This makes it innappropriate to do the efficient thing NS_IF_ADDREF(*result = mThing); because this makes the assignment happen multiple times. Additionally, you want |result| to only be dereferenced once which you can't get from *result = mThing; NS_IF_ADDREF(*result); So if the first form can be made to work, it is preferred. These efficiency considerations were pointed out by <jband@netscape.com>, though the form recommended in the first example above does not exactly match his desired coding style. The solution must work even if |mThing| in the example above is an |nsCOMPtr|. The solution should not, in my opinion, compromise the safety/conversion feature of |nsCOMPtr| that denies clients the ability to |AddRef()| them directly, e.g., nsCOMPtr<nsIThing> mThing; // ... NS_IF_ADDREF(mThing); // error: can't |AddRef()| an |nsCOMPtr| NS_IF_ADDREF(*result = mThing); // OK Note: |NS_ADDREF(expr)| already has the property that |expr| is only evaluated once in non-logging builds.

Assuming this builds and tests OK then r=jband

Patch tested and checked in. Marking FIXED as of "nsISupportsUtils.h" 1.56. Note the small change to the template (omitting the |*| from |T*|) to improve the error message given when the macro is mistakenly applied to an |nsCOMPtr|.

Adding verifyme keyword.

- Per last comments, age of bug, and no reopen - Marking Verified/Fixed. Please reopen if still a problem.