When working with multiple inheritances, a diamond problem can surface in programming languages, especially in C++. When dealing with extensive codebases, multiple inheritances are commonly employed as a strategy in C++. To structure the program and source code effectively, classes are used. However, improper handling of multiple inheritances can lead to complications. The DIAMOND problem mainly manifests in such scenarios. This tutorial sheds light on the main factors contributing to the diamond problem, its emergence from multiple inheritances, and the necessary solutions to address it.
System Requirement for the code
You must ensure that an Ubuntu operating system is set up and operational on the virtual machine to execute the software related to "C++ diamond inheritance" within the Linux environment. This process requires two essential tools. The first tool is a text editing application, for which we will make use of the integrated "text editor" in Linux. Alternatively, you can opt for other preferred text editing tools. The second tool is the Ubuntu terminal, which you will utilize to execute the program and observe the generated output. Given that the "diamond problem" emerges when inheritance relationships are established within the source code, we will initially delve into the concept of multiple inheritances in this guide.
Multiple Inheritances
For example, consider a real-life situation where a child inherits traits from both parents. In this scenario, the child is considered to be a subclass derived from the mother and father. Transitioning back to the concept of inheritance, let's delve into it further. Within this discussion, the term "constructors" will play a significant role. In cases of multiple inheritances, the constructors of a derived class (child class) are executed following the order of the parent classes. In contrast, for destructors, the inheritance sequence is reversed. To demonstrate the concept of inheritance in C++, we will now present a simple example.
A class in C++ can leverage Multiple Inheritance to inherit from more than one class. The constructors of the inherited classes are invoked in the exact sequence of inheritance. For example, in the given program, the constructor of the second class is invoked prior to class A.
A class can descend from many base classes.
Example:
- The FATHER and MOTHER classes are the ancestors of the CHILD class.
- The LIQUID and FUEL classes give rise to the PETROL class.
class A
{
// ,,,,,,,,,,,,,
};
class B
{
// ,,,,,,,,,,,,,,
};
class C: public A,public B
{
// ,,,,,,,,,,,,,
};
Examining a sample will aid in gaining a clearer grasp of the idea:
# include < iostream >
using namespace std ;
class one
{
public:
one ( ) { cout << " one's constructor is called here " << endl ; }
} ;
class two
{
public:
two ( ) { cout << "two's constructor is called here " << endl ; }
} ;
class three : public two , public one // Note the order
{
public:
three ( ) { cout << " C's constructor called " << endl ; }
} ;
int main()
{
three th ;
return 0 ;
}
OUTPUT:
two's constructor is called here
one's constructor is called here
three's constructor is called here
?????????????????..
Process executed in 0.11 seconds
Press any key continue.
The destructors are invoked following the constructors, in the opposite sequence.
The Diamond Problem occurs when a class has two superclasses that have a common base class, leading to conflicts. In the scenario depicted below, the TA class inherits duplicate attributes from the Person class, causing ambiguity. Consider the program provided as an example.
# include < iostream >
using namespace std ;
class Base {
// Data members of Base class
public :
Base ( int a ) { cout < < " Base :: Base ( int ) called " < < endl ; }
} ;
class Derived : public Base {
// data members of Derived
public :
Derived ( int a ) : Base ( a ) {
cout < < " Derived :: Derived ( int ) called " < < endl ;
}
} ;
class Derived_2 : public Base {
// data members of Derived_2
public :
Derived_2 ( int a ) : Base ( a ) {
cout < < " Derived_2 :: Derived_2 ( int ) called " < < endl ;
}
} ;
class Result : public Derived , public Derived_2 {
public :
Result ( int a ) : Derived_2 ( a ) , Derived ( a ) {
cout < < " Result :: Result ( int ) called " < < endl ;
}
} ;
int main ( ) {
Result rs1 ( 30 ) ;
}
OUTPUT:
Base :: Base ( int ) called
Derived :: Derived ( int ) called
Base :: Base ( int ) called
Derived_2 :: Derived_2 ( int ) called
Result :: Result ( int ) called
?????????????????..
Process executed in 0.11 seconds
Press any key continue.
Explanation
The class named "Base" is referenced twice within the provided code. Consequently, when the object "rs1" is deleted, the destructor of "Base" will also be called twice. This duplication occurs because object "rs1" includes two instances of the "Base" element, leading to potential confusion. Introducing the keyword "virtual" can resolve this issue. By designating the classes "Derived" and "Derived_2" as virtual base classes, we can avoid having two copies of the "Base" class within the "Result" class. Consider the following example program for illustration purposes.
# include < iostream >
using namespace std ;
class Base {
public :
Base ( int x ) { cout < < " Base :: Base ( int ) is called " < < endl ; }
Base ( ) { cout < < "Base :: Base ( ) is called " < < endl ; }
} ;
class Derived : virtual public Base {
public :
Derived ( int a ) :: Base ( a ) {
cout < < "Derived :: Derived ( int ) is called " < < endl ;
}
} ;
class Derived_1 : virtual public Derived {
public :
Derived_1 ( int a ) : Derived ( a ) {
cout < < " Derived_1 :: Derived_1 ( int ) is called " < < endl ;
}
} ;
class Result : public Derived_1 , public Derived {
public :
Result ( int a ) : Derived_1 ( a ) , Derived ( a ) {
cout < < " Result :: Result ( int ) is called " < < endl ;
}
} ;
int main ( ) {
Result rs1 ( 30 ) ;
}
OUTPUT:
Base :: Base ( ) is called
Derived :: Derived ( int ) is called
Derived_1 :: Derived_1 ( int ) is called
Result :: Result ( int ) is called
?????????????????..
Process executed in 0.11 seconds
Press any key continue.
Explanation
A sole invocation to the function Object { [native code] } for the "Person" class occurs within the provided program. It is essential to observe that in the mentioned output, the invocation of the default function Object { [native code] } for "Person" is significant. When employing the "virtual" keyword, the default function Object { [native code] } from the superclass is automatically called, even if the parent classes explicitly invoke a parameterized function Object { [native code] }. To access the parameterized function Object { [native code] } of the 'Person' class, you can do so within the "TA" class by explicitly invoking it. The subsequent software serves as an example to illustrate this concept.
# include < iostream >
using namespace std ;
class Base {
public :
Base ( int a ) { cout < < " Base :: Base ( int ) is called " < < endl ; }
Base () { cout < < " Base :: Base ( ) is called " < < endl ; }
} ;
class Derived : virtual public Base {
public :
Derived ( int a ) : Person ( a ) {
cout < < " Derived :: Derived ( int ) is called " < < endl ;
}
} ;
Class Derived_1 : virtual public Derived {
public :
Derived_1 ( int a ) : Derived ( a ) {
cout < < " Derived_1 :: Derived_1 ( int ) is called " < < endl ;
}
} ;
class Result : public Derived_1 , public Derived {
public :
Result ( int a ) : Derived ( a ) , Derived ( a ) , Base ( a ) {
cout < < " Result :: Result ( int ) is called " < < endl ;
}
} ;
int main ( ) {
Result rs1 ( 30 ) ;
}
OUTPUT:
Base :: Base ( int ) is called
Derived :: Derived ( int ) is called
Derived_1 :: Derived_1 ( int ) is called
Result :: Result ( int ) is called
?????????????????..
Process executed in 0.11 seconds
Press any key continue.
Explanation
The Object function within a grandparent class is usually invoked via the parent class instead of directly. Direct calling is only allowed when the "virtual" keyword is utilized.
