Question

In Java, where all classes are really references, I do this:

Car getCar(int mileage)
{
  Car car = new Car();

  car.setMileage(mileage);
  return car;
}

In C++, how do I do this? I can put it into a reference:

void getCar(int mileage, Car& car)
{
  car.setMileage(mileage);
  return true;
}

Or, I can create a new object:

Car* getCar(int mileage)
{
  Car* car = new Car;

  car.setMileage(mileage);
  return car;
}

But then, the caller is also responsible for deleting car.

I don’t want to return a pointer. I want to return a Car:

Car getCar(int mileage)
{
  Car car;

  car.setMileage(mileage);
  return car;
}

But of course, car is a local variable which will be deleted once the function finishes.

What’s generally the ‘standard’ way of doing this? Which is the best way, and why?

Answer 1

Your final example is the correct and idiomatic way to return an object:

Car getCar(int mileage)
{
  Car car;

  car.setMileage(mileage);
  return car;
}

Yes, car will be deleted at the end of the function, but not before it is copied into the returned object. You might invoke this function like so:

{
    Car myCar;
    myCar = getCar(42);
}

The car that is local to getCar is copied into the calling environment’s myCar.

---

What you can’t and must not do is to return a reference to or a pointer to a local variable.
This is WRONG:

Car& getCar(int mileage)
{
  Car car;
  return car;
}

This is also WRONG:

Car* getCar(int mileage)
{
  Car car;
  return &car;
}

In each of these cases, you are allowing the calling function access to an object which no longer exists.

You mustn’t return a pointer or reference to a local. You may return a copy of a local.