To interface or not to interface

Michel Fortin michel.fortin at
Wed May 26 15:43:34 PDT 2010

On 2010-05-26 15:44:58 -0400, Jacob Carlborg <doob at> said:

> On 2010-05-26 16.20, Michel Fortin wrote:
>> In D you can't do that currently, but here's how it could be added.
>> First add an optional attribute for interface members:
>> interface TableDelegate {
>> ...
>> @optional NSColor backgroundColorForRow(NSTableView table, size_t row);
>> }
>> Classes implementing the interface can omit the @optional methods. When
>> omitted, the interface function is null. When using the interface, you
>> must check first if the optional function is implemented by making sure
>> the address isn't null:
>> TableDelegate delegate = this.delegate;
>> if (&delegate.backgroundColorForRow != null) {
>> NSColor color = delegate.backgroundColorForRow(this, row);
>> ...draw background color...
>> }
>> This would work somewhat similarily to weak-linked symbols.
> That is quite a clever idea, to check for null. The question is then 
> how much trouble would that be to implement.

If I had to guess those things would need to be done:

1. Add the "@optional" attribute token to the lexer
2. Allow @optional to be attached to functions in an interface
3. When checking if a class correctly implements an interface, allow 
unimplemented @optional functions
4. When generating the interface's vtable, use null for unimplemented 
@optional functions

I think that's all. Checking for null is easily done by retrieving the 
delegate pointer (which you can already do), and calling the function 
uses the usual calling mechanism for interfaces. I expect that trying 
to call an unimplemented interface function would be like trying to 
call a null function pointer.

Michel Fortin
michel.fortin at

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