如何从一个类中获取变量，然后将其放入另一个类中，然后将其放入构造函数中？

Question

I have 2 h and cpp files. I wanna know how to call variables from the Abe.h file or the Abe class to the Bob.h or the Bob class. Please help.

Abe.h

#include <iostream>
using namespace std;
#ifndef ABE
#define ABE

class Abe
{
    private:
    int num;

    public:
        Abe();
        Abe(int);
        void showNumber();

};
#endif // ABE

Abe.cpp

#include "Abe.h"
#include <iostream>

using namespace std;

Abe::Abe()
{
    num=45;
}

Abe::Abe(int n)
{
    num=n;
}

void Abe::showNumber()
{
    cout<<num;
}

BOB.h

#include "Abe.h"
#include <iostream>

using namespace std;
#ifndef BOB
#define BOB

class Bob
{
private:
    Abe a;

public:
    Bob(Abe);
    void showNum();
};

#endif // BOB

BOB.cpp

#include "Abe.h"
#include "Bob.h"
#include <iostream>

using namespace std;

Bob::Bob(Abe a1)
{
    a=a1;
  //^not sure what a=a1 is doing but if you could explain in simple terms or in deatil that would help.
}

void Bob::showNum()
{
    //how do I display it here??
}

so how do I get "num" from the Abe class and use it in the Bob class? please help. Thank You!

Answer 1

The assignment in the Bob constructor is just like any other assignment, it copies the data from a1 into a the same way like eg this code would:

int a1 = 6;

...

int a;
a = a1;

---

Once you have an initialized the a object in the Bob class, you can call the member functions of a like any other objects. However, you can't access private members.

One solution is to add a getter member function to the Abe class which returns the num member, something like

int Abe::getNumber() const
{
    return num;
};

Then you can use it in the Bob class like

std::cout << "the number is " << a.getNumber() << '\n';

Answer 2

a=a1 assigns the Abe passed into Bob 's constructor to Bob 's a member. Probably a good idea to use more descriptive variable names and there are some optimizations to be gained by using the initializer list. Example:

Bob::Bob(Abe a1):a(a1)
{
}

Now that Bob has an Abe , it can... Do nothing to get the number from Abe. Abe has to allow Bob to get the number. To do this redefine Abe in one of two ways:

class Abe
{
    private:
    int num;

    public:
        Abe();
        Abe(int);
        void showNumber();
        int getNumber()
        {
            return num;
        }
};

Now Bob can

int number = a.getNumber();

or (and this is second because it is the less preferred option because it tightly couples Bob and Abe .

class Abe
{
    friend class Bob;
    private:
    int num;

    public:
        Abe();
        Abe(int);
        void showNumber();
};

Bob now has total access to the internals of Abe and can really mess the poor guy up if he wants to. Which is not nice. Anyway, Bob can now

int number = a.num;

Bob can read num , change num , set it to hopelessly bad values and generally be a jerk to Abe . Good thing Bob is a friend. The other downside is Bob now needs to know how Abe works on the inside and if Abe 's code is changed, Bob will have to change as well.