The problem is I need Base-class's Private members into Derived-class Public members but there I can find only two main types of keywords to Inherit private which is the default type and public which Inherit all things as same as they are in Base-class. But I need to inherit only Private members of the Base-class into public members of the Derived-class.
Can someone help me out with this!
You wants a class Base has publicData and privateData, but only privateData is inherited in derived class.
This is might be an XY problem.
I presume you want two classes to have "some data" in common, but achieving it by inheritance in inappropriate is this case.
Partially inheritance break the concept of inheritance, and I suggest to use composition.
Most of time composition and aggregation are the better solution.
Here's a composition example for using the same private data without inheritance.
If you really need to inherit private data, make them protected in Base and no public data. Or multiple inheritances which I suggest only use interface style won't bite you less in the future.
If this cannot answer your question, please update your question for detailed use case. Eg: you need to refactor something or some limitation.
struct Data{
int val;
};
class Base{
public:
void SomeFunction();
protected:
Data m_Data;
};
class Derived: public Base{
// You will have protected m_Data here.
};
class AnotherClass{ // A class has data in common
private:
Data m_Data;
};
I have an abstract class base with private member variable base_var. I want to create a derived class, which also has base_var as a private member.
To me, this seems like an obvious thing you would want to do. base is abstract so it will never be instantiated. The only time I will create a base-object is if it is actually a derived object, so obviously when I give ´base´ a private member variable, what I am really trying to do is give that variable to all of its derived objects.
However, the below diagram seems to suggest that this is not doable with inheritance?
Why not? What would then even be the point of having private stuff in an abstract class? That class will never be instantiated, so all that private stuff is essentially useless?
However, the below diagram seems to suggest that this is not doable with inheritance?
Correct, private members of a class can not be accessed by derived classes. If You want a member of a class to be accessible by its derived classes but not by the outside, then You have to make it protected.
Why not? What would then even be the point of having private stuff in an abstract class? That class will never be instantiated, so all that private stuff is essentially useless?
Even an abstract class can have member functions which act on a (private) member variable. Consider (somewhat silly example, but well):
class MaxCached
{
private:
int cache = std::numeric_limits<int>::min();
public:
bool put(int value)
{
if (value > cache)
{
cache = value;
return true;
}
return false;
}
int get() const
{
return cache;
}
virtual void someInterface() const = 0;
};
Deriving from this class gives You the functionality of the base class (put and get) without the danger of breaking it (by for example writing a wrong value to cache).
Side note: Above is a purely made up example! You shouldn't add such a cache (which is independent of Your interface) into the abstract base class. As it stands the example breaks with the "Single Responsibility Principle"!
Just because a class is abstract doesn't mean there cannot be code implemented in that class that might access that variable. When you declare an item in a class to be private, the compiler assumes you had a good reason and will not change the access just because it there is a pure virtual function in the class.
If you want your derived classes to have access to a base class member declare the member as protected.
I have an abstract class base with private member variable base_var
class foo {
public:
virtual void a_pure_virtual_method() = 0;
int get_var() { base_var; }
virtual ~foo(){}
private:
int base_var;
};
Note that a class is said to be abstract when it has at least one pure virtual (aka abstract) method. There is nothing that forbids an abstract class to have non-pure virtual or even non-virtual methods.
I want to create a derived class, which also has base_var as a private member.
class derived : public foo {};
To me, this seems like an obvious thing you would want to do.
Sure, no problem so far.
The only time I will create a base-object is if it is actually a derived object, so obviously when I give ´base´ a private member variable, what I am really trying to do is give that variable to all of its derived objects.
Still fine.
Why not?
You are confusing access rights that are display in the image you included with the mere presence of the members in the derived. The derived class has no access to members that are private in the base class. Period. This is just according to the definition of what is private.
What would then even be the point of having private stuff in an abstract class? That class will never be instantiated, so all that private stuff is essentially useless?
It is not useless at all. Derived classes inherit all members, they just cannot access all of them. The private stuff is there you just cannot access it directly. Thats the whole point of encapsulation. Consider this example:
class bar : public foo {
void test() {
std::cout << base_var; // error base_var is private in foo
std::cout << get_var(); // fine
}
};
I have created two classes A and B where B inherits from class A. As you can see, I have a vector in class A that is currently under the protected section of the class. I am unsure if using protected is bad practice?
#include <vector>
class A
{
public :
A();
protected:
std::vector <std::string> a;
};
class B : A
{
public :
B();
void accessVector()
{
a.size();
}
private:
};
When A makes a data member a protected, it is offering the following guarantee to all classes that derive from it:
"You may do anything you like to a without telling me. This includes appending to it, modifying its contents, removing items, sorting it, moving from it, moving to it and otherwise making its state undefined and/or unknowable to me".
