Related
Below is my base class:
Vehicle
{
protected: //members being inherited
public:
void getDetails();
};
This is my derived class:
class Car : protected Vehicle
{}
now I am trying to access the public function of base class through my derived class object. according to my understanding the public functions of base class are inherited by the derived class, so it should work.
Car obj2;
obj2.getDetails(); /*Public func of base class*/
But I am getting error like:
'Vehicle::getDetails' not accessible because 'Car' uses 'protected' to inherit from 'Vehicle'
Can someone explain this? what i am missing?
You're using the "protected" access specifier when Car inherits from Vehicle. This means that protected members are inherited as they are, but public members are inherited as protected too, because that's the highest accessibility level you're allowing. So the getDetails function is protected in Car, meaning you can't access it outside the class.
Using public instead should solve your problem:
class Car : public Vehicle
You might better understand this in therms of pseudo OOP in C.
class Car : protected Vehicle
{};
Is similar to
class Car
{
protected:
Vehicle vehicle; // hidden or unnamed member.
};
The call
obj2.getDetails();
Can be imaging like a call to the hidden member
obj2.vehicle.getDetails();
obj2.vehicle is not accessible because it is protected and the public getDetails is not accessible in the protected member.
You can replace protected: with public: or private:, and see how the access modifiers work.
What is the difference between public, private, and protected inheritance in C++?
All of the questions I've found on SO deal with specific cases.
class A
{
public:
int x;
protected:
int y;
private:
int z;
};
class B : public A
{
// x is public
// y is protected
// z is not accessible from B
};
class C : protected A
{
// x is protected
// y is protected
// z is not accessible from C
};
class D : private A // 'private' is default for classes
{
// x is private
// y is private
// z is not accessible from D
};
IMPORTANT NOTE: Classes B, C and D all contain the variables x, y and z. It is just question of access.
About usage of protected and private inheritance you could read here.
To answer that question, I'd like to describe member's accessors first in my own words. If you already know this, skip to the heading "next:".
There are three accessors that I'm aware of: public, protected and private.
Let:
class Base {
public:
int publicMember;
protected:
int protectedMember;
private:
int privateMember;
};
Everything that is aware of Base is also aware that Base contains publicMember.
Only the children (and their children) are aware that Base contains protectedMember.
No one but Base is aware of privateMember.
By "is aware of", I mean "acknowledge the existence of, and thus be able to access".
next:
The same happens with public, private and protected inheritance. Let's consider a class Base and a class Child that inherits from Base.
If the inheritance is public, everything that is aware of Base and Child is also aware that Child inherits from Base.
If the inheritance is protected, only Child, and its children, are aware that they inherit from Base.
If the inheritance is private, no one other than Child is aware of the inheritance.
Limiting the visibility of inheritance will make code not able to see that some class inherits another class: Implicit conversions from the derived to the base won't work, and static_cast from the base to the derived won't work either.
Only members/friends of a class can see private inheritance, and only members/friends and derived classes can see protected inheritance.
public inheritance
IS-A inheritance. A button is-a window, and anywhere where a window is needed, a button can be passed too.
class button : public window { };
protected inheritance
Protected implemented-in-terms-of. Rarely useful. Used in boost::compressed_pair to derive from empty classes and save memory using empty base class optimization (example below doesn't use template to keep being at the point):
struct empty_pair_impl : protected empty_class_1
{ non_empty_class_2 second; };
struct pair : private empty_pair_impl {
non_empty_class_2 &second() {
return this->second;
}
empty_class_1 &first() {
return *this; // notice we return *this!
}
};
private inheritance
Implemented-in-terms-of. The usage of the base class is only for implementing the derived class. Useful with traits and if size matters (empty traits that only contain functions will make use of the empty base class optimization). Often containment is the better solution, though. The size for strings is critical, so it's an often seen usage here
template<typename StorageModel>
struct string : private StorageModel {
public:
void realloc() {
// uses inherited function
StorageModel::realloc();
}
};
public member
Aggregate
class pair {
public:
First first;
Second second;
};
Accessors
class window {
public:
int getWidth() const;
};
protected member
Providing enhanced access for derived classes
class stack {
protected:
vector<element> c;
};
class window {
protected:
void registerClass(window_descriptor w);
};
private member
Keep implementation details
class window {
private:
int width;
};
Note that C-style casts purposely allows casting a derived class to a protected or private base class in a defined and safe manner and to cast into the other direction too. This should be avoided at all costs, because it can make code dependent on implementation details - but if necessary, you can make use of this technique.
These three keywords are also used in a completely different context to specify the visibility inheritance model.
This table gathers all of the possible combinations of the component declaration and inheritance model presenting the resulting access to the components when the subclass is completely defined.
The table above is interpreted in the following way (take a look at the first row):
if a component is declared as public and its class is inherited as public the resulting access is public.
