Related
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.
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.
Suppose I want to have an inheritance hierarchy like this.
class Base
class DerivedOne : public Base
class DerivedTwo : public Base
The base class is not meant to be instantiated, and thus has some pure virtual functions that the derived classes must define, making it an abstract base class.
However, there are some functions that you would like your derived classes to get from your base class. These functions modify private data members that both DerivedOne and DerivedTwo will have.
class Base {
public:
virtual void MustDefine() =0; // Function that derived classes must define
void UseThis(); // Function that derived classes are meant to use
};
However, the UseThis() function is meant to modify private data members. That's where the question comes in. Should I give the Base class dummy private data members? Should I give it protected data members (and thus the derived classes won't declare their own private data members). I know the second approach will decrease encapsulation.
What is the best approach to a situation like this? If a more detailed explanation is needed I'd be happy to provide it.
If those member variables are supposed to exist in all derived classes then you should declare them in the base class. If you are worried about encapsulation, you can make them private and provide protected accessor methods for derived classes.
Another five cents: the good practice is to have abstract interface class which has no other members, but only public pure virtual methods and often public virtual destructor. Then you create base implementation which can also be abstract but can have protected fields, etc.
In you case it would be something like:
class IBlaBla;
class BlaBlaBase : public IBlaBla;
class DerivedOne : public BlaBlaBase
class DerivedTwo : public BlaBlaBase
This allows you to have more flexibility in the future if you decide that Base is no longer good for some specialized task.
Should I give the Base class dummy
private data members?
If you can implement a part of functionality without exposing the details to the derived classes, then do it in base class. If your derived classes would need access to these members, provide setters and getters. However, it is not convenient to have setters available for derived classes because your code becomes tightly coupled.
Encapsulation is sometimes overrated. If your base class and derived classes need to access those members, then they should probably be protected, not private. If it really is something that needs to be encapsulated, then you may want to make them private but provide getters and setters (either make them private to Base, with getters and setters defined there, or private to the derived classes, with pure virtual getters and setters in Base).
It's a bit hard to give you more specific advice without knowing about the actual problem you're trying to solve.
You will have to define Base::UseThis(), in the body of which you will make use of Base's fields (which you will also need to declare in the class definition above). If you only need to access them in UseThis, they can be private. If DerivedOne/Two will need access to them, you should make them protected.
Here is a possible resolution to your dilemna:
class Base {
public:
virtual ~Base() {}
virtual void redefine_me() = 0;
void utility_function();
private:
virtual int get_data_member() = 0;
virtual void set_data_member(int v) = 0;
};
class Derived1 : public Base {
public:
virtual void redefine_me() { do_d1_stuff(); }
private:
int my_private_idaho_;
virtual int get_data_member() { return my_private_idaho_; }
virtual void set_data_member(int v) { my_rpviate_idaho_ = v; }
};
class Derived2 : public Base {
public:
virtual void redefine_me() { do_d2_stuff(); }
private:
int gomer_pyle_;
virtual int get_data_member() { return gomer_pyle_; }
virtual void set_data_member(int v) { gomer_pyle_ = v; }
};
void Base::utility_function()
{
set_data_member(get_data_member() + 1);
}
It's biggest disadvantage is that now access to the private data member is mediated by a virtual function call, which isn't the cheapest thing around. It's also hidden from the optimizer.
This means that if you choose it, you should adopt a pattern where you fetch the private data member into a local variable at the beginning of your utility function and set it from the local variable before you return. Of course some utility functions may call out to functions that require the object state to be updated before they're called, and this pattern would then have to be modified to account for that. But then again, such utility functions are likely not to be able to satisfy the strong exception handling guarantee and should be rethought anyway.
It looks as if you need some interface for client code, and some 'convenient' functionality for implementors of the interface, which they can only use if they follow the rule of calling the useThis function of the convenience layer, which will tweak their private members.
Whenever I gave in to the temptation of putting the convenience functionality in my abstract base class, I regretted it (soon!) afterwards. It takes away a lot of flexibility. The solution proposed by AlexKR makes this situation slightly better.
An alternative way of doing this is providing some convenience class that implementers of the interface can aggregate instead of inheriting it. It can provide a function taking the implementer's members as arguments.
class Interface { public: virtual void f() = 0; };
class Convenience {
public:
void tweakMyMembers( int& member1, float& member2 );
bool somestate;
};
class Implementor : public Interface {
int m1; float m2;
public: Implementor( bool b ): conv( b ) {}
virtual void f() { conv.tweakMyMembers( m1, m2 ); if( m1<m2 ) dothis(); }
};