Why can this member function access a private vector

Question

Now I have here a code snippet that I am trying to understand. I thought that only friend functions could access private member variables, so why is the method 'grad' able to access 'a' here to get its size?

#include <vector>

using namespace std;

class Polynom{
  private:
    vector<double> a;

  public:
    Polynom(const vector<double>& v): a(v) {}

    int grad() { return a.size()-1; }
};

int main()
  {
    return 0;
  }

The grad function belongs to the Polynom class. Member functions of a class can access the private variables of that same class. — Harrand, Mar 23 '17 at 14:52
All members of a class can access all other members of the class. — NathanOliver, Mar 23 '17 at 14:52
Actually, `private` primarily exists for exactly this use case: to limit the access to other member functions. `friend` is just a construct to extend this to non-member functions. — Ferdinand Beyer, Mar 23 '17 at 14:53
if this wasnt possible, what would be the use of private fields? — 463035818_is_not_an_ai, Mar 23 '17 at 14:53
If `grad()` that is a member function of `Polynom` could not access `a`, how do you intend to change `a` contents? — Amadeus, Mar 23 '17 at 14:53
You should read your C++ book more carefully, this is a very basic concept. — Baum mit Augen, Mar 23 '17 at 14:53
@Amadeus I thought private member functions were for accessing private member variables. — azureai, Mar 23 '17 at 14:58
Your class has no C++ textbook? That's terrible. [Here, pick one](http://stackoverflow.com/q/388242/10077). — Fred Larson, Mar 23 '17 at 15:02

score 0 · Answer 1 · answered Mar 23 '17 at 16:34

Ask yourself, if private member access disqualified the rest of the class from access, what would be the point of a private field? You declare a potentially complex object, and then literally can do nothing with it? You have a private region that can interact with itself, but which cannot have any effect on any public facing functionality? You use the private structures as middlemen to write to public fields that you then use? None of these scenarios makes that much sense. Access modifiers protect the class from external influences, but the OOP model assumes the programmer will take care of themselves within the class. It is noteworthy that there is one condition in which private members can not be accessed: Inheritance. The base class private variables are there in a derived class, but cannot be referenced directly. To be clear,these are the base class' private variables. The child class has its own private scope that it can declare within and access normally.

Why can this member function access a private vector

1 Answers1