PVS-Studio warning V801 for Geant4

class G4String : public std::string {
  ....
};

void G4NeutronHPChannel::
Init(G4Element * anElement, const G4String dirName,
     const G4String aFSType)
{
  theFSType = aFSType;
  Init(anElement, dirName);
}

This suspicious code was found in Geant4 project by PVS-Studio static code analyzer.
Warning message is:
V801 Decreased performance. It is better to redefine the second function argument as a reference. Consider replacing 'const .. dirName' with 'const .. &dirName'. g4neutronhpchannel.cc 55

PVS-Studio is a static analyzer for detecting bugs in the source code of applications written in C, C++, C++11, C++/CX. Site: http://www.viva64.com/en/pvs-studio/