An example:
class Super {
public: int p;
private: int q;
protected: int r;
};
class Sub : private Super {};
class Subsub : public Sub {};
The resulting access for variables p, q, r in class Subsub is none.
Another example:
class Super {
private: int x;
protected: int y;
public: int z;
};
class Sub : protected Super {};
The resulting access for variables y, z in class Sub is protected and for variable x is none.
A more detailed example:
class Super {
private:
int storage;
public:
void put(int val) { storage = val; }
int get(void) { return storage; }
};
int main(void) {
Super object;
object.put(100);
object.put(object.get());
cout << object.get() << endl;
return 0;
}
Now lets define a subclass:
class Sub : Super { };
int main(void) {
Sub object;
object.put(100);
object.put(object.get());
cout << object.get() << endl;
return 0;
}
The defined class named Sub which is a subclass of class named Super or that Sub class is derived from the Super class.
The Sub class introduces neither new variables nor new functions. Does it mean that any object of the Sub class inherits all the traits after the Super class being in fact a copy of a Super class’ objects?
No. It doesn’t.
If we compile the following code, we will get nothing but compilation errors saying that put and get methods are inaccessible. Why?
When we omit the visibility specifier, the compiler assumes that we are going to apply the so-called private inheritance. It means that all public superclass components turn into private access, private superclass components won't be accessible at all. It consequently means that you are not allowed to use the latter inside the subclass.
We have to inform the compiler that we want to preserve the previously used access policy.
class Sub : public Super { };
Don’t be misled: it doesn’t mean that private components of the Super
class (like the storage variable) will turn into public ones in a
somewhat magical way. Private components will remain private, public
will remain public.
Objects of the Sub class may do "almost" the same things as their older siblings created from the Super class. "Almost" because the fact of being a subclass also means that the class lost access to the private components of the superclass. We cannot write a member function of the Sub class which would be able to directly manipulate the storage variable.
This is a very serious restriction. Is there any workaround?
Yes.
The third access level is called protected. The keyword protected means that the component marked with it behaves like a public one when used by any of the subclasses and looks like a private one to the rest of the world. -- This is true only for the publicly inherited classes (like the Super class in our example) --
class Super {
protected:
int storage;
public:
void put(int val) { storage = val; }
int get(void) { return storage; }
};
class Sub : public Super {
public:
void print(void) {cout << "storage = " << storage;}
};
int main(void) {
Sub object;
object.put(100);
object.put(object.get() + 1);
object.print();
return 0;
}
As you see in the example code we a new functionality to the Sub class and it does one important thing: it accesses the storage variable from the Super class.
It wouldn’t be possible if the variable was declared as private.
In the main function scope the variable remains hidden anyway so if you write anything like:
object.storage = 0;
The compiler will inform you that it is an error: 'int Super::storage' is protected.
Finally, the last program will produce the following output:
storage = 101
It has to do with how the public members of the base class are exposed from the derived class.
public -> base class's public members will be public (usually the default)
protected -> base class's public members will be protected
private -> base class's public members will be private
As litb points out, public inheritance is traditional inheritance that you'll see in most programming languages. That is it models an "IS-A" relationship. Private inheritance, something AFAIK peculiar to C++, is an "IMPLEMENTED IN TERMS OF" relationship. That is you want to use the public interface in the derived class, but don't want the user of the derived class to have access to that interface. Many argue that in this case you should aggregate the base class, that is instead of having the base class as a private base, make in a member of derived in order to reuse base class's functionality.
Member in base class : Private Protected Public
Inheritance type : Object inherited as:
Private : Inaccessible Private Private
Protected : Inaccessible Protected Protected
Public : Inaccessible Protected Public
1) Public Inheritance:
a. Private members of Base class are not accessible in Derived class.
b. Protected members of Base class remain protected in Derived class.
c. Public members of Base class remain public in Derived class.
So, other classes can use public members of Base class through Derived class object.
2) Protected Inheritance:
a. Private members of Base class are not accessible in Derived class.
b. Protected members of Base class remain protected in Derived class.
c. Public members of Base class too become protected members of Derived class.
So, other classes can't use public members of Base class through Derived class object; but they are available to subclass of Derived.
3) Private Inheritance:
a. Private members of Base class are not accessible in Derived class.
b. Protected & public members of Base class become private members of Derived class.
So, no members of Base class can be accessed by other classes through Derived class object as they are private in Derived class. So, even subclass of Derived
class can't access them.
Public inheritance models an IS-A relationship. With
class B {};
class D : public B {};
every D is a B.
Private inheritance models an IS-IMPLEMENTED-USING relationship (or whatever that's called). With
class B {};
class D : private B {};
a D is not a B, but every D uses its B in its implementation. Private inheritance can always be eliminated by using containment instead:
class B {};
class D {
private:
B b_;
};
This D, too, can be implemented using B, in this case using its b_. Containment is a less tight coupling between types than inheritance, so in general it should be preferred. Sometimes using containment instead of private inheritance is not as convenient as private inheritance. Often that's a lame excuse for being lazy.
