This confuses me.

I do understand that a default-constructed *context* object is useful to be able to switch context from any place in the code.

But I'm now asking about the usefulness of a default-constructed *stack* object. A default-constructed *context* object is not passed a stack object.

because og the move-semantics supported by the stack class an default constructed stack instance means not-a-stack. It doesn't allocates memory.

If you're saying that a default-constructed context object must nonetheless store a default-constructed stack object... this starts to feel like an implementation detail leaking out into the API.

It is not leaking - all classes providing move-semantics are used in this ways. Please look into boost.thread - boost::thread has a default ctor -> constructs a boost::thread instance that refers to Not-a-Thread.

In that case, what further Context work is needed to support Mac OS X?

assembler implementation/fcontext - the source code has already an assembler file for MAC OS X (x86-64/i386). I wasn't able to test the code because I haven't a MAC OS X installation. Oliver -- NEU: FreePhone - kostenlos mobil telefonieren und surfen! Jetzt informieren: http://www.gmx.net/de/go/freephone