libstc++-v3 vector not convertible to pointer

[nbecker@fred.net]

I see that in libstdc++-v3 (current cvs) a vector::iterator is no
longer compatible with a pointer.  Specifically, this won't work:

reinterpret_cast<fftw_real*>(in.begin())

where in is vector.begin() and fftw_real is double.

Is there any alternative class I could use?  This seems to be a
fundamental need for high performance computing, since we still we
need to interoperate with C for some time to come.