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digitalmars.D.learn - Template Inheritance

reply %u <blm768 gmail.com> writes:
I've been working on porting an old D library to D2, and I'm running into a
nasty issue with templates and inheritance. I've got a base class like this:

class Reader {
    void get(T)(ref T[] buffer);
}

and a subclass like this:

class SubReader {
    void get()(SomeClass param);
}

The problem is that by creating a new form of the template in the subclass, I
made the base class's version invisible. If I try to use "alias Reader.get
get" like I would do for functions, the compiler complains that the symbols
clash (no musical puns intended). Does anyone know how to get this to work?
Feb 18 2012
next sibling parent reply %u <blm768 gmail.com> writes:
In the interim, I'm just redefining the template in the base class, but that's a
really annoying hack to have to perform every single time I have to make a new
form of the template.
Feb 18 2012
parent %u <blm768 gmail.com> writes:
Correction: redefining in the *subclass*. Silly me.
Feb 18 2012
prev sibling next sibling parent reply Jonathan M Davis <jmdavisProg gmx.com> writes:
On Sunday, February 19, 2012 00:55:59 %u wrote:
 I've been working on porting an old D library to D2, and I'm running into a
 nasty issue with templates and inheritance. I've got a base class like this:
 
 class Reader {
     void get(T)(ref T[] buffer);
 }
 
 and a subclass like this:
 
 class SubReader {
     void get()(SomeClass param);
 }
 
 The problem is that by creating a new form of the template in the subclass,
 I made the base class's version invisible. If I try to use "alias
 Reader.get get" like I would do for functions, the compiler complains that
 the symbols clash (no musical puns intended). Does anyone know how to get
 this to work?

Template functions are non-virtual. You can't derive from them. If you want the derived classes to have the same functions, you must redefine them in the derived class. - Jonathan M Davis
Feb 18 2012
next sibling parent reply %u <blm768 gmail.com> writes:
Thanks! I guess I'll just have to live with redefining the functions, do some
sort
of interface/mixin thing, or change the class interface. It makes sense that
template functions aren't virtual (how are you supposed to deal with vtables?),
but I wish that at least an alias declaration could work. Maybe if there were
some
way to alias the base template and then modify it...

Templates inheriting from templates would be a very interesting way to
accomplish
that, but it would be a very strange system...
Feb 18 2012
next sibling parent reply %u <blm768 gmail.com> writes:
I think I got it! This seems to work:

class Derived {
    //Pulls in all the template forms in the base class
    template get(args ...) {
        alias Base.get!args get;
    }

    //Create new versions of get() here.
}
Feb 18 2012
parent reply Timon Gehr <timon.gehr gmx.ch> writes:
On 02/19/2012 02:23 AM, %u wrote:
 I think I got it! This seems to work:

 class Derived {
      //Pulls in all the template forms in the base class
      template get(args ...) {
          alias Base.get!args get;
      }

      //Create new versions of get() here.
 }

This kills IFTI for the base class templates. It would be nice if it would work. Do you want to file an enhancement request? Otherwise I'll do it. This one lets you keep IFTI, but does not forward perfectly for some special implicit conversions. auto get(T...)(T a){return super.get!T(a);}
Feb 18 2012
parent reply BLM <blm768 gmail.com> writes:
That last one looks a lot better than my solution. It's certainly a lot clearer.

One problem I discovered with using templates was that I ended up needing
virtual
functions, which means that I had to convert the template functions to mixins
and
just instantiate them for each type (at least there were only two types to
handle!) in the base class. The problem I've got now is that if I create
versions
of get() that take different types in subclasses, I lose access to the
superclass's overload set. If I try to use "alias Base.get get", DMD complains
that the alias and the functions conflict. It looks like I can override existing
overloads but not create new ones. I guess I might have to put a hold on the
project until the language gets modified (assuming that actually happens). How
would I go about filing an enhancement request?
Feb 21 2012
parent Timon Gehr <timon.gehr gmx.ch> writes:
On 02/22/2012 01:13 AM, BLM wrote:
 That last one looks a lot better than my solution. It's certainly a lot
clearer.

 One problem I discovered with using templates was that I ended up needing
virtual
 functions, which means that I had to convert the template functions to mixins
and
 just instantiate them for each type (at least there were only two types to
 handle!) in the base class. The problem I've got now is that if I create
versions
 of get() that take different types in subclasses, I lose access to the
 superclass's overload set. If I try to use "alias Base.get get", DMD complains
 that the alias and the functions conflict. It looks like I can override
existing
 overloads but not create new ones. I guess I might have to put a hold on the
 project until the language gets modified (assuming that actually happens). How
 would I go about filing an enhancement request?

You can post it in the bug tracker and choose severity as 'enhancement': http://d.puremagic.com/issues/
Feb 25 2012
prev sibling parent Jonathan M Davis <jmdavisProg gmx.com> writes:
On Sunday, February 19, 2012 01:23:13 %u wrote:
 I think I got it! This seems to work:
 
 class Derived {
     //Pulls in all the template forms in the base class
     template get(args ...) {
         alias Base.get!args get;
     }
 
     //Create new versions of get() here.
 }

That seems like it could work. - Jonathan M Davis
Feb 18 2012
prev sibling parent Jacob Carlborg <doob me.com> writes:
On 2012-02-19 02:07, Jonathan M Davis wrote:
 On Sunday, February 19, 2012 00:55:59 %u wrote:
 I've been working on porting an old D library to D2, and I'm running into a
 nasty issue with templates and inheritance. I've got a base class like this:

 class Reader {
      void get(T)(ref T[] buffer);
 }

 and a subclass like this:

 class SubReader {
      void get()(SomeClass param);
 }

 The problem is that by creating a new form of the template in the subclass,
 I made the base class's version invisible. If I try to use "alias
 Reader.get get" like I would do for functions, the compiler complains that
 the symbols clash (no musical puns intended). Does anyone know how to get
 this to work?

Template functions are non-virtual. You can't derive from them. If you want the derived classes to have the same functions, you must redefine them in the derived class. - Jonathan M Davis

Yeah, but isn't that an overload, or specialization, in the subclass? -- /Jacob Carlborg
Feb 19 2012
prev sibling next sibling parent Jonathan M Davis <jmdavisProg gmx.com> writes:
On Sunday, February 19, 2012 01:17:42 %u wrote:
 Thanks! I guess I'll just have to live with redefining the functions, do
 some sort of interface/mixin thing, or change the class interface. It makes
 sense that template functions aren't virtual (how are you supposed to deal
 with vtables?), but I wish that at least an alias declaration could work.
 Maybe if there were some way to alias the base template and then modify
 it...
 
 Templates inheriting from templates would be a very interesting way to
 accomplish that, but it would be a very strange system...

aliases only work with actual, concrete functions. So, you could alias an specific instantiation of a templated function, but not the templated function as a whole. e.g. alias get!int getInt; works, but alias get getVal; doesn't. - Jonathan M Davis
Feb 18 2012
prev sibling next sibling parent Timon Gehr <timon.gehr gmx.ch> writes:
On 02/19/2012 01:55 AM, %u wrote:
 I've been working on porting an old D library to D2, and I'm running into a
 nasty issue with templates and inheritance. I've got a base class like this:

 class Reader {
      void get(T)(ref T[] buffer);
 }

 and a subclass like this:

 class SubReader {
      void get()(SomeClass param);
 }

 The problem is that by creating a new form of the template in the subclass, I
 made the base class's version invisible. If I try to use "alias Reader.get
 get" like I would do for functions, the compiler complains that the symbols
 clash (no musical puns intended).

That is a bug.
 Does anyone know how to get this to work?

There seems to be no perfect way in the current implementation.
Feb 18 2012
prev sibling parent Jonathan M Davis <jmdavisProg gmx.com> writes:
On Sunday, February 19, 2012 12:34:07 Jacob Carlborg wrote:
 Template functions are non-virtual. You can't derive from them. If you
 want
 the derived classes to have the same functions, you must redefine them in
 the derived class.
 
 - Jonathan M Davis

Yeah, but isn't that an overload, or specialization, in the subclass?

Yes. But if you're trying to give the function different behavior in the derived class, what else can you do? Of course, it won't work with polymorphism though (the version that gets called will depend on the reference that you're using), so it may not be such a good idea from that standpoint. Really, templated functions and class hierarchies don't get along very well. - Jonathan M Davis
Feb 19 2012