Numerous warnings about improper header definitions, finally I get this:

g++ -D__unix__ -I../include/snmp++ -I./ -O -pipe -march=pentium3 -c  -fPIC -DPIC oid.cpp -o .libs/oid.lo
In file included from /usr/include/g++/backward/iostream.h:31,
                 from ../include/snmp++/smival.h:49,
                 from ../include/snmp++/oid.h:62,
                 from oid.cpp:59:
/usr/include/g++/backward/backward_warning.h:32:2: warning: #warning This file includes at least one deprecated or antiquated header. Please consider using one of the 32 headers found in section 17.4.1.2 of the C++ standard. Examples include substituting the <X> header for the <X.h> header for C++ includes, or <sstream> instead of the deprecated header <strstream.h>. To disable this warning use -Wno-deprecated.
In file included from oid.cpp:59:
../include/snmp++/oid.h:181: `vector' was not declared in this scope
../include/snmp++/oid.h:181: syntax error before `>' token
oid.cpp:503: `vector' was not declared in this scope
oid.cpp:503: syntax error before `>' token
oid.cpp: In member function `void Oid::get_oid(...) const':
oid.cpp:505: `outVect' undeclared (first use this function)
oid.cpp:505: (Each undeclared identifier is reported only once for each
   function it appears in.)
gmake[3]: *** [oid.lo] Error 1
gmake[3]: Leaving directory `/usr/ports/net/cflowd/work/cflowd-2-1-b1/snmp++/classes/src'
gmake[2]: *** [all] Error 2
gmake[2]: Leaving directory `/usr/ports/net/cflowd/work/cflowd-2-1-b1/snmp++/classes'
gmake[1]: *** [classes/lib/libsnmp++.a] Error 2
gmake[1]: Leaving directory `/usr/ports/net/cflowd/work/cflowd-2-1-b1/snmp++'
gmake: *** [all] Error 2
*** Error code 2

Stop in /usr/ports/net/cflowd.

Fix: 

I am guessing this is a byproduct of moving to gcc 3.2.1 - I do not encounter this problem on a 4-STABLE machine using 2.95. I don't get any of the warnings either.
How-To-Repeat: I have caused this to repeat several times on this machine and another 5.0-RELEASE i386 machine (using SMP). I have a 5.0-RELEASE machine that is not using SMP that I have not had the opportunity to test on.