How to use class G4StackManager?

Hi!
Does anyone know which example uses the G4StackManager class? I did not find. I want to use “GetNTotalTrack” but it’s not working. I did like this:

MyStackManager.hh:
#pragma once
#include “G4StackManager.hh”
class MyStackManager: public G4StackManager
{
public:
MyStackManager(G4StackManager* trackContainer);
~MyStackManager();

inline G4int GetNTotalTrack() {return m_primaries; }

private:
G4int m_primaries;
};

MyStackManager.cpp:
MyStackManager::MyStackManager(G4StackManager* trackContainer):

m_primaries (trackContainer->GetNTotalTrack())

{
G4cout
<< trackContainer->GetNTotalTrack() << " primaries." << G4endl;
}

MyStackManager::~MyStackManager()
{};