Tomer Gabel's annoying spot on the 'net RSS 2.0
# Sunday, 21 August 2005
I always enjoy researching languages (a passion that's only intensified since I cowrote our internal C#->java source-level compiler), so when a colleague approached me with an issue he had while creating a design in C# I was immediately intrigued.

Consider the following scenario:

class a1
{
public virtual int ret()
{
return 1;
}
}

class a2 : a1
{
public override int ret()
{
return 2;
}
}

Now, suppose I want to create another class, a3, which overrides ret but instead of implementing it on its own, it calls the a1 implementation - effectively skipping two generations of inheritence. If I wanted to call a2's implementation I could write a simple base.ret() call, so for a1 it's a simple case of base.base.ret(), right?

Quite wrong. Apparently the C# language specification defines the base keyword in a way that simply does not allow this. I was curious of this is the case with java as well, and lo and behold: java doesn't support super.super.member constructs either.

While researching the issue, I've come across two particularly interesting notes: Mads Torgersen, the new Microsoft program manager on C# Compiler and Language, has this to say in an interview:

Q: why only the first immediate base class is allowed in c#? e.g. cant do base.base.ToString()
A: The behaviour of your base class is determined by the writer of that class. It would open up a hole for breach of contract if you could bypass this and access something that this writer has decided to hide from you.

I'm not sure I'm convinced by this; it assumes the person who writes the instance class is not the same person who wrote the base class, which is not always the case. Other people around the 'net have said that this is a sort of "OOP no-no"; I can't say I've ever thought about this particular issue very deeply so I'll have to do some more thinking. If anyone has any references (no books, please - they're expensive and difficult to get) I would be very interested in hearing about it.

Brian Maso, posting to a similar discussion regarding java, suggests that this limitation derives from the java virtual machine design:

"super" is not an object reference. It's a Java language fiction. It basically indicates to the compiler that the target method call uses the "invokespecial" bytecode, not the usual method invocation bytecode "invokevirtual". This is as opposed to "this", which is an actual object reference variable.

If that is indeed the case, I can imagine the CLR shares similar design limitations, however this is research I'll leave for another time. For the time being suffice to say that the only way to do this is to use reflection, which is a sloppy solution which hinders performance, readability and safety. Imagine accessing runtime information, creating class instances (generating additional work for the GC in the process) etc. instead of a simple vtable indirection - it's not even funny!

Sunday, 21 August 2005 14:05:41 (Jerusalem Standard Time, UTC+02:00)  #    -
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