Question

So I was looking through some boost source code and came across this:

(from <boost/checked_delete.hpp>)

template<class T> inline void checked_delete(T * x)
{
    // intentionally complex - simplification causes regressions
    typedef char type_must_be_complete[ sizeof(T)? 1: -1 ];
    (void) sizeof(type_must_be_complete);
    delete x;
}

Anyone happen to know why it is implemented in this way? Wouldn’t sizeof(T) (for example) already be enough?

Answer 1

Someone asked the same question earlier. This post written by Peter Dimov (one of the writers of boost/checked_delete.hpp) pretty much speaks for itself:

• What’s the result of applying sizeof to an incomplete type?

A compile-time error, unless the compiler chooses to return 0 as a
nonstandard extension.

• Why is sizeof called twice?

The second sizeof is a workaround for a Metrowerks CodeWarrior bug in
which the first typeof is never instantiated unless used.

• Why is the result of sizeof cast to void? What exactly does that
  line
  do?

Silences a compiler warning.