The argument func in quad3d is templated so that you can supply the 3-d function to be integrated either as a function or functor. However, the code as given works only with functions. Corrected code is given in this post:
http://www.nr.com/forum/showthread.php?t=1663&p=3458
This code also templates the 4 functions that supply the limits of integration so that you can use functors there as well.