This is how you solve it with boost::optional:

#include <boost/optional.hpp>

boost::optional<int> getConfigParam(std::string name);  // return either an int or a `not-an-int`

int main()
{
  if (boost::optional<int> oi = getConfigParam("MaxValue")) // did I get a real int?
    runWithMax(*oi);                                        // use my int
  else
    runWithNoMax();
}