I don't think anyone knows what protected inheritance models. At least I haven't seen any convincing explanation yet.
If you inherit publicly from another class, everybody knows you are inheriting and you can be used polymorphically by anyone through a base class pointer.
If you inherit protectedly only your children classes will be able to use you polymorphically.
If you inherit privately only yourself will be able to execute parent class methods.
Which basically symbolizes the knowledge the rest of the classes have about your relationship with your parent class
Accessors | Base Class | Derived Class | World
—————————————+————————————+———————————————+———————
public | y | y | y
—————————————+————————————+———————————————+———————
protected | y | y | n
—————————————+————————————+———————————————+———————
private | | |
or | y | n | n
no accessor | | |
y: accessible
n: not accessible
Based on this example for java... I think a little table worth a thousand words :)
Protected data members can be accessed by any classes that inherit from your class. Private data members, however, cannot. Let's say we have the following:
class MyClass {
private:
int myPrivateMember; // lol
protected:
int myProtectedMember;
};
From within your extension to this class, referencing this.myPrivateMember won't work. However, this.myProtectedMember will. The value is still encapsulated, so if we have an instantiation of this class called myObj, then myObj.myProtectedMember won't work, so it is similar in function to a private data member.
Summary:
Private: no one can see it except for within the class
Protected: Private + derived classes can see it
Public: the world can see it
When inheriting, you can (in some languages) change the protection type of a data member in certain direction, e.g. from protected to public.
Private:
The private members of a base class can only be accessed by members of that base class .
Public:
The public members of a base class can be accessed by members of that base class, members of its derived class as well as the members which are outside the base class and derived class.
Protected:
The protected members of a base class can be accessed by members of base class as well as members of its derived class.
In short:
private: base
protected: base + derived
public: base + derived + any other member
I have tried explaining inheritance using a picture below.
The main gist is that the private members of parent class are never directly accessible from derived/child class but you can use parent class's member function to access the private members of parent class.
Private variables are always present in derived class but it cannot be accessed by derived class. Its like its their but you cannot see with your own eyes but if you ask someone form the parent class then he can describe it to you.
I found an easy answer and so thought of posting it for my future reference too.
Its from the links http://www.learncpp.com/cpp-tutorial/115-inheritance-and-access-specifiers/
class Base
{
public:
int m_nPublic; // can be accessed by anybody
private:
int m_nPrivate; // can only be accessed by Base member functions (but not derived classes)
protected:
int m_nProtected; // can be accessed by Base member functions, or derived classes.
};
class Derived: public Base
{
public:
Derived()
{
// Derived's access to Base members is not influenced by the type of inheritance used,
// so the following is always true:
m_nPublic = 1; // allowed: can access public base members from derived class
m_nPrivate = 2; // not allowed: can not access private base members from derived class
m_nProtected = 3; // allowed: can access protected base members from derived class
}
};
int main()
{
Base cBase;
cBase.m_nPublic = 1; // allowed: can access public members from outside class
cBase.m_nPrivate = 2; // not allowed: can not access private members from outside class
cBase.m_nProtected = 3; // not allowed: can not access protected members from outside class
}
It's essentially the access protection of the public and protected members of the base class in the derived class. With public inheritance, the derived class can see public and protected members of the base. With private inheritance, it can't. With protected, the derived class and any classes derived from that can see them.
What is the difference between public, private, and protected inheritance in C++?
All of the questions I've found on SO deal with specific cases.
class A
{
public:
int x;
protected:
int y;
private:
int z;
};
class B : public A
{
// x is public
// y is protected
// z is not accessible from B
};
class C : protected A
{
// x is protected
// y is protected
// z is not accessible from C
};
class D : private A // 'private' is default for classes
{
// x is private
// y is private
// z is not accessible from D
};
IMPORTANT NOTE: Classes B, C and D all contain the variables x, y and z. It is just question of access.
About usage of protected and private inheritance you could read here.
To answer that question, I'd like to describe member's accessors first in my own words. If you already know this, skip to the heading "next:".
There are three accessors that I'm aware of: public, protected and private.
Let:
class Base {
public:
int publicMember;
protected:
int protectedMember;
private:
int privateMember;
};
Everything that is aware of Base is also aware that Base contains publicMember.
Only the children (and their children) are aware that Base contains protectedMember.
No one but Base is aware of privateMember.
By "is aware of", I mean "acknowledge the existence of, and thus be able to access".
next:
The same happens with public, private and protected inheritance. Let's consider a class Base and a class Child that inherits from Base.
If the inheritance is public, everything that is aware of Base and Child is also aware that Child inherits from Base.
If the inheritance is protected, only Child, and its children, are aware that they inherit from Base.
If the inheritance is private, no one other than Child is aware of the inheritance.
Limiting the visibility of inheritance will make code not able to see that some class inherits another class: Implicit conversions from the derived to the base won't work, and static_cast from the base to the derived won't work either.
