在C ++中将变量从一个类访问到另一个

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;
}

Ah

#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;
}

Bh

#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

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.

Answer 2

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";
};