Accessing variables from one class to another in C++ - c++

Question

I am trying to access a variable declared in class A from class B, without using static variables. I have the classes separated in header and source files.
I have seen different people using pass by reference (I assume "const &a" declared in the class definition) but it doesn't work for me.
Update:When I tried to pass in the A object to the B::print as a const-reference parameter I got an error. In my example, I am trying to access string a from the function void print declared in class B. The problem now is that I am getting an error in B.cpp.
main.cpp
#include <iostream>
#include <string>
#include "A.h"
#include "B.h"
using namespace std;
int main()
{
A first;
B second;
second.print(cout, first);
return 0;
}
A.h
#include <string>
using namespace std;
class A
{
string a = "abc";
public:
A();
void print(ostream& o) const;
~A();
};
A.cpp
#include <iostream>
#include <string>
#include "A.h"
#include "B.h"
using namespace std;
A::A()
{
}
A::~A()
{
}
void A::print(ostream& o) const
{
o << a;
}
ostream& operator<<(ostream& o, A const& a)
{
a.print(o);
return o;
}
B.h
#include <iostream>
#include <string>
#include "A.h"
using namespace std;
class B
{
public:
B();
void print(ostream&, A const&) const;
~B();
};
B.cpp
#include "B.h"
#include "A.h"
#include <iostream>
#include <string>
using namespace std;
B::B()
{
}
B::~B()
{
}
void B::print(ostream& o, A const& a) const
{
o << a << endl;
//^^ error no operator "<<" mathes these operands
}

Answer 1

Since a is not a static member, it can't be accessed without an instance of class A. You can, however, pass one in the function:
class B {
void print(const A &o) {
cout << o.a << endl;
}
};
In addition, if a member is private, you can declare class B as friend, which means it can access private and protected members of class A.
class A {
friend class B;
private:
std::string a = "abc";
};

Answer 2

The way I'd do it is to pass in the A object to the B::print as a const-reference parameter. I'd also pass in the ostream as a reference parameter. And I'd take advantage of C++'s streaming output operator (<<).
Like this:
#include <iostream>
#include <string>
using std::cout;
using std::endl;
using std::ostream;
using std::string;
class A
{
std::string s = "abc";
public:
void print(ostream& o) const;
};
void A::print(ostream& o) const
{
o << s;
}
ostream& operator<<(ostream& o, A const& a)
{
a.print(o);
return o;
}
class B
{
public:
void print(ostream&, A const&) const;
};
void B::print(ostream& o, A const& a) const
{
o << a << endl;
}
int main()
{
A first;
B second;
second.print(cout, first);
}
UPDATE: given the comments above, I'm not not sure if the problem is "How does one split up code into separate .h and .cpp files?" or if it is "How do I access A member variables from B, without using static variables in A?"
UPDATE: I changed A's member variable from a to s to disambiguate from other a identifiers.