btw, in another code base, you may familiar with, the comment is the variable name: http://www.google.com/codesearch?q=type_must_be_complete&exact_package=chromium

On Thu, Nov 11, 2010 at 12:24 AM, Finnur Thorarinsson <finnur@chromium.org> wrote:

Umm... shouldn't this behavior be commented so that people are not left wondering why it fails and trying to "fix" it?

On Wed, Nov 10, 2010 at 16:04, Darin Adler <darin@apple.com> wrote:

On Nov 10, 2010, at 2:33 PM, Daebarkee Jung wrote:

> I found that the following lines made errors:
> // OwnPtrCommon.h
> template <typename T> inline void deleteOwnedPtr(T* ptr)
> {
> typedef char known[sizeof(T) ? 1 : -1];
> if (sizeof(known))
> delete ptr;
> }
>
> I am very curious about why the author wrote like the above.
> What could be the author's intention?

The code is to prevent issues like the ones described on these websites:

http://stackoverflow.com/questions/1767679/incomplete-type-memory-leaks
http://bytes.com/topic/c/answers/611877-gcc-class-forward-declarations-destructor-calls
http://connect.microsoft.com/VisualStudio/feedback/details/231177/delete-of-pointer-to-incomplete-class

If we delete a pointer and the object has incomplete type, we get undefined behavior. Instead this code causes compilation to fail if the object has incomplete type. The use of a negative number for the size of an array is a way to guarantee we get a compilation error.

Your alternate version might also work; I’m not sure.

-- Darin

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