c++ - Why can I access private variables in the copy constructor?

Question

I have learned that I can never access a private variable, only with a get-function in the class. But then why can I access it in the copy constructor?

Example:

Field::Field(const Field& f)
{
  pFirst = new T[f.capacity()];

  pLast = pFirst + (f.pLast - f.pFirst);
  pEnd  = pFirst + (f.pEnd - f.pFirst);
  std::copy(f.pFirst, f.pLast, pFirst);
}

My declaration:

private:
  T *pFirst,*pLast,*pEnd;

Answer 1

The access modifiers work on class level, and not on object level.

That is, two objects of the same class can access each others private data.

Why:

Primarily due to efficiency. It would be a non-negligible runtime overhead to check if this == other each time you access other.x which you would have to if the access modifiers worked on object level.

It's also kind of semantically logical if you think of it in terms of scoping: "How big part of the code do I need to keep in mind when modifying a private variable?" – You need to keep the code of the whole class in mind, and this is orthogonal to which objects exist in runtime.

^{_{And it's incredibly convenient when writing copy constructors and assignment operators.}}