it could be not good question and i know i need more time to learn about it
but i'm really wondering how to make it work
here is my code

#include <bits/stdc  .h>
using namespace std;
class Parent{
    protected:
        int value;
        int size;
    public:
        Parent();
        Parent(const Parent &p);
};
Parent::Parent()
{
    this->value = 0;
    this->size = 0;
}
Parent::Parent(const Parent &p)
{
    this->value = p.value;
    this->size = p.size;
}
class Child:public Parent{
    public:
        Child();
        Child(const Parent& p);
};
Child::Child()
{
    this->value = 0;
    this->size = 0;
}
Child::Child(const Parent& p)
{
    this->value = ??
    this->size = ?? 
}

as i want to use parent class as a parameter in an inherited child class,
the problem is i can't use p.size or p.value in child class.
is there any way to solve this problem?
thank u for read this.

CodePudding user response：

When you implement a class, you also have to use its constructors in each of your derived class constructors.

In your case: Child::Child(const Parent& p) : Parent(p) {}

CodePudding user response：

This Child(const Parent& p); is not a proper copy constructor. A copy constructor for a class T takes a &T (possibly with CV-qualifier) as argument. In this case it should be Child(const Child& p);.

Furthermore, if we look at https://en.cppreference.com/w/cpp/language/access, then we can see that:

A protected member of a class is only accessible

1. to the members and friends of that class;
2. to the members and friends (until C 17) of any derived class of that class, but only when the class of the object through which the protected member is accessed is that derived class or a derived class of that derived class

So a function in Child that takes a Child as argument has access to that other Childs protected members, but a function that takes a Parent does not have access to that Parents protected members.

For example, if you have a Button and TextBox that both inherit from UIWidget, then the Button can access UIWidget protected members in other Buttons, but not in TextBoxes.

Edit:

If you really want to have a constructor that takes a Parent as argument, then you can do as Roy Avidan suggests in his answer, and do this:

Child::Child(const Parent& p)
: Parent(p)  // <- Call the Parent copy constructor
{
  // No access to protected members of p here!
}

This works because it calls the copy constructor of Parent with a Parent argument, which means that any access to the protected members of Parent happens in Parent. Note that any access to a protected member of p in the body is an access violation of p.