Remember that anyone may create a class that derives from A.
For this reason, to all intents and purposes, a protected member is a public member, since a derived class may simply say the following:
public:
using A::a;
Starting here and working forward, you'll find that there are only two sensible use-cases for protected:
When a base class defines a virtual member function that may need to be called from an overridden version of the same function in a derived class.
When the base class wants to expose 'data as interface' to a derived class, but not to the world.
I have not used C++ for a really (really) long time and this question may be stupid but I could really use some help.
If my base class has a private data member and my derived class is derived publicly, the private members of the base class are NOT inherited. But, they can still be accessed via the inherited public functions. Eg:
class B{
int a,b;
public:
void SetA(int);
int GetA();
};
class D:public B{
public:
SetAAttribute(int x)
{ SetA(x); }
}
Now, my question is as follows:
Technically, the derived class objects do not have the 'a' attribute defined on them. In layman terms, I am basically setting an attribute on an entity when the attributes does not even exist in the first place. It seems unintuitive to imagine such a concept. Did I understand this correctly?
Any extra explanation or correction would be greatly appreciated.
It's not that the attribute doesn't exist anymore, it's just that it's hidden from you. The base class methods can still access the base class members because they're not hidden from each other.
The private base members are inherited, the compiler will just give you an error if you try to access them, since you aren't supposed to access them directly.
What is the difference between private and protected members in C++ classes?
I understand from best practice conventions that variables and functions which are not called outside the class should be made private—but looking at my MFC project, MFC seems to favor protected.
What's the difference and which should I use?
Private members are only accessible within the class defining them.
Protected members are accessible in the class that defines them and in classes that inherit from that class.
Edit: Both are also accessible by friends of their class, and in the case of protected members, by friends of their derived classes.
Edit 2: Use whatever makes sense in the context of your problem. You should try to make members private whenever you can to reduce coupling and protect the implementation of the base class, but if that's not possible then use protected members. Check C++ FAQ for a better understanding of the issue. This question about protected variables might also help.
Public members of a class A are accessible for all and everyone.
Protected members of a class A are not accessible outside of A's code, but is accessible from the code of any class derived from A.
Private members of a class A are not accessible outside of A's code, or from the code of any class derived from A.
So, in the end, choosing between protected or private is answering the following questions: How much trust are you willing to put into the programmer of the derived class?
By default, assume the derived class is not to be trusted, and make your members private. If you have a very good reason to give free access of the mother class' internals to its derived classes, then you can make them protected.
Protected members can be accessed from derived classes. Private ones can't.
class Base {
private:
int MyPrivateInt;
protected:
int MyProtectedInt;
public:
int MyPublicInt;
};
class Derived : Base
{
public:
int foo1() { return MyPrivateInt;} // Won't compile!
int foo2() { return MyProtectedInt;} // OK
int foo3() { return MyPublicInt;} // OK
};
class Unrelated
{
private:
Base B;
public:
int foo1() { return B.MyPrivateInt;} // Won't compile!
int foo2() { return B.MyProtectedInt;} // Won't compile
int foo3() { return B.MyPublicInt;} // OK
};
In terms of "best practice", it depends. If there's even a faint possibility that someone might want to derive a new class from your existing one and need access to internal members, make them Protected, not Private. If they're private, your class may become difficult to inherit from easily.
The reason that MFC favors protected, is because it is a framework. You probably want to subclass the MFC classes and in that case a protected interface is needed to access methods that are not visible to general use of the class.
It all depends on what you want to do, and what you want the derived classes to be able to see.
class A
{
private:
int _privInt = 0;
int privFunc(){return 0;}
virtual int privVirtFunc(){return 0;}
protected:
int _protInt = 0;
int protFunc(){return 0;}
public:
int _publInt = 0;
int publFunc()
{
return privVirtFunc();
}
};
class B : public A
{
private:
virtual int privVirtFunc(){return 1;}
public:
void func()
{
_privInt = 1; // wont work
_protInt = 1; // will work
_publInt = 1; // will work
privFunc(); // wont work
privVirtFunc(); // will work, simply calls the derived version.
protFunc(); // will work
publFunc(); // will return 1 since it's overridden in this class
}
}
Attributes and methods marked as protected are -- unlike private ones -- still visible in subclasses.
Unless you don't want to use or provide the possibility to override the method in possible subclasses, I'd make them private.
Sure take a look at the Protected Member Variables question. It is recommended to use private as a default (just like C++ classses do) to reduce coupling. Protected member variables are most always a bad idea, protected member functions can be used for e.g. the Template Method pattern.
Protected members can only be accessed by descendants of the class, and by code in the same module. Private members can only be accessed by the class they're declared in, and by code in the same module.
