It is possible to send class reference as an argument to a function?

Question

I stumbled across this piece of code when I researched for a good example for Observer Design pattern. In main, it gets error, taking address of temporary[-fpermissive] which I dont dont understand what it is frankly. Sending a class refference to an function? Is this real life?

#include <vector>
#include <iostream>
using namespace std;

class AlarmListener
{
  public:
    virtual void alarm() = 0;
};

class SensorSystem
{
    vector < AlarmListener * > listeners;
  public:
    void attach(AlarmListener *al)
    {
        listeners.push_back(al);
    }
    void soundTheAlarm()
    {
        for (int i = 0; i < listeners.size(); i++)
          listeners[i]->alarm();
    }
};

class Lighting: public AlarmListener
{
  public:
     /*virtual*/void alarm()
    {
        cout << "lights up" << '\n';
    }
};

class Gates: public AlarmListener
{
  public:
     /*virtual*/void alarm()
    {
        cout << "gates close" << '\n';
    }
};

class CheckList
{
    virtual void localize()
    {
        cout << "   establish a perimeter" << '\n';
    }
    virtual void isolate()
    {
        cout << "   isolate the grid" << '\n';
    }
    virtual void identify()
    {
        cout << "   identify the source" << '\n';
    }
  public:
    void byTheNumbers()
    {
        // Template Method design pattern
        localize();
        isolate();
        identify();
    }
};
// class inheri.  // type inheritance
class Surveillance: public CheckList, public AlarmListener
{
     /*virtual*/void isolate()
    {
        cout << "   train the cameras" << '\n';
    }
  public:
     /*virtual*/void alarm()
    {
        cout << "Surveillance - by the numbers:" << '\n';
        byTheNumbers();
    }
};

int main()
{
  SensorSystem ss;
  ss.attach(&Gates());
  ss.attach(&Lighting());
  ss.attach(&Surveillance());
  ss.soundTheAlarm();
}

Where are you using references? Everything I see is you using pointers. — NathanOliver, Mar 03 '16 at 14:40

score 2 · Answer 1 · answered Mar 03 '16 at 14:42

This is ill-formed:

ss.attach(&Gates());
         ^^^

Gates() is an rvalue (specifically, a prvalue). You cannot take the address of an rvalue. It's not an object that has identity, so it doesn't really have an address that you can take. The language is preventing you from doing something that doesn't make sense to do. If you did store a pointer to this temporary, you'd just end up with a dangling pointer since at the end of this line the temporary Gates would be destroyed.

Since SensorSystem doesn't own its AlarmListeners, you'll have to create them up front:

Gates gates;
Lighting lighting;
Surveillance surveillance;

SensorSystem ss;
ss.attach(&gates);
ss.attach(&lighting);
ss.attach(&surveillance);

It is possible to send class reference as an argument to a function?

1 Answers1