A private variable can be accessed from another object of the same type? [duplicate]

Answer 1

Any access outside of the class is "prohibited". Unless you for example use the friend class. (Which let's any other class access to the private members of the class)

You can define friend-class like this:

class MyClass {
    friend class MyClassB;
public:
    MyClass();
    ~MyClass();
private:
    int number;
};

Then MyClassB will have access to the private variables of "MyClass".

If you do something like this:

class MyClass {
public:
    MyClass();
    ~MyClass();
private:
    int number;
};


int main(int argc, char *argv[])
{
    MyClass A;
    A.number = 11; // You can't do this
    if(A.number > 10) { // Either you can't do this.
        qDebug() << "It's more than 10"; // Qt. well.
    }


    return 0;
}

You'll get an "error" because you're trying to access A.number from outside of the class.

But if you want to access from some function inside the class:

class MyClass {
public:
    myClass() {
       number = 10;
       if(number > 10) {
           qDebug() << "It's more than 10"; // Qt. well.
       }
   }
    ~MyClass();
private:
    int number;
};

Answer 2

When a variable is designated private, it means that only methods of this class and optionally classes and methods designated as friends can access it. Any instance of the class can access private variables of all other instances of the same class.

Answer 3

Any snippet of code that "belongs" to the type Person will be able to access public, protected and private variables of any Person object.

The type matters here, not the instance.

Answer 4

if(this->age < p.age) return false;

p.age: age is private, and can access like this because this line is inside the method of class, so it can access all private member.

if you put p.age outside method of class, it will notice error. For example:

int main() 
{ 
    Person p("Hello, world!", 23);
    // will be error, because main is not inside class People
    if (p.age < 18) {   
       cout << "You are not adult" << endl;
    }
    return 0; 
} 

Hope this help :)

Answer 5

Methods of a class can access its private attributes throughout all class instances, at least in C++.