FooBase::foo', but is not marked 'new' or 'override'; 'new' (and 'virtual') is assumed 1> .\Dlg.cpp(22) : see declaration of 'FooBase::foo' 1> Specify 'override' (and 'virtual') to override the ref class virtual method 1> Specify 'new' (and 'virtual') to hide the ref class virtual method with a new virtual method 1> Position for 'new' and 'override' keywords is after method parameter list but if I add new

ref class FooDerived : public FooBase
{
public:
   virtual void foo() new
   {
   }
};

I get Dlg.cpp(30) : error C2059: syntax error : 'string' Dlg.cpp(31) : error C2091: function returns function also

ref class FooDerived : public FooBase
{
public:
   virtual void foo() new = FooBase::foo
   {
   }
};

results in 1>.\Dlg.cpp(30) : error C2059: syntax error : 'string' 1>.\Dlg.cpp(30) : error C2091: function returns function 1>.\Dlg.cpp(31) : warning C4569: 'FooBase::foo' : no members match the signature of the explicit override 1>.\Dlg.cpp(31) : error C3671: 'FooDerived::foo' : function does not override 'FooBase::foo' and

ref class FooDerived : public FooBase, public IFoo
{
public:
   virtual void foo() new = IFoo::foo
   {
   }
};

generates 1>.\Dlg.cpp(30) : error C2059: syntax error : 'string' 1>.\Dlg.cpp(30) : error C2091: function returns function 1>.\Dlg.cpp(31) : warning C4569: 'IFoo::foo' : no members match the signature of the explicit override 1>.\Dlg.cpp(31) : error C3671: 'FooDerived::foo' : function does not override 'IFoo::foo' What I'm trying to do is overriding

John Schroedl

You could use Renamed Overriding which will work if you're always called through an interface handle.

interface class IFoo
{
public:
void foo();
};

ref class FooBase : public IFoo
{
public:
virtual void foo() sealed = IFoo::foo
{
Console::WriteLine("FooBase");
}
};

ref class FooDerived : public FooBase
{
public:
// derivedFoo is the implementation of IFoo::foo
virtual void derivedFoo() = IFoo::foo
{
Console::WriteLine("FooDerived");
}
};

int main(array ^args)
{
FooBase^ fb = gcnew FooBase();
fb->foo(); // Prints "FooBase"

FooDerived^ fd = gcnew FooDerived();
fd->foo();					// Prints "FooBase"

IFoo^ iface = dynamic\_cast(fb);
iface->foo();					// Prints "FooBase"

iface = dynamic\_cast(fd);
iface->foo();					// Prints "FooDerived"

return 0;

}

John Schroedl

Perhaps better. This seems to be what you're after to me.

ref class FooDerived : public FooBase
{
public:
virtual void foo() new = IFoo::foo
{
Console::WriteLine("FooDerived");
}
};

John

Marius Bancila

Yes. This seem to be what I was looking for. It's curious that out of all my attempts, this one I missed. :( BTW, there is a little error in your sample.

FooDerived^ fd = gcnew FooDerived();
fd->foo(); // Prints "FooBase"

This obviously prints FooDerived. And the first example, where you say virtual void foo() = IFoo::foo does not compile. But thanks again for the help.

Marius Bancila

It doesn't work in MFC apps with /clr support.

1>.\mfc_mm_2008.cpp(38) : error C2059: syntax error : 'string'
1>.\mfc_mm_2008.cpp(38) : error C2091: function returns function
1>.\mfc_mm_2008.cpp(39) : warning C4569: 'IFoo::foo' : no members match the signature of the explicit override
1>.\mfc_mm_2008.cpp(39) : error C3671: 'FooDerived::foo' : function does not override 'IFoo::foo'

The reason is MFC is rewriting the new operator with

#ifdef _DEBUG
#define new DEBUG_NEW
#endif

So anyone wanting to do this in MFC, make sure you don't have that replacement of new before your declaration of the new function definition.

John Schroedl

Actually, that's not a error in the output, that's a direct paste from running the code and the output. There is also no compile error for me (this was directly from running code). Notice the function name in FooDerived; it's "derivedFoo()" not "foo()". FooDerived can only print "FooDerived" when fd->derivedFoo() is called OR when called via an IFoo^. So, calling fd->foo() does call the base class implementation.

virtual void derivedFoo() = IFoo::foo

John Schroedl

Good tip about the DEBUG_NEW screwing up the new keyword. I tried the code in my MFC + /clr application but we've had to remove those DEBUG_NEW defines for other reasons so I didn't run into this issue. Good to know! John

John Schroedl

BTW, my kids have spent many hours playing your 'Alchemy' game so thanks for that :-)