Only members/friends of a class can see private inheritance, and only members/friends and derived classes can see protected inheritance.
public inheritance
IS-A inheritance. A button is-a window, and anywhere where a window is needed, a button can be passed too.
class button : public window { };
protected inheritance
Protected implemented-in-terms-of. Rarely useful. Used in boost::compressed_pair to derive from empty classes and save memory using empty base class optimization (example below doesn't use template to keep being at the point):
struct empty_pair_impl : protected empty_class_1
{ non_empty_class_2 second; };
struct pair : private empty_pair_impl {
non_empty_class_2 &second() {
return this->second;
}
empty_class_1 &first() {
return *this; // notice we return *this!
}
};
private inheritance
Implemented-in-terms-of. The usage of the base class is only for implementing the derived class. Useful with traits and if size matters (empty traits that only contain functions will make use of the empty base class optimization). Often containment is the better solution, though. The size for strings is critical, so it's an often seen usage here
template<typename StorageModel>
struct string : private StorageModel {
public:
void realloc() {
// uses inherited function
StorageModel::realloc();
}
};
public member
Aggregate
class pair {
public:
First first;
Second second;
};
Accessors
class window {
public:
int getWidth() const;
};
protected member
Providing enhanced access for derived classes
class stack {
protected:
vector<element> c;
};
class window {
protected:
void registerClass(window_descriptor w);
};
private member
Keep implementation details
class window {
private:
int width;
};
Note that C-style casts purposely allows casting a derived class to a protected or private base class in a defined and safe manner and to cast into the other direction too. This should be avoided at all costs, because it can make code dependent on implementation details - but if necessary, you can make use of this technique.
These three keywords are also used in a completely different context to specify the visibility inheritance model.
This table gathers all of the possible combinations of the component declaration and inheritance model presenting the resulting access to the components when the subclass is completely defined.
The table above is interpreted in the following way (take a look at the first row):
if a component is declared as public and its class is inherited as public the resulting access is public.
An example:
class Super {
public: int p;
private: int q;
protected: int r;
};
class Sub : private Super {};
class Subsub : public Sub {};
The resulting access for variables p, q, r in class Subsub is none.
Another example:
class Super {
private: int x;
protected: int y;
public: int z;
};
class Sub : protected Super {};
The resulting access for variables y, z in class Sub is protected and for variable x is none.
A more detailed example:
class Super {
private:
int storage;
public:
void put(int val) { storage = val; }
int get(void) { return storage; }
};
int main(void) {
Super object;
object.put(100);
object.put(object.get());
cout << object.get() << endl;
return 0;
}
Now lets define a subclass:
class Sub : Super { };
int main(void) {
Sub object;
object.put(100);
object.put(object.get());
cout << object.get() << endl;
return 0;
}
The defined class named Sub which is a subclass of class named Super or that Sub class is derived from the Super class.
The Sub class introduces neither new variables nor new functions. Does it mean that any object of the Sub class inherits all the traits after the Super class being in fact a copy of a Super class’ objects?
No. It doesn’t.
If we compile the following code, we will get nothing but compilation errors saying that put and get methods are inaccessible. Why?
When we omit the visibility specifier, the compiler assumes that we are going to apply the so-called private inheritance. It means that all public superclass components turn into private access, private superclass components won't be accessible at all. It consequently means that you are not allowed to use the latter inside the subclass.
We have to inform the compiler that we want to preserve the previously used access policy.
class Sub : public Super { };
Don’t be misled: it doesn’t mean that private components of the Super
class (like the storage variable) will turn into public ones in a
somewhat magical way. Private components will remain private, public
will remain public.
Objects of the Sub class may do "almost" the same things as their older siblings created from the Super class. "Almost" because the fact of being a subclass also means that the class lost access to the private components of the superclass. We cannot write a member function of the Sub class which would be able to directly manipulate the storage variable.
This is a very serious restriction. Is there any workaround?
Yes.
The third access level is called protected. The keyword protected means that the component marked with it behaves like a public one when used by any of the subclasses and looks like a private one to the rest of the world. -- This is true only for the publicly inherited classes (like the Super class in our example) --
class Super {
protected:
int storage;
public:
void put(int val) { storage = val; }
int get(void) { return storage; }
};
class Sub : public Super {
public:
void print(void) {cout << "storage = " << storage;}
};
int main(void) {
Sub object;
object.put(100);
object.put(object.get() + 1);
object.print();
return 0;
}
As you see in the example code we a new functionality to the Sub class and it does one important thing: it accesses the storage variable from the Super class.
It wouldn’t be possible if the variable was declared as private.
In the main function scope the variable remains hidden anyway so if you write anything like:
object.storage = 0;
The compiler will inform you that it is an error: 'int Super::storage' is protected.
