The Override Problem
The term “override problem” describes a problem that occurs when a virtual function of a base object becomes unreachable because an interface method overrides the implementation of the base class. For example, all CLI objects derive from System.Object. If an interface has a method that has the same signature as one of System.Object's methods, then the respective method of System.Object is unreachable if the interface method is virtual.
For example, consider the following declaration of the interface XFoo and its implementing class :
using namespace System;
public __gc __interface XFoo
{
public:
virtual String* ToString();
};
public __gc class Foo : public XFoo
{
public:
virtual String * ToString()
{
return NULL;
}
};
If the method ToString of an instance is called, then the implementation of the interface method is invoked. For example:
int main(void)
{
Foo * f = new Foo();
Object * o = f;
f->ToString(); // calls Foo.ToString
o->ToString(); // calls Foo.ToString
return 0;
}
This may not be intended, because the interface method likely has a different semantic than its namesake of System.Object.
A solution is to prevent the interface method from overriding the method of System.Object without making the interface method non-virtual. The CLI provides a remedy by way of the “newslot” flag, which is attached to the method header in the IL code. CLI languages may have different means for denoting a method with “newslot”.
The following examples show ways of implementing XFoo in different languages, so that Object.ToString can still be called.
//C++
//interface methods should be qualified with the interface they belong to
public __gc class A: public XFoo
{
public:
virtual String* XFoo::ToString()
{
Console::WriteLine("A::foo");
return NULL;
}
};
 |
Although XFoo::ToString is virtual, it cannot be overridden in an inheriting class, because the CLI method header contains the final attribute. In an inheriting class one can, however, derive again from XFoo and provide an implementation. |
In C# there are different ways provide an implementation:
// IL contains: newslot final virtual
public new string ToString()
{
}
The keyword new inserts the newslot attribute in the CLI method header. This implementation cannot be overridden in an inheriting class.
//IL contains: newslot virtual
public new virtual string ToString()
{
}
This method can be overridden in a derived class.
// Using a qualified method name for the implementation. The virtual
//modifier is not allowed
string XFoo.ToString()
{
return null;
}
This implementation cannot be overridden in a derived class. An instance of the implementing class must be cast to XFoo before the method can be called.
'VB .NET
Public Shadows Function ToString() As String Implements XFoo.ToString
Console.WriteLine("Foo.toString")
End Function
This implementation cannot be overridden in a derived class.
Public Overridable Shadows Function ToString() As String _
Implements XFoo.ToString
Console.WriteLine("Foo.toString")
End Function
This method can be overridden.
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