• Mandel (15/15) Apr 15 2007 Hi,
• Frits van Bommel (13/32) Apr 15 2007 What possible use could this have? Interface methods are by definition
• BCS (11/27) Apr 15 2007 An interface can't have static methouds so it needs to have an instance ...
• Mandel (16/25) Apr 15 2007 I want to call methods on objects that implement a specific interface.

Mandel <mandel mailinator.com> writes:

Hi,

I like to have a pointer to an interface method,
like this:

interface Item
{
	void work();
}

int main(char[][] args)
{
	
	auto s = &Item.work
	return 0;
}

But this example fails because of an "undefined symbol" for Item.work.
Is this a bug or intended behaviour?

Frits van Bommel <fvbommel REMwOVExCAPSs.nl> writes:

Mandel wrote:
 Hi,
 
 I like to have a pointer to an interface method,

What possible use could this have? Interface methods are by definition 
abstract, and are thus never directly defined.

(Note that D doesn't support the concept of a "member pointer" as C++ does)

 like this:
 
 interface Item
 {
 	void work();
 }
 
 int main(char[][] args)
 {
 	
 	auto s = &Item.work
 	return 0;
 }
 
 But this example fails because of an "undefined symbol" for Item.work.

Actually, it fails with "semicolon expected following auto declaration, 
not 'return'" :).

 Is this a bug or intended behaviour?

What happens here is that the compiler tells the linker to fill in a 
pointer to the relevant method at link time, but in the case of an 
abstract method the symbol simply doesn't exist. There's nothing 
sensible to do here other than produce an error.

The fact that the error isn't produced until link-time instead of at 
compile-time could be considered a bug though, since the compiler can 
determine the function can't possibly be defined in a legal program.

BCS <ao pathlink.com> writes:

Reply to Mandel,

 Hi,
 
 I like to have a pointer to an interface method,
 like this:
 interface Item
 {
 void work();
 }
 int main(char[][] args)
 {
 auto s = &Item.work
 return 0;
 }
 But this example fails because of an "undefined symbol" for Item.work.
 Is this a bug or intended behaviour?
 

An interface can't have static methouds so it needs to have an instance to 
work on:

class CItem: Item
{
  void work(){}
}

Item c = new CItem;
auto s = &c.work;

somthing like that should work. Note that the delegate you get will depend 
on the class you use.

Mandel <mandel mailinator.com> writes:

Frits van Bommel Wrote:

 Mandel wrote:
 Hi,
 
 I like to have a pointer to an interface method,

 
 What possible use could this have? Interface methods are by definition 
 abstract, and are thus never directly defined.

I want to call methods on objects that implement a specific interface.
In normal cases you would simple call the function by it's name and also by
interface - trivial.

But it would also mean that I have to write code for every interface method
class
implements and also every interface.
In my case I have many interfaces that may also change during development.
The behaviour on all functions is determined by their return type (it will be a
RPC class). Therefore I would like to create a class that registers interface
methods (passed as template argument).
This way, all code that handle objects based on a registered interface method
will be created at compile time.
No need for specific implementations for every interface in this case. :)

For example, if I have interfaces A, B, C, D and some classes that implement at
least one of these interfaces. I have to write handler code (e.g. a class) for
every interface.
If I would have a class that  allows to store (wrapped) accessors to interface
methods,
only one class needs to be written - replacing all handler classes for each
interface.

I hope I was not too confusing. ;-)

I have done this in C++. Would be nice if the same could be achieved in D.
 
 (Note that D doesn't support the concept of a "member pointer" as C++ does)

Maybe there is something similar in D to achieve the same concept?