How do I copy the contents of a map from one class into a new vector from another class

Question

In my C++ code, I have a class that has a map of pairs as a private member, and I need to copy those pairs into a new vector that will be held in a different class (also private). If these containers were both in main(), I could do it pretty easily using copy(), but copy() won't work on my class. What's the most straightforward approach given my constraints?

Answer 1

If you have access to the class definitions, you could create a special friend function that can be used to access the private members of each class and copy the private members from one class to another.

For instance:

class B; //forward declaration

class A
{
    private:
        //... private data members

    public:
        //... methods, etc.

        friend void copy(const A&, B&);
};

class B
{
    private:
        //... private data members

    public:
        //... methods, etc.

        friend void copy(const A&, B&);
};

void copy(const A& rhs, B& lhs)
{
    //... copy the private elements out of class A into class B
}

Of course then again, if you had access to the class definitions, there's a number of things you could do including adding a simple method to one of the classes, declaring one class the friend of the other, etc. If someone else created the code and declared those data-members private, then either they missed a use-case or you shouldn't be touching that data ...