std::Friendship and inheritance
Friend functions
In principle, private and protected members of a class cannot be accessed from outside the same class in which they are declared. However, this rule does not affect
friends.
Friends are functions or classes declared with the
friend
keyword.
If we want to declare an external function as friend of a class, thus allowing this function to have access to the private and protected members of this class, we do it by declaring a prototype of this external function within the class, and preceding it with the keyword
friend:
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// friend functions
#include <iostream>
using namespace std;
class CRectangle {
int width, height;
public:
void set_values (int, int);
int area () {return (width * height);}
friend CRectangle duplicate (CRectangle);
};
void CRectangle::set_values (int a, int b) {
width = a;
height = b;
}
CRectangle duplicate (CRectangle rectparam)
{
CRectangle rectres;
rectres.width = rectparam.width*2;
rectres.height = rectparam.height*2;
return (rectres);
}
int main () {
CRectangle rect, rectb;
rect.set_values (2,3);
rectb = duplicate (rect);
cout << rectb.area();
return 0;
}
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The
duplicate function is a friend of
CRectangle. From within that function we have been able to access the members
width and
height of different objects of type
CRectangle, which are private members. Notice that neither in the declaration of
duplicate() nor in its later use in
main() have we considered
duplicate a member of class
CRectangle. It isn't! It simply has access to its private and protected members without being a member.
The friend functions can serve, for example, to conduct operations between two different classes. Generally, the use of friend functions is out of an object-oriented programming methodology, so whenever possible it is better to use members of the same class to perform operations with them. Such as in the previous example, it would have been shorter to integrate
duplicate() within the class
CRectangle.
Friend classes
Just as we have the possibility to define a friend function, we can also define a class as friend of another one, granting that first class access to the protected and private members of the second one.
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// friend class
#include <iostream>
using namespace std;
class CSquare;
class CRectangle {
int width, height;
public:
int area ()
{return (width * height);}
void convert (CSquare a);
};
class CSquare {
private:
int side;
public:
void set_side (int a)
{side=a;}
friend class CRectangle;
};
void CRectangle::convert (CSquare a) {
width = a.side;
height = a.side;
}
int main () {
CSquare sqr;
CRectangle rect;
sqr.set_side(4);
rect.convert(sqr);
cout << rect.area();
return 0;
}
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In this example, we have declared
CRectangle as a friend of
CSquare so that
CRectangle member functions could have access to the protected and private members of
CSquare, more concretely to
CSquare::side, which describes the side width of the square.
You may also see something new at the beginning of the program: an empty declaration of class
CSquare. This is necessary because within the declaration of
CRectangle we refer to CSquare (as a parameter in
convert()). The definition of
CSquare is included later, so if we did not include a previous empty declaration for
CSquare this class would not be visible from within the definition of
CRectangle.
Consider that friendships are not corresponded if we do not explicitly specify so. In our example,
CRectangle is considered as a friend class by
CSquare, but
CRectangle does not consider
CSquare to be a friend, so
CRectangle can access the protected and private members of
CSquare but not the reverse way. Of course, we could have declared also
CSquare as friend of
CRectangle if we wanted to.
Another property of friendships is that they are
not transitive: The friend of a friend is not considered to be a friend unless explicitly specified.
Inheritance between classes
A key feature of C++ classes is inheritance. Inheritance allows to create classes which are derived from other classes, so that they automatically include some of its "parent's" members, plus its own. For example, we are going to suppose that we want to declare a series of classes that describe polygons like our
CRectangle, or like
CTriangle. They have certain common properties, such as both can be described by means of only two sides: height and base.
This could be represented in the world of classes with a class
CPolygon from which we would derive the two other ones:
CRectangle and
CTriangle.
The class
CPolygon would contain members that are common for both types of polygon. In our case:
width and
height. And
CRectangle and
CTriangle would be its derived classes, with specific features that are different from one type of polygon to the other.
Classes that are derived from others inherit all the accessible members of the base class. That means that if a base class includes a member
A and we derive it to another class with another member called
B, the derived class will contain both members
A and
B.
In order to derive a class from another, we use a colon (
:) in the declaration of the derived class using the following format:
class derived_class_name: public base_class_name
{ /*...*/ };
Where
derived_class_name is the name of the derived class and
base_class_name is the name of the class on which it is based. The
public access specifier may be replaced by any one of the other access specifiers
protected and
private. This access specifier limits the most accessible level for the members inherited from the base class: The members with a more accessible level are inherited with this level instead, while the members with an equal or more restrictive access level keep their restrictive level in the derived class.
