什么是访问说明符?我应该以私有、受保护还是公共继承?
I am confused about the meaning of access modifiers with respect to inheritance. What is the difference between inheritance involving the private
, protected
and public
keywords?
what are Access Specifiers?
There are 3 access specifiers
for a class/struct/Union in C++. These access specifiers define how the members of the class can be accessed. Of course, any member of a class is accessible within that class(Inside any member function of that same class). Moving ahead to type of access specifiers, they are:
Public - The members declared as Public are accessible from outside the Class through an object of the class.
Protected - The members declared as Protected are accessible from outside the class BUT only in a class derived from it.
Private - These members are only accessible from within the class. No outside Access is allowed.
An Source Code Example:
class MyClass
{
public:
int a;
protected:
int b;
private:
int c;
};
int main()
{
MyClass obj;
obj.a = 10; //Allowed
obj.b = 20; //Not Allowed, gives compiler error
obj.c = 30; //Not Allowed, gives compiler error
}
Inheritance and Access Specifiers
Inheritance in C++ can be one of the following types:
Private
InheritancePublic
InheritanceProtected
inheritance
Here are the member access rules with respect to each of these:
First and most important rule
Private
members of a class are never accessible from anywhere except the members of the same class.
Public Inheritance:
All
Public
members of the Base Class becomePublic
Members of the derived class &
AllProtected
members of the Base Class becomeProtected
Members of the Derived Class.
i.e. No change in the Access of the members. The access rules we discussed before are further then applied to these members.
Code Example:
Class Base
{
public:
int a;
protected:
int b;
private:
int c;
};
class Derived:public Base
{
void doSomething()
{
a = 10; //Allowed
b = 20; //Allowed
c = 30; //Not Allowed, Compiler Error
}
};
int main()
{
Derived obj;
obj.a = 10; //Allowed
obj.b = 20; //Not Allowed, Compiler Error
obj.c = 30; //Not Allowed, Compiler Error
}
Private Inheritance:
All
Public
members of the Base Class becomePrivate
Members of the Derived class &
AllProtected
members of the Base Class becomePrivate
Members of the Derived Class.
An code Example:
Class Base
{
public:
int a;
protected:
int b;
private:
int c;
};
class Derived:private Base //Not mentioning private is OK because for classes it defaults to private
{
void doSomething()
{
a = 10; //Allowed
b = 20; //Allowed
c = 30; //Not Allowed, Compiler Error
}
};
class Derived2:public Derived
{
void doSomethingMore()
{
a = 10; //Not Allowed, Compiler Error, a is private member of Derived now
b = 20; //Not Allowed, Compiler Error, b is private member of Derived now
c = 30; //Not Allowed, Compiler Error
}
};
int main()
{
Derived obj;
obj.a = 10; //Not Allowed, Compiler Error
obj.b = 20; //Not Allowed, Compiler Error
obj.c = 30; //Not Allowed, Compiler Error
}
Protected Inheritance:
All
Public
members of the Base Class becomeProtected
Members of the derived class &
AllProtected
members of the Base Class becomeProtected
Members of the Derived Class.
A Code Example:
Class Base
{
public:
int a;
protected:
int b;
private:
int c;
};
class Derived:protected Base
{
void doSomething()
{
a = 10; //Allowed
b = 20; //Allowed
c = 30; //Not Allowed, Compiler Error
}
};
class Derived2:public Derived
{
void doSomethingMore()
{
a = 10; //Allowed, a is protected member inside Derived & Derived2 is public derivation from Derived, a is now protected member of Derived2
b = 20; //Allowed, b is protected member inside Derived & Derived2 is public derivation from Derived, b is now protected member of Derived2
c = 30; //Not Allowed, Compiler Error
}
};
int main()
{
Derived obj;
obj.a = 10; //Not Allowed, Compiler Error
obj.b = 20; //Not Allowed, Compiler Error
obj.c = 30; //Not Allowed, Compiler Error
}
Remember the same access rules apply to the classes and members down the inheritance hierarchy.
Important points to note:
- Access Specification is per-Class not per-Object
Note that the access specification C++ work on per-Class basis and not per-object basis.
A good example of this is that in a copy constructor or Copy Assignment operator function, all the members of the object being passed can be accessed.
- A Derived class can only access members of its own Base class
Consider the following code example:
class Myclass
{
protected:
int x;
};
class derived : public Myclass
{
public:
void f( Myclass& obj )
{
obj.x = 5;
}
};
int main()
{
return 0;
}
It gives an compilation error:
prog.cpp:4: error: ‘int Myclass::x’ is protected
Because the derived class can only access members of its own Base Class. Note that the object obj
being passed here is no way related to the derived
class function in which it is being accessed, it is an altogether different object and hence derived
member function cannot access its members.
What is a friend
? How does friend
affect access specification rules?
You can declare a function or class as friend
of another class. When you do so the access specification rules do not apply to the friend
ed class/function. The class or function can access all the members of that particular class.
So do
friend
s break Encapsulation?
No they don't, On the contrary they enhance Encapsulation!
friend
ship is used to indicate a intentional strong coupling between two entities.
If there exists a special relationship between two entities such that one needs access to others private
or protected
members but You do not want everyone to have access by using the public
access specifier then you should use friend
ship.
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