whatnot.cpp

// .h
#ifdef whatnot
    std::unique_ptr<classA> theclass;
#else
    std::unique_ptr<classB> theclass;
#endif

// .cpp
// How can I avoid using the actual classnames here
#ifdef whatnot
	theclass.reset(new classA(blargh));
#else
	theclass.reset(new classB(blargh));
#endif

using

#ifdef whatnot
using classX = classA;
#else
using classX = classB;
#endif

std::unique_ptr<classX> theclass;
theclass.reset(new classX(blargh));