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// derived classes
#include <iostream>
using namespace std;
class CPolygon {
protected:
int width, height;
public:
void set_values (int a, int b)
{ width=a; height=b;}
};
class CRectangle: public CPolygon {
public:
int area ()
{ return (width * height); }
};
class CTriangle: public CPolygon {
public:
int area ()
{ return (width * height / 2); }
};
int main () {
CRectangle rect;
CTriangle trgl;
rect.set_values (4,5);
trgl.set_values (4,5);
cout << rect.area() << endl;
cout << trgl.area() << endl;
return 0;
}
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The objects of the classes
CRectangle and
CTriangle each contain members inherited from
CPolygon. These are:
width,
height and
set_values().
The
protected access specifier is similar to
private. Its only difference occurs in fact with inheritance. When a class inherits from another one, the members of the derived class can access the protected members inherited from the base class, but not its private members.
Since we wanted
width and
height to be accessible from members of the derived classes
CRectangle and
CTriangle and not only by members of
CPolygon, we have used
protected access instead of
private.
We can summarize the different access types according to who can access them in the following way:
Access | public | protected | private |
members of the same class | yes | yes | yes |
members of derived classes | yes | yes | no |
not members | yes | no | no |
Where "not members" represent any access from outside the class, such as from
main(), from another class or from a function.
In our example, the members inherited by
CRectangle and
CTriangle have the same access permissions as they had in their base class
CPolygon:
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CPolygon::width // protected access
CRectangle::width // protected access
CPolygon::set_values() // public access
CRectangle::set_values() // public access
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This is because we have used the
public keyword to define the inheritance relationship on each of the derived classes:
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class CRectangle: public CPolygon { ... }
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This
public keyword after the colon (
:) denotes the most accessible level the members inherited from the class that follows it (in this case
CPolygon) will have. Since
public is the most accessible level, by specifying this keyword the derived class will inherit all the members with the same levels they had in the base class.
If we specify a more restrictive access level like
protected, all public members of the base class are inherited as protected in the derived class. Whereas if we specify the most restricting of all access levels:
private, all the base class members are inherited as private.
For example, if
daughter was a class derived from
mother that we defined as:
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class daughter: protected mother;
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This would set
protected as the maximum access level for the members of
daughter that it inherited from
mother. That is, all members that were public in
mother would become protected in
daughter. Of course, this would not restrict
daughter to declare its own public members. That maximum access level is only set for the members inherited from
mother.
If we do not explicitly specify any access level for the inheritance, the compiler assumes private for classes declared with
class keyword and public for those declared with
struct.
What is inherited from the base class?
In principle, a derived class inherits every member of a base class except:
- its constructor and its destructor
- its operator=() members
- its friends
Although the constructors and destructors of the base class are not inherited themselves, its default constructor (i.e., its constructor with no parameters) and its destructor are always called when a new object of a derived class is created or destroyed.
If the base class has no default constructor or you want that an overloaded constructor is called when a new derived object is created, you can specify it in each constructor definition of the derived class:
derived_constructor_name (parameters) : base_constructor_name (parameters) {...}
For example:
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// constructors and derived classes
#include <iostream>
using namespace std;
class mother {
public:
mother ()
{ cout << "mother: no parameters\n"; }
mother (int a)
{ cout << "mother: int parameter\n"; }
};
class daughter : public mother {
public:
daughter (int a)
{ cout << "daughter: int parameter\n\n"; }
};
class son : public mother {
public:
son (int a) : mother (a)
{ cout << "son: int parameter\n\n"; }
};
int main () {
daughter cynthia (0);
son daniel(0);
return 0;
}
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mother: no parameters
daughter: int parameter
mother: int parameter
son: int parameter |
Notice the difference between which
mother's constructor is called when a new
daughter object is created and which when it is a
son object. The difference is because the constructor declaration of
daughter and
son:
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daughter (int a) // nothing specified: call default
son (int a) : mother (a) // constructor specified: call this
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Multiple inheritance
In C++ it is perfectly possible that a class inherits members from more than one class. This is done by simply separating the different base classes with commas in the derived class declaration. For example, if we had a specific class to print on screen (
COutput) and we wanted our classes
CRectangle and
CTriangle to also inherit its members in addition to those of
CPolygon we could write:
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class CRectangle: public CPolygon, public COutput;
class CTriangle: public CPolygon, public COutput;
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here is the complete example:
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// multiple inheritance
#include <iostream>
using namespace std;
class CPolygon {
protected:
int width, height;
public:
void set_values (int a, int b)
{ width=a; height=b;}
};
class COutput {
public:
void output (int i);
};
void COutput::output (int i) {
cout << i << endl;
}
class CRectangle: public CPolygon, public COutput {
public:
int area ()
{ return (width * height); }
};
class CTriangle: public CPolygon, public COutput {
public:
int area ()
{ return (width * height / 2); }
};
int main () {
CRectangle rect;
CTriangle trgl;
rect.set_values (4,5);
trgl.set_values (4,5);
rect.output (rect.area());
trgl.output (trgl.area());
return 0;
}
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