ruby.h:620
static inline VALUE
#if defined(HAVE_PROTOTYPES)
rb_class_of(VALUE obj)
#else
rb_class_of(obj)
    VALUE obj;
#endif
{
    if (FIXNUM_P(obj)) return rb_cFixnum;
    if (obj == Qnil) return rb_cNilClass;
    if (obj == Qfalse) return rb_cFalseClass;
    if (obj == Qtrue) return rb_cTrueClass;
    if (SYMBOL_P(obj)) return rb_cSymbol;

    return RBASIC(obj)->klass;
}
