C++ Address-of nullchecking

Impl& impl;

That little motherfucking line of code in a header is pissing me off.  Why?  Because it eventually causes an access violation at the line:

if(!impl.mProcessTimer.checkExpirationAndReset(SECS_BETWEEN_PROCESS))

And I have no idea how to check for nulls in an address-of situation.  Regular pointers?  Sure, but whenever I try if(!impl) or some other more convoluted variation of that, MSVC screams about invalid operators. 

Halp plz.

>>32
Wake me up before I change again
Remind me the story that I won´t get insane
Tell me why it´s always the same
Explain me the reason why I´m so much in pain