From: "Berger, Daniel" <Daniel.Berger@...>
Date: 2005-03-12T08:47:24+09:00
Subject: Problem with rb_block_given_p() calling context?

Hi all,

Ruby 1.8.2
Solaris 9

I'm having problem with rb_block_given_p() in a C extension.  It seems
to be evaluated by a constructor when it shouldn't be.  Here's the C
code:

/* fab.c */
#include "ruby.h"

VALUE cFoo, cFab;

static VALUE foo_init(VALUE self){
   if(rb_block_given_p())
      rb_yield(self);

   return self;
}

static VALUE fab_get_foos(VALUE klass, VALUE rbNum){
   VALUE argv, rbFoo;
   int i = 0;
   int n = NUM2INT(rbNum);

   for(i = 0; i < n; i++){
      rbFoo = rb_class_new_instance(0,&argv,cFoo);
      rb_iv_set(rbFoo,"@some_arg",INT2FIX(i));
      
      if(rb_block_given_p()){
         rb_yield(rbFoo);
      }
   }
   return Qnil;
}

void Init_fab(){
   cFab = rb_define_class("Fab",rb_cObject);
   cFoo = rb_define_class("Foo",rb_cObject);

   rb_define_method(cFoo, "initialize", foo_init, 0);
   rb_define_attr(cFoo,"some_arg", 1, 1);

   rb_define_singleton_method(cFab, "get_foos", fab_get_foos, 1);
}

Now, if I run a sample script, I get this as the result:

# test1.rb
require "fab"
Fab.get_foos(3){ |f| p f.some_arg }

# result
nil
0
nil
1
nil
2

Why am I getting the nil?  It looks like it's evaluating the call to
rb_block_given_p() on the Foo constructor, i.e. in foo_init, even though
I'm not explicitly passing it there.

A pure Ruby version doesn't seem to have this problem:

# test2.rb
class Foo
   attr_accessor :some_arg
   def initialize
      yield self if block_given?
   end
end

class Fab
   def self.get_foos(num)
      0.upto(num){ |i|
         f = Foo.new
         f.some_arg = i
         yield f if block_given?
      }
   end
end

if $0 == __FILE__
   Fab.get_foos(5){ |f|
      p f.some_arg
   }
end

# result
0
1
2

What have I done wrong?

Regards,

Dan