Re: Order of evaluation of expression

From:

"Igor Tandetnik" <itandetnik@mvps.org>

Newsgroups:

microsoft.public.vc.language

Date:

Tue, 3 Jun 2008 12:02:42 -0400

Message-ID:

<uWnGHRZxIHA.5520@TK2MSFTNGP06.phx.gbl>

Paul <vhr@newsgroups.nospam> wrote:

struct Lock {
   void lock();
   void unlock();
};

struct Latch {
   Latch(Lock& lck) : lock(lck) { lock.lock(); }
   ~Latch() { lock.unlock(); }
private:
   Lock& lock;
};

inline std::wostream& operator <<(std::wostream& os, const Latch&)
{
   return os;
}

int main()
{
   Lock lck;

   std::wcout << Latch(lck) << L"Something else 1.\n" << L"Something
else
2.\n";

   return 0;
}

The code seems to be working all right. Is there nothing wrong with
this idea?

It depends on what exactly you expect to be protected by the lock. What
else tries to lock and unlock it? For example, if you have something
like

std::wcout << Latch(lck) << FunctionSupposedlyProtectedByLock();

then it's not guaranteed that Latch constructor runs before
FunctionSupposedlyProtectedByLock. You can only be sure that two
operator<<()'s will run in the order written, but since your
operator<<(Latch) implementation is a no-op, that doesn't help any.
--
With best wishes,
Igor Tandetnik

With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
overhead. -- RFC 1925