Getting an object's array index in C++

Question

Let's say i have an array of objects and i want to set variables within those objects based on their array index (0, 1, 4... etc.). Is there a way to get the object's index (within its parent array) though a member function, that is, without passing an integer?

Made up example:

class Car
{
    public:
        void init();
    private:
        short weight;
};

void Car::init()
{
    // affect 'weight' based on object's array index
}

Car myCars[7];

myCars[2].init();

Is there a way to retrieve myCars' index (i.e. 2) from within init() without the function having received an integer from the outside?

I know this is not necessary, but i was curious whether it was possible.

Thank you.

Simply no! It's not possible unless you store the indices along with your Car instances. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Jan 16, 2013 at 16:49

RageD · Accepted Answer · 2013-01-16 16:54:18Z

4

Are you willing to give more information to the init method? You can do something as follows with some pointer arithmetic:

#include <iostream>

using namespace std;

class Car
{
    public:
        void init(const Car*);
    private:
        short weight;
};

void Car::init(const Car* arr)
{
    // affect 'weight' based on object's array index
    int idx = this - arr;
    cout<< "My index: " << idx << endl;
}

int main()
{
  Car myCars[7];

  for(int i = 0 ; i < 7 ; ++i)
    myCars[i].init(myCars);
  return 0;
}

answered Jan 16, 2013 at 16:54

RageD

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2 Comments

Csq Over a year ago

why is myCars[i].init(myCars); better than passing it directly as myCars[i].init(i);.

RageD Over a year ago

It's not necessarily better, although, potentially more robust if you don't actually know i or want to do sanity checks. For instance, if there was a pointer flying around Car* myCar = &myCars[x]; for some arbitrary x, then you do not need to hold that value in order to do this computation. As for sanity checks, this way you can check if &arr[idx] == this or not to see if the calculation is being done on the correct array. A lot of this would depend on implementation, really and what the OP wants to guarantee.

Mark B · Accepted Answer · 2013-01-16 16:49:36Z

2

No, the C++ language doesn't provide such a capability. If you really need that (double check your design) you'll have to pass it in and maintain it if the index ever changes for your object (for example if you insert in the middle of a container).

answered Jan 16, 2013 at 16:49

Mark B

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1 Comment

Adrian Over a year ago

Thank you, that's all i wanted to know. It it not crucial for my design, where nothing prevents me from passing variables the usual way. It was just a question of principle.

Csq · Accepted Answer · 2013-01-16 16:49:29Z

1

No.

But as an alternative, you could move the initialization to the constructor (if applicable). That gets called automatically when you create the array. But this also won't let you adjust the weight based on the index though.

answered Jan 16, 2013 at 16:49

Csq

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2 Comments

user405725 Over a year ago

How having a constructor is an alternative to finding the index of some array at which the object is placed?

Csq Over a year ago

No, the name init misleaded me. Updated the answer.

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