Finally, the last program will produce the following output:
storage = 101
It has to do with how the public members of the base class are exposed from the derived class.
public -> base class's public members will be public (usually the default)
protected -> base class's public members will be protected
private -> base class's public members will be private
As litb points out, public inheritance is traditional inheritance that you'll see in most programming languages. That is it models an "IS-A" relationship. Private inheritance, something AFAIK peculiar to C++, is an "IMPLEMENTED IN TERMS OF" relationship. That is you want to use the public interface in the derived class, but don't want the user of the derived class to have access to that interface. Many argue that in this case you should aggregate the base class, that is instead of having the base class as a private base, make in a member of derived in order to reuse base class's functionality.
Member in base class : Private Protected Public
Inheritance type : Object inherited as:
Private : Inaccessible Private Private
Protected : Inaccessible Protected Protected
Public : Inaccessible Protected Public
1) Public Inheritance:
a. Private members of Base class are not accessible in Derived class.
b. Protected members of Base class remain protected in Derived class.
c. Public members of Base class remain public in Derived class.
So, other classes can use public members of Base class through Derived class object.
2) Protected Inheritance:
a. Private members of Base class are not accessible in Derived class.
b. Protected members of Base class remain protected in Derived class.
c. Public members of Base class too become protected members of Derived class.
So, other classes can't use public members of Base class through Derived class object; but they are available to subclass of Derived.
3) Private Inheritance:
a. Private members of Base class are not accessible in Derived class.
b. Protected & public members of Base class become private members of Derived class.
So, no members of Base class can be accessed by other classes through Derived class object as they are private in Derived class. So, even subclass of Derived
class can't access them.
Public inheritance models an IS-A relationship. With
class B {};
class D : public B {};
every D is a B.
Private inheritance models an IS-IMPLEMENTED-USING relationship (or whatever that's called). With
class B {};
class D : private B {};
a D is not a B, but every D uses its B in its implementation. Private inheritance can always be eliminated by using containment instead:
class B {};
class D {
private:
B b_;
};
This D, too, can be implemented using B, in this case using its b_. Containment is a less tight coupling between types than inheritance, so in general it should be preferred. Sometimes using containment instead of private inheritance is not as convenient as private inheritance. Often that's a lame excuse for being lazy.
I don't think anyone knows what protected inheritance models. At least I haven't seen any convincing explanation yet.
If you inherit publicly from another class, everybody knows you are inheriting and you can be used polymorphically by anyone through a base class pointer.
If you inherit protectedly only your children classes will be able to use you polymorphically.
If you inherit privately only yourself will be able to execute parent class methods.
Which basically symbolizes the knowledge the rest of the classes have about your relationship with your parent class
Accessors | Base Class | Derived Class | World
—————————————+————————————+———————————————+———————
public | y | y | y
—————————————+————————————+———————————————+———————
protected | y | y | n
—————————————+————————————+———————————————+———————
private | | |
or | y | n | n
no accessor | | |
y: accessible
n: not accessible
Based on this example for java... I think a little table worth a thousand words :)
Protected data members can be accessed by any classes that inherit from your class. Private data members, however, cannot. Let's say we have the following:
class MyClass {
private:
int myPrivateMember; // lol
protected:
int myProtectedMember;
};
From within your extension to this class, referencing this.myPrivateMember won't work. However, this.myProtectedMember will. The value is still encapsulated, so if we have an instantiation of this class called myObj, then myObj.myProtectedMember won't work, so it is similar in function to a private data member.
Summary:
Private: no one can see it except for within the class
Protected: Private + derived classes can see it
Public: the world can see it
When inheriting, you can (in some languages) change the protection type of a data member in certain direction, e.g. from protected to public.
Private:
The private members of a base class can only be accessed by members of that base class .
Public:
The public members of a base class can be accessed by members of that base class, members of its derived class as well as the members which are outside the base class and derived class.
Protected:
The protected members of a base class can be accessed by members of base class as well as members of its derived class.
In short:
private: base
protected: base + derived
public: base + derived + any other member
I have tried explaining inheritance using a picture below.
The main gist is that the private members of parent class are never directly accessible from derived/child class but you can use parent class's member function to access the private members of parent class.
Private variables are always present in derived class but it cannot be accessed by derived class. Its like its their but you cannot see with your own eyes but if you ask someone form the parent class then he can describe it to you.
I found an easy answer and so thought of posting it for my future reference too.
Its from the links http://www.learncpp.com/cpp-tutorial/115-inheritance-and-access-specifiers/
class Base
{
public:
int m_nPublic; // can be accessed by anybody
private:
int m_nPrivate; // can only be accessed by Base member functions (but not derived classes)
protected:
int m_nProtected; // can be accessed by Base member functions, or derived classes.
