Python Tutorial: Class vs. Instance Attributes
In C++, you've already seen that structs and classes can have data members of A good real-life example of a composition is the relationship. Is a uses-a relationship one where a specific method of a class will use (if the used class has virtual functions that you have to override, for example). . regarding any other c/c++ compiler/IDE or incompatibilities with VS. For example suppose we have two classes then these two classes are said to be “has-a” relationship if both of these entities share each other's.
For example, a car is built using a metal frame, an engine, some tires, a transmission, a steering wheel, and a large number of other parts. A personal computer is built from a CPU, a motherboard, some memory, etc… Even you are built from smaller parts: This process of building complex objects from simpler ones is called object composition.
The complex object is sometimes called the whole, or the parent.
10.2 — Composition
The simpler object is often called the part, child, or component. For this reason, structs and classes are sometimes referred to as composite types. This reduces complexity, and allows us to write code faster and with less errors because we can reuse code that has already been written, tested, and verified as working. Types of object composition There are two basic subtypes of object composition: A note on terminology: Composition To qualify as a composition, an object and a part must have the following relationship: Composition relationships are part-whole relationships where the part must constitute part of the whole object.Class Diagram: Association & Aggregation Relationships - Georgia Tech - Software Development Process
We could use instance methods for this purpose: RobotInstances This is not a good idea for two reasons: First of all, because the number of robots has nothing to do with a single robot instance and secondly because we can't inquire the number of robots before we haven't created an instance. If we try to invoke the method with the class name Robot. RobotInstanceswe get an error message, because it needs an instance as an argument: RobotInstances Traceback most recent call last: RobotInstances takes exactly 1 argument 0 given The next idea, which still doesn't solve our problem, consists in omitting the parameter "self": RobotInstances " is treated as an instance method call and an instance method needs a reference to the instance as the first parameter.
c++ - What do "has-a" and "is-a" mean? - Stack Overflow
So, what do we want? We want a method, which we can call via the class name or via the instance name without the necessity of passing a reference to an instance to it. The solution consists in static methods, which don't need a reference to an instance.
It's easy to turn a method into a static method. All we have to do is to add a line with " staticmethod" directly in front of the method header. It's the decorator syntax. You can see in the following example that we can now use our method RobotInstances the way we wanted: RobotInstances We will get the following output: Like static methods class methods are not bound to instances, but unlike static methods class methods are bound to a class.
The first parameter of a class method is a reference to a class, i.
Relations between Classes
They can be called via an instance or the class name. RobotInstances The output looks like this: They are often used, where we have static methods, which have to call other static methods. To do this, we would have to hard code the class name, if we had to use static methods.
This is a problem, if we are in a use case, where we have inherited classes.
The following program contains a fraction class, which is still not complete. If you work with fractions, you need to be capable of reducing fractions, e. We can reduce a fraction to lowest terms by dividing both the numerator and denominator by the Greatest Common Divisor GCD. We have defined a static gcd function to calculate the greatest common divisor of two numbers. For example, the 'GCD of 8 and 24 is 8. The static method "gcd" is called by our class method "reduce" with "cls.
We define a class "Pets" with a method "about". This class will be inherited in a subclass "Dogs" and "Cats". The method "about" will be inherited as well. We will define the method "about" as a "staticmethod" in our first implementation to show the disadvantage of this approach: