Alias singleton method from C

[Ross Bamford]

Hi,

I think I must have missed something obvious, but I can't see how to
alias a singleton method from a C extension. I think I can grab the
singleton class, then pass that to rb_alias, but is that the right /
best way to do it? Is there a define_singleton_alias somewhere that I've
failed to spot?

Any help would be much appreciated.

Thanks,

 

[Daniel Berger]

Hi,

I think I must have missed something obvious, but I can't see how to
alias a singleton method from a C extension. I think I can grab the
singleton class, then pass that to rb_alias, but is that the right /
best way to do it? Is there a define_singleton_alias somewhere that I've
failed to spot?

Any help would be much appreciated.

Good question. I'm not sure why this code fails:

#include <ruby.h>

static VALUE foo_bar(){
return rb_str_new2("hello");
}

void Init_foo(){
VALUE cFoo, singleton;

cFoo = rb_define_class("Foo", rb_cObject);

rb_define_singleton_method(cFoo, "bar", foo_bar, 0);

singleton = rb_const_get(rb_cObject, rb_intern("Foo"));
rb_define_alias(singleton, "baz", "bar");
}

When I compile and run that it fails with "undefined method `bar' for
class `Foo'" even though it clearly is defined. I can see in class.c
that rb_define_alias is just calling rb_alias from eval.c. It looks
like rb_alias handles singletons differently, but why it's not working
in the example I provided I'm not sure.

Hopefully Matz or Guy will pipe up.

Regards,

Dan

 

[Tim Pease]

Good question. I'm not sure why this code fails:

#include <ruby.h>

static VALUE foo_bar(){
return rb_str_new2("hello");
}

void Init_foo(){
VALUE cFoo, singleton;

cFoo = rb_define_class("Foo", rb_cObject);

rb_define_singleton_method(cFoo, "bar", foo_bar, 0);

singleton = rb_const_get(rb_cObject, rb_intern("Foo"));
rb_define_alias(singleton, "baz", "bar");
}

When I compile and run that it fails with "undefined method `bar' for
class `Foo'" even though it clearly is defined. I can see in class.c
that rb_define_alias is just calling rb_alias from eval.c. It looks
like rb_alias handles singletons differently, but why it's not working
in the example I provided I'm not sure.

rb_define_alias only works for instance methods.
rb_define_singleton_method is the same as saying

class Foo
class << self
def bar( ) "hello" end
end
alias :baz :bar
end

That is going to fail because you have not defined a method "bar" in
the Foo class. It is defined in the Foo singleton class.

Does that make sense or help at all? I've got to run to a meeting, so
I don't have time to work out the correct way of doing it. That's
left as an exercise for the reader

Blessings,
TwP

 

[Daniel Berger]

rb_define_alias only works for instance methods.

It should work for either singletons or instance methods. If it
didn't, then this code wouldn't work:

class Foo
class << self
def bar
"hello"
end
alias baz bar
end
end

p Foo.bar => "hello"
p Foo.baz => "hello"

The C example I pasted is meant to do what I've done there.

Internally rb_define_alias calls rb_alias (in eval.c) which checks to
see if klass is a singleton or not. So, in theory, I should be able to
do:

rb_define_alias(singleton, baz, bar);

But obviously I'm doing something wrong - I'm just not sure what.

Regards,

Dan

PS - A rb_define_singleton_alias() would be a handy function.

 

[Ross Bamford]

It should work for either singletons or instance methods. If it
didn't, then this code wouldn't work:

class Foo
class << self
def bar
"hello"
end
alias baz bar
end
end

p Foo.bar => "hello"
p Foo.baz => "hello"

The C example I pasted is meant to do what I've done there.

Internally rb_define_alias calls rb_alias (in eval.c) which checks to
see if klass is a singleton or not. So, in theory, I should be able to
do:

rb_define_alias(singleton, baz, bar);

But obviously I'm doing something wrong - I'm just not sure what.

This is similar to what I was going to do - you just need to get the
singleton class from the class. I _think_ in your example, cFoo ==
singleton. Something like this works:

/* define singleton methods as normal */

VALUE singleton = rb_singleton_class(cXMLNode);
rb_define_alias(singleton, "new_element", "new");

I guess that would work with rb_alias too, but I thought there had to be
a wrapper for this somewhere that I'd missed.

PS - A rb_define_singleton_alias() would be a handy function.

Agreed. Assuming it's not already lurking somewhere, that is.

Cheers,

 

[Tim Pease]

Your line of code above that grabs the singleton class is incorrect.

singleton = rb_const_get( rb_cObject, rb_intern("Foo"));

This will just return the class object for your "Foo" class. In this
case singleton is equivalent to cFoo. Try this out in your code ...

singleton == cFoo

That should equate to true. Here is the correct way to grab the
singleton class from C code ...

cat foo.c

#include <ruby.h>

static VALUE
foo_bar( VALUE self ) {
return rb_str_new2( "hello" );
}


void
Init_foo( ) {
VALUE cFoo, singleton;

cFoo = rb_define_class("Foo", rb_cObject);

rb_define_singleton_method(cFoo, "bar", foo_bar, 0);

singleton = rb_singleton_class(cFoo);
rb_define_alias(singleton, "baz", "bar");
}

Now you can do this ...

Foo.bar #=> "hello"
Foo.baz #=> "hello"


The magic syntax is the line ...

singleton = rb_singleton_class(cFoo);

That gives me Foo's singleton.

Blessings,
TwP

 

[Daniel Berger]

Ross Bamford wrote:

/* define singleton methods as normal */

VALUE singleton = rb_singleton_class(cXMLNode);
rb_define_alias(singleton, "new_element", "new");

Bingo. That's what you want. I was just getting the singleton wrong.

- Dan