};
class Derived: public Base
{
public:
Derived()
{
// Derived's access to Base members is not influenced by the type of inheritance used,
// so the following is always true:
m_nPublic = 1; // allowed: can access public base members from derived class
m_nPrivate = 2; // not allowed: can not access private base members from derived class
m_nProtected = 3; // allowed: can access protected base members from derived class
}
};
int main()
{
Base cBase;
cBase.m_nPublic = 1; // allowed: can access public members from outside class
cBase.m_nPrivate = 2; // not allowed: can not access private members from outside class
cBase.m_nProtected = 3; // not allowed: can not access protected members from outside class
}
It's essentially the access protection of the public and protected members of the base class in the derived class. With public inheritance, the derived class can see public and protected members of the base. With private inheritance, it can't. With protected, the derived class and any classes derived from that can see them.
What is the difference between public, private, and protected inheritance in C++?
All of the questions I've found on SO deal with specific cases.
class A
{
public:
int x;
protected:
int y;
private:
int z;
};
class B : public A
{
// x is public
// y is protected
// z is not accessible from B
};
class C : protected A
{
// x is protected
// y is protected
// z is not accessible from C
};
class D : private A // 'private' is default for classes
{
// x is private
// y is private
// z is not accessible from D
};
IMPORTANT NOTE: Classes B, C and D all contain the variables x, y and z. It is just question of access.
About usage of protected and private inheritance you could read here.
To answer that question, I'd like to describe member's accessors first in my own words. If you already know this, skip to the heading "next:".
There are three accessors that I'm aware of: public, protected and private.
Let:
class Base {
public:
int publicMember;
protected:
int protectedMember;
private:
int privateMember;
};
Everything that is aware of Base is also aware that Base contains publicMember.
Only the children (and their children) are aware that Base contains protectedMember.
No one but Base is aware of privateMember.
By "is aware of", I mean "acknowledge the existence of, and thus be able to access".
next:
The same happens with public, private and protected inheritance. Let's consider a class Base and a class Child that inherits from Base.
If the inheritance is public, everything that is aware of Base and Child is also aware that Child inherits from Base.
If the inheritance is protected, only Child, and its children, are aware that they inherit from Base.
If the inheritance is private, no one other than Child is aware of the inheritance.
Limiting the visibility of inheritance will make code not able to see that some class inherits another class: Implicit conversions from the derived to the base won't work, and static_cast from the base to the derived won't work either.
Only members/friends of a class can see private inheritance, and only members/friends and derived classes can see protected inheritance.
public inheritance
IS-A inheritance. A button is-a window, and anywhere where a window is needed, a button can be passed too.
class button : public window { };
protected inheritance
Protected implemented-in-terms-of. Rarely useful. Used in boost::compressed_pair to derive from empty classes and save memory using empty base class optimization (example below doesn't use template to keep being at the point):
struct empty_pair_impl : protected empty_class_1
{ non_empty_class_2 second; };
struct pair : private empty_pair_impl {
non_empty_class_2 &second() {
return this->second;
}
empty_class_1 &first() {
return *this; // notice we return *this!
}
};
private inheritance
Implemented-in-terms-of. The usage of the base class is only for implementing the derived class. Useful with traits and if size matters (empty traits that only contain functions will make use of the empty base class optimization). Often containment is the better solution, though. The size for strings is critical, so it's an often seen usage here
template<typename StorageModel>
struct string : private StorageModel {
public:
void realloc() {
// uses inherited function
StorageModel::realloc();
}
};
public member
Aggregate
class pair {
public:
First first;
Second second;
};
Accessors
class window {
public:
int getWidth() const;
};
protected member
Providing enhanced access for derived classes
class stack {
protected:
vector<element> c;
};
class window {
protected:
void registerClass(window_descriptor w);
};
private member
Keep implementation details
class window {
private:
int width;
};
Note that C-style casts purposely allows casting a derived class to a protected or private base class in a defined and safe manner and to cast into the other direction too. This should be avoided at all costs, because it can make code dependent on implementation details - but if necessary, you can make use of this technique.
These three keywords are also used in a completely different context to specify the visibility inheritance model.
This table gathers all of the possible combinations of the component declaration and inheritance model presenting the resulting access to the components when the subclass is completely defined.
The table above is interpreted in the following way (take a look at the first row):
if a component is declared as public and its class is inherited as public the resulting access is public.
An example:
class Super {
public: int p;
private: int q;
protected: int r;
};
class Sub : private Super {};
class Subsub : public Sub {};
The resulting access for variables p, q, r in class Subsub is none.
Another example:
class Super {
private: int x;
protected: int y;
public: int z;
};
class Sub : protected Super {};
The resulting access for variables y, z in class Sub is protected and for variable x is none.
A more detailed example:
class Super {
private:
int storage;
public:
void put(int val) { storage = val; }
int get(void) { return storage; }
};
int main(void) {
Super object;
object.put(100);
object.put(object.get());
cout << object.get() << endl;
return 0;
}
Now lets define a subclass:
class Sub : Super { };
int main(void) {
Sub object;
object.put(100);
object.put(object.get());
cout << object.get() << endl;
return 0;
}
The defined class named Sub which is a subclass of class named Super or that Sub class is derived from the Super class.
The Sub class introduces neither new variables nor new functions. Does it mean that any object of the Sub class inherits all the traits after the Super class being in fact a copy of a Super class’ objects?
No. It doesn’t.
If we compile the following code, we will get nothing but compilation errors saying that put and get methods are inaccessible. Why?
When we omit the visibility specifier, the compiler assumes that we are going to apply the so-called private inheritance. It means that all public superclass components turn into private access, private superclass components won't be accessible at all. It consequently means that you are not allowed to use the latter inside the subclass.
We have to inform the compiler that we want to preserve the previously used access policy.
class Sub : public Super { };
Don’t be misled: it doesn’t mean that private components of the Super
class (like the storage variable) will turn into public ones in a
somewhat magical way. Private components will remain private, public
will remain public.
Objects of the Sub class may do "almost" the same things as their older siblings created from the Super class. "Almost" because the fact of being a subclass also means that the class lost access to the private components of the superclass. We cannot write a member function of the Sub class which would be able to directly manipulate the storage variable.
This is a very serious restriction. Is there any workaround?
Yes.
The third access level is called protected. The keyword protected means that the component marked with it behaves like a public one when used by any of the subclasses and looks like a private one to the rest of the world. -- This is true only for the publicly inherited classes (like the Super class in our example) --
class Super {
protected:
int storage;
public:
void put(int val) { storage = val; }
int get(void) { return storage; }
};
class Sub : public Super {
public:
void print(void) {cout << "storage = " << storage;}
};
int main(void) {
Sub object;
object.put(100);
object.put(object.get() + 1);
object.print();
return 0;
}
As you see in the example code we a new functionality to the Sub class and it does one important thing: it accesses the storage variable from the Super class.
It wouldn’t be possible if the variable was declared as private.
In the main function scope the variable remains hidden anyway so if you write anything like:
object.storage = 0;
The compiler will inform you that it is an error: 'int Super::storage' is protected.
Finally, the last program will produce the following output:
storage = 101
It has to do with how the public members of the base class are exposed from the derived class.
public -> base class's public members will be public (usually the default)
protected -> base class's public members will be protected
private -> base class's public members will be private
As litb points out, public inheritance is traditional inheritance that you'll see in most programming languages. That is it models an "IS-A" relationship. Private inheritance, something AFAIK peculiar to C++, is an "IMPLEMENTED IN TERMS OF" relationship. That is you want to use the public interface in the derived class, but don't want the user of the derived class to have access to that interface. Many argue that in this case you should aggregate the base class, that is instead of having the base class as a private base, make in a member of derived in order to reuse base class's functionality.
Member in base class : Private Protected Public
Inheritance type : Object inherited as:
Private : Inaccessible Private Private
Protected : Inaccessible Protected Protected
Public : Inaccessible Protected Public
1) Public Inheritance:
a. Private members of Base class are not accessible in Derived class.
b. Protected members of Base class remain protected in Derived class.
c. Public members of Base class remain public in Derived class.
So, other classes can use public members of Base class through Derived class object.
2) Protected Inheritance:
a. Private members of Base class are not accessible in Derived class.
b. Protected members of Base class remain protected in Derived class.
c. Public members of Base class too become protected members of Derived class.
So, other classes can't use public members of Base class through Derived class object; but they are available to subclass of Derived.
3) Private Inheritance:
a. Private members of Base class are not accessible in Derived class.
b. Protected & public members of Base class become private members of Derived class.
So, no members of Base class can be accessed by other classes through Derived class object as they are private in Derived class. So, even subclass of Derived
class can't access them.
Public inheritance models an IS-A relationship. With
class B {};
class D : public B {};
every D is a B.
Private inheritance models an IS-IMPLEMENTED-USING relationship (or whatever that's called). With
class B {};
class D : private B {};
a D is not a B, but every D uses its B in its implementation. Private inheritance can always be eliminated by using containment instead:
class B {};
class D {
private:
B b_;
};
This D, too, can be implemented using B, in this case using its b_. Containment is a less tight coupling between types than inheritance, so in general it should be preferred. Sometimes using containment instead of private inheritance is not as convenient as private inheritance. Often that's a lame excuse for being lazy.
I don't think anyone knows what protected inheritance models. At least I haven't seen any convincing explanation yet.
If you inherit publicly from another class, everybody knows you are inheriting and you can be used polymorphically by anyone through a base class pointer.
If you inherit protectedly only your children classes will be able to use you polymorphically.
If you inherit privately only yourself will be able to execute parent class methods.
