Intentionally return NaN

return std::numeric_limits<double>::quiet_NaN();

I know it's an old question, but with C++11 you have the nan(const char*) family of functions (nan for doubles, nanl for long doubles and nanf for floats). The argument is implementation specific, but passing an empty string (e.g. nan("")) returns a generic NaN value.

This should work:

#include <limits>

return std::numeric_limits<double>::quiet_NaN();