The terms “universal” and “existential” here come from the similarly-named quantifiers in predicate logic. Universal quantification is normally written as ∀, which you can read as “for all”, and means roughly what it sounds like: in a logical statement resembling “∀x. …” whatever is in place of the “…” is true for all possible “x” … Read more
Check the Wikipedia example: it is very helpful at a high level: class Animal: def __init__(self, name): # Constructor of the class self.name = name def talk(self): # Abstract method, defined by convention only raise NotImplementedError(“Subclass must implement abstract method”) class Cat(Animal): def talk(self): return ‘Meow!’ class Dog(Animal): def talk(self): return ‘Woof! Woof!’ animals = … Read more
For me, the link from CKing post and the example in Wikipedia are clear enough, but I’ll try to give you a new example. As they said, Strategy Pattern is mostly a way to change the behaviour of an algorithm at runtime. Of course you can achieve this in many different ways (such as holding … Read more
Syntax errors non-withstanding, you cannot dynamic_cast a non-polymorphic type. static_cast is the cast you would use in this case, if you know that it is in fact an object of the target type. The reason why: static_cast basically has the compiler perform a check at compile time “Could the input be cast to the output?” … Read more
Two objects respond to the same message with different behaviors; the sender doesn’t have to care.
This is a problem called “slicing.” Dog() creates a Dog object. If you were to call Dog().makeSound(), it would print “bark” as you expect it to. The problem is that you are initializing the badDog, which is an object of type Animal, with this Dog. Since the Animal can only contain an Animal and not … Read more
I simply prefer to repeat the property() as well as you will repeat the @classmethod decorator when overriding a class method. While this seems very verbose, at least for Python standards, you may notice: 1) for read only properties, property can be used as a decorator: class Foo(object): @property def age(self): return 11 class Bar(Foo): … Read more
The is operator in Python checks that the two arguments refer