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.

I enjoy Infected Mushroom. So go fuck yourself >>30.