c++ 中没有 getter setter 的访问成员

Question

I am clusmy at using English. Hope you can uderstand.

How can we access private member wihtout getter? I can't understand it.

When we learned about C++ private, public, protected, private member or function can be accessed at class scope.

class TestClass {
public:
   TestClass() {
     testMem = 10;
   }

   void testFunction() {
     cout << testMem << endl;
   }
private:
   int testMem;
}

but, i'm really confused when i saw this code.

class B {
public:
    B() {
          testNumber = 10;
        }

        void printMember() {
          cout << testNumber << endl;
        }

private:
    int testNumber;
}

class Money {
public:
    Money();
        bool operator < (Money& amount, B& b) const {
           // this code doesn't make any error
           // why same type member can access their private member directly without getter and setter?
           amount.cents = 10;
           amount.dollors = 10;

           // this Code make a error
           // b.testNumber = 10;

           // below
           // compare code
        } 

private:
    int dollars;
    int cents;
};

i'm really confused.. why Money& amount parameter can access their private member without getter and setter? But B class can't access their private member without getter. how..? why..?

Answer 1

Line 25: b.testNumber = 10;

This code generates an error because the Money class is trying to directly access the private data member of a different class.

Private data members can only be accessed by the class itself. You could fix this issue by making a function in B to change the data member and then call that function to change it.

An example:

class B {
public:
    B() {
          testNumber = 10;
        }

        void changeMember(int newData)
        {
            testNumber = newData;
        }

        void printMember() {
          cout << testNumber << endl;
        }

private:
    int testNumber;
}

class Money {
    friend class B;
public:
    Money();
        bool operator < (Money& amount, B& b) const {
           // this code doesn't make any error
           amount.cents = 10;
           amount.dollars = 10;

           // this Code make a error
           b.changeMember(10);

           // below
           // compare code
        } 

private:
    int dollars;
    int cents;
};