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2012-05-04T07:51:29.693
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2012-05-04T07:51:29.693
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If you trace through the 1.9.3 source, you'll find that <code>defined?</code> is implemented in <a href="https://github.com/ruby/ruby/blob/ruby_1_9_3/insns.def#L749" rel="nofollow"><code>insns.def</code></a>: <pre><code>DEFINE_INSN defined (rb_num_t op_type, VALUE obj, VALUE needstr) /* ... */ switch (type) { /* ... */ case DEFINED_CONST: klass = v; if (vm_get_ev_const(th, GET_ISEQ(), klass, SYM2ID(obj), 1)) { expr_type = "constant"; } break; </code></pre> So when you <code>defined? SomeConstant</code>, you trickle through that big <code>switch</code> and end up calling <code>vm_get_ev_const</code>. The function is defined in <a href="https://github.com/ruby/ruby/blob/ruby_1_9_3/vm_insnhelper.c#L1160" rel="nofollow"><code>vm_insnhelper.c</code></a>: <pre><code>static inline VALUE vm_get_ev_const(rb_thread_t *th, const rb_iseq_t *iseq, VALUE orig_klass, ID id, int is_defined) </code></pre> That function happens to be static so you can't get at it. Looks like <code>vm_get_ev_const</code> is defined in terms of <a href="https://github.com/ruby/ruby/blob/ruby_1_9_3/variable.c#L1846" rel="nofollow"><code>rb_const_defined</code></a> and <a href="https://github.com/ruby/ruby/blob/ruby_1_9_3/variable.c#L1858" rel="nofollow"><code>rb_const_defined_from</code></a> and both of those should be available in your C so you could try those; but you'd have to find the right <code>klass</code> for those. Or you could go with your idea and just use <code>Object.const_defined?</code>. One problem with that is that it won't do The Right Thing with things like <code>A::B</code>, you'd have to say <code>Object.const_defined? :A && A.const_defined? :B</code> for that as <code>Object.const_defined? :'A::B'</code> will just throw an exception in your face. A general solution here would require iteration and class lookups. However, if the classes that you're looking at are all in the top level namespace, then a simple <code>Object.const_defined?</code> should do the trick.
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