Static int with array C++

Question

I'm trying to static count my driver user. however it always give me same value instead

class Driver {
private:
  static int ID;
  string name;
public : 
  void displayDriver(string n) {
    cout << ID << endl;
  }
  void createDriver(string n) {
    name = n ;
    ID++;
  }
}

int Driver::id=0;

int main() {
  Driver driver[10];
  Driver[0].createDriver("first");
  Driver[1].createDriver("second");
  Driver[2].createDriver("first");                 
  Driver[0].displayDriver();
  Driver[1].displayDriver();
  Driver[2].displayDriver();
}

my expected output should be :

 1
 2
 3

but system shows me :

 3
 3
 3

You literally asked it to do that.

Lightness Races in Orbit
– Lightness Races in Orbit

2015-05-30 04:15:42 +00:00
Commented May 30, 2015 at 4:15 — Lightness Races in Orbit
– Lightness Races in Orbit, Commented May 30, 2015 at 4:15

LeleDumbo · Accepted Answer · 2015-05-30 03:04:24Z

4

private:
 static int ID;

Denotes that ID is shared among Driver instances. Any change to it (from createDriver in your current code) will be reflected in ALL instances. Don't use static field if you want each instance has its own unique field.

answered May 30, 2015 at 3:04

LeleDumbo

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Comments

gae123 · Accepted Answer · 2015-05-30 03:08:41Z

1

Do something like I show here to get the results you expect. The idea is to keep the next counter in ID but every instance to have its own id that get initialized when you create the driver.

class Driver
{
  private:
     static int ID;
     int my_id;
     string name;
  public : 
  void displayDriver(string n)
  {
   cout << my_id << endl;
  }
  void createDriver(string n)
  {
    name = n ;
    my_id = ID++;
  }
}
int Driver::id=0;
int main
{
  Driver driver[10];
  Driver[0].createDriver("first");
  Driver[1].createDriver("second");
  Driver[2].createDriver("first");                 
  Driver[0].displayDriver();
  Driver[1].displayDriver();
  Driver[2].displayDriver();
}

answered May 30, 2015 at 3:08

gae123

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Comments

Shivam · Accepted Answer · 2015-05-30 03:25:12Z

0

It is because the value of ID gets incremented each time u createDriver("");

(no matter for which Driver[]) as it is a static variable

So u r incrementing ID progressively to 3 in first 3 lines while displaying value of ID in next 3 lines

To get the expected output

I think u need to try it like this :

 Driver[0].createDriver("");
 Driver[0].displayDriver();
 Driver[1].createDriver("");
 .
 .  
 .

So on!

Note that all drivers have the same ID (static field).

edited May 30, 2015 at 3:25

answered May 30, 2015 at 3:08

Shivam

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