Which basically symbolizes the knowledge the rest of the classes have about your relationship with your parent class
Accessors | Base Class | Derived Class | World
—————————————+————————————+———————————————+———————
public | y | y | y
—————————————+————————————+———————————————+———————
protected | y | y | n
—————————————+————————————+———————————————+———————
private | | |
or | y | n | n
no accessor | | |
y: accessible
n: not accessible
Based on this example for java... I think a little table worth a thousand words :)
Protected data members can be accessed by any classes that inherit from your class. Private data members, however, cannot. Let's say we have the following:
class MyClass {
private:
int myPrivateMember; // lol
protected:
int myProtectedMember;
};
From within your extension to this class, referencing this.myPrivateMember won't work. However, this.myProtectedMember will. The value is still encapsulated, so if we have an instantiation of this class called myObj, then myObj.myProtectedMember won't work, so it is similar in function to a private data member.
Summary:
Private: no one can see it except for within the class
Protected: Private + derived classes can see it
Public: the world can see it
When inheriting, you can (in some languages) change the protection type of a data member in certain direction, e.g. from protected to public.
Private:
The private members of a base class can only be accessed by members of that base class .
Public:
The public members of a base class can be accessed by members of that base class, members of its derived class as well as the members which are outside the base class and derived class.
Protected:
The protected members of a base class can be accessed by members of base class as well as members of its derived class.
In short:
private: base
protected: base + derived
public: base + derived + any other member
I have tried explaining inheritance using a picture below.
The main gist is that the private members of parent class are never directly accessible from derived/child class but you can use parent class's member function to access the private members of parent class.
Private variables are always present in derived class but it cannot be accessed by derived class. Its like its their but you cannot see with your own eyes but if you ask someone form the parent class then he can describe it to you.
I found an easy answer and so thought of posting it for my future reference too.
Its from the links http://www.learncpp.com/cpp-tutorial/115-inheritance-and-access-specifiers/
class Base
{
public:
int m_nPublic; // can be accessed by anybody
private:
int m_nPrivate; // can only be accessed by Base member functions (but not derived classes)
protected:
int m_nProtected; // can be accessed by Base member functions, or derived classes.
};
class Derived: public Base
{
public:
Derived()
{
// Derived's access to Base members is not influenced by the type of inheritance used,
// so the following is always true:
m_nPublic = 1; // allowed: can access public base members from derived class
m_nPrivate = 2; // not allowed: can not access private base members from derived class
m_nProtected = 3; // allowed: can access protected base members from derived class
}
};
int main()
{
Base cBase;
cBase.m_nPublic = 1; // allowed: can access public members from outside class
cBase.m_nPrivate = 2; // not allowed: can not access private members from outside class
cBase.m_nProtected = 3; // not allowed: can not access protected members from outside class
}
It's essentially the access protection of the public and protected members of the base class in the derived class. With public inheritance, the derived class can see public and protected members of the base. With private inheritance, it can't. With protected, the derived class and any classes derived from that can see them.
I have a class, it has several public methods that use one private field that they all changed and use this field in their computations.
I need subclass from this class with few new methods, new methods won't use this field. I expect those methods from the base class behave the same as they did, but a little bit confused that they use private field.
Are private field accessible in the subclass?
Should I also declare a field with same type and name in my subclass?
Private member data is not accessible to derived classes. If you want to access this data, change it to protected. Public member functions of the base class that are used to change the private member will still be available to your derived class but direct access of base class private data is not allowed. All public and protected fields of the base class will be available to your derived class (assuming you are using public inheritance)
Private fields are never accessible to derived classes. If you declared your methods which access the private field as public, the derived classes can use them.
Most posters seem to misunderstood your question.
YES, you can use those public methods. That is the whole point of private members - they can be accessed by any member of the same class be they public, protected or private
You could use the public functions (which use a private field) in you drived class.
You should not declare a field with same type!!
But you could not direct access a private field from the base class in the drived class. Use for this a public function from your base class (e.g. setter and getter functions)
Sample:
//***********************************
//Base Class
//***********************************
class Base
{
public:
Base();
~Base();
void increment();
void decrement();
void get();
private:
int id_;
};
Base::Base()
: id_(0)
{
}
Base::~Base()
{
}
void Base::increment()
{
++id_;
}
void Base::decrement()
{
--id_;
}
int Base::get()
{
return id_;
}
void Base::set(id)
{
id_ = id;
}
//***********************************
// Drived Class
//***********************************
class Drived() : public Base
{
public:
Drived();
~Drived();
void plus(int a);
void minus(int b);
};
Drived::Drived()
: Base()
{
}
Drived::~Drived()
{
}
void Drived::plus(int a)
{
int i = get();
set(i + a); // <- OK
}
void Drived::minus(int b)
{
id_ -= b; // <- Not allowd; id is private
}
In the derived class members, you cannot access the private field. However, the base class members, which you have inherited through the virtue of them being public, can of course still access it. The base member functions will be visible through the derived class if you used public inheritance, but only to the derived class if you used protected or private inheritance. protected and private inheritance is rarely used, though default if you are using the class keyword.
For further clarification, if you did want to access the field, you should define accessors in the base class with protected access. This would allow you to control the access to the private field.
However, you should ask yourself whether polymorphism is the correct solution here? Does the derived class pass the is-a test? Is the base class's destructor virtual?