Of course friend functions throw this out the window, but oh well.
private members are only accessible from within the class, protected members are accessible in the class and derived classes. It's a feature of inheritance in OO languages.
You can have private, protected and public inheritance in C++, which will determine what derived classes can access in the inheritance hierarchy. C# for example only has public inheritance.
private = accessible by the mothership (base class) only
(ie only my parent can go into my parent's bedroom)
protected = accessible by mothership (base class), and her daughters
(ie only my parent can go into my parent's bedroom, but gave son/daughter permission to walk into parent's bedroom)
public = accessible by mothership (base class), daughter, and everyone else
(ie only my parent can go into my parent's bedroom, but it's a house party - mi casa su casa)
Since no public member function is needed to fetch and update protected members in the derived class, this increases the efficiency of code and reduces the amount of code we need to write. However, programmer of the derived class is supposed to be aware of what he is doing.
private is preferred for member data. Members in C++ classes are private by default.
public is preferred for member functions, though it is a matter of opinion. At least some methods must be accessible. public is accessible to all. It is the most flexible option and least safe. Anybody can use them, and anybody can misuse them.
private is not accessible at all. Nobody can use them outside the class, and nobody can misuse them. Not even in derived classes.
protected is a compromise because it can be used in derived classes. When you derive from a class, you have a good understanding of the base class, and you are careful not to misuse these members.
MFC is a C++ wrapper for Windows API, it prefers public and protected. Classes generated by Visual Studio wizard have an ugly mix of protected, public, and private members. But there is some logic to MFC classes themselves.
Members such as SetWindowText are public because you often need to access these members.
Members such as OnLButtonDown, handle notifications received by the window. They should not be accessed, therefore they are protected. You can still access them in the derived class to override these functions.
Some members have to do threads and message loops, they should not be accessed or override, so they are declared as private
In C++ structures, members are public by default. Structures are usually used for data only, not methods, therefore public declaration is considered safe.
Private : Accessible by class member functions & friend function or friend class.
For C++ class this is default access specifier.
Protected: Accessible by class member functions, friend function or friend class & derived classes.
You can keep class member variable or function (even typedefs or inner classes) as private or protected as per your requirement.
Most of the time you keep class member as a private and add get/set functions to encapsulate. This helps in maintenance of code.
Generally private function is used when you want to keep your public functions modular or to eliminate repeated code instead of writing whole code in to single function. This helps in maintenance of code.
Refer this link for more detail.
Private: It is an access specifier. By default the instance (member) variables or the methods of a class in c++/java are private. During inheritance, the code and the data are always inherited but is not accessible outside the class. We can declare our data members as private so that no one can make direct changes to our member variables and we can provide public getters and setters in order to change our private members. And this concept is always applied in the business rule.
Protected: It is also an access specifier. In C++, the protected members are accessible within the class and to the inherited class but not outside the class. In Java, the protected members are accessible within the class, to the inherited class as well as to all the classes within the same package.
Private member can be accessed only in same class where it has declared where as protected member can be accessed in class where it is declared along with the classes which are inherited by it .
A protected nonstatic base class member can be accessed by members and friends of any classes derived from that base class by using one of the following:
A pointer to a directly or indirectly derived class
A reference to a directly or indirectly derived class
An object of a directly or indirectly derived class
The protected keyword specifies access to class members in the
member-list up to the next access specifier (public or private) or the
end of the class definition. Class members declared as protected can
be used only by the following:
Member functions of the class that originally declared these members.
Friends of the class that originally declared these members.
Classes derived with public or protected access from the class that originally declared these members.
Direct privately derived classes that also have private access to protected members.
When preceding the name of a base class, the protected keyword
specifies that the public and protected members of the base class are
protected members of its derived classes.
Protected members are not as private as private members, which are
accessible only to members of the class in which they are declared,
but they are not as public as public members, which are accessible in
any function.
Protected members that are also declared as static are accessible to
any friend or member function of a derived class. Protected members
that are not declared as static are accessible to friends and member
functions in a derived class only through a pointer to, reference to,
or object of the derived class.
protected (C++)
What is the difference between private and protected members in C++ classes?
Other answers have stated:
public - accessible by all.
protected - accessible by derived classes (and friends).
private - restricted.
What's the difference and which should I use?
The C++ core guidelines gives the advice that data should always be private. I think this is good advice as it makes for 'data spaghetti' when you have derived classes that can access protected data. It makes much more sense for functions to be protected, but it depends on the use case.
For functions you have a choice. For data, you should make it private and provide protected accessor functions if needed. This gives more control over the class data.
private and protected access modifiers are one and same only that protected members of the base class can be accessed outside the scope of the base class in the child(derived)class.
It also applies the same to inheritance .
But with the private modifier the members of the base class can only be accessed in the scope or code of the base class and its friend functions only''''