Sorting a vector in C++ by accessing a private member

Question

I'm working on a project and I have to sort a vector, but I'm facing some difficulties.

class CService {
    private:
        string m_strSeller;
    public:
        // Other stuff.
};

class CAnalizeTime : public CService {
    private:
        void sortSellerVector () {
            vector<CService>m_vData;
            m_vData.push_back(m_strSeller);
            sort(m_vData.begin(), m_vData.end());
        }
};

I'm getting 2 errors on my void sortSellerVector() function:

1. Cannot access private member declared in class 'CService'
2. Cannot convert from 'class std::basic_string,class std::allocator >' to 'const class CService'

My questions are:

1. How do you access a private member from another class?
2. What exactly does the second error mean? I don't understand it.

Answer 1

To be able to sort a vector<CService>, the easiest way is to just give CService an operator< overload that std::sort can use to sort the elements. Of course, this operator< overload will have access to m_strSeller if it is a member function:

class CService {
  private:
    string m_strSeller;
  public:
    bool operator<(const CService& other) const
    {
      return m_strSeller < other.m_strSeller;
    }
    /////..... (other stuff)
};

Then you can just do:

vector<CService> vData;
// Push some CServices into the vector
sort(vData.begin(), vData.end());

An alternative method is to have a comparison function that you give to std::sort to use. You can make it a friend of CService so that it can access the private member:

class CService {
  private:
    string m_strSeller;
  public:
    friend bool compare(const CService&, const CService&);
    /////..... (other stuff)
};

bool compare(const CService& a, const CService& b)
{
  return a.m_strSeller < b.m_strSeller;
}

Which you then use like so:

vector<CService> vData;
// Push some CServices into the vector
sort(vData.begin(), vData.end(), compare);