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    <head>
      <base href="http://llvm.org/bugs/" />
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    <body><table border="1" cellspacing="0" cellpadding="8">
        <tr>
          <th>Bug ID</th>
          <td><a class="bz_bug_link 
          bz_status_NEW "
   title="NEW --- - Objective-C method writeback parameter marked "out" does not allow passing more generic pointer type, unless -fobjc-arc is enabled"
   href="http://llvm.org/bugs/show_bug.cgi?id=16626">16626</a>
          </td>
        </tr>

        <tr>
          <th>Summary</th>
          <td>Objective-C method writeback parameter marked "out" does not allow passing more generic pointer type, unless -fobjc-arc is enabled
          </td>
        </tr>

        <tr>
          <th>Product</th>
          <td>clang
          </td>
        </tr>

        <tr>
          <th>Version</th>
          <td>3.3
          </td>
        </tr>

        <tr>
          <th>Hardware</th>
          <td>Macintosh
          </td>
        </tr>

        <tr>
          <th>OS</th>
          <td>MacOS X
          </td>
        </tr>

        <tr>
          <th>Status</th>
          <td>NEW
          </td>
        </tr>

        <tr>
          <th>Severity</th>
          <td>enhancement
          </td>
        </tr>

        <tr>
          <th>Priority</th>
          <td>P
          </td>
        </tr>

        <tr>
          <th>Component</th>
          <td>-New Bugs
          </td>
        </tr>

        <tr>
          <th>Assignee</th>
          <td>unassignedclangbugs@nondot.org
          </td>
        </tr>

        <tr>
          <th>Reporter</th>
          <td>jtbandes@gmail.com
          </td>
        </tr>

        <tr>
          <th>CC</th>
          <td>llvmbugs@cs.uiuc.edu
          </td>
        </tr>

        <tr>
          <th>Classification</th>
          <td>Unclassified
          </td>
        </tr></table>
      <p>
        <div>
        <pre>With the following classes:

    @interface Foo @end
    @implementation Foo @end

    @interface Bar : Foo @end
    @implementation Bar @end

    @interface Nothing
    + (void)blah:(out Bar **)b;
    @end
    @implementation Nothing
    + (void)blah:(out Bar **)b { }
    @end

One would expect to be able to pass the address of a Foo* object in for the
writeback parameter, since the +blah: method is expected to write an object of
type Bar*, a subtype of Foo*. For example:

    int main (int argc, char const *argv[])
    {
      Foo *f;
      [Nothing blah:&f];

      return 0;
    }

This works when -fobjc-arc is enabled, but otherwise, the following warning is
given:

    warning: incompatible pointer types sending 'Foo **' to parameter of type
'Bar **' [-Wincompatible-pointer-types]

Note: the "in"/"out"/"inout" annotations have historically been part of
Distributed Objects, but still have meaning nowadays, as seen in Clang's ARC
documentation:

<a href="http://clang.llvm.org/docs/AutomaticReferenceCounting.html#passing-to-an-out-parameter-by-writeback">http://clang.llvm.org/docs/AutomaticReferenceCounting.html#passing-to-an-out-parameter-by-writeback</a>

"A pass-by-writeback is evaluated as follows:
...
4. If the parameter is not an Objective-C method parameter marked out, then *p
is read, and the result is written into the temporary with primitive
semantics."

In my opinion these annotations/qualifiers should be enough for the compiler to
realize, in all cases, that passing a Foo** value to an "out Bar**" parameter
is completely valid. (This issue was inspired by the minor outrage against the
new Dropbox Obj-C API, which provides DBError, a subclass of NSError, and has
some DBError** writeback parameters. Dropbox did not mark these parameters
"out", but if they had, it should have not caused any problems to pass an
NSError**.)</pre>
        </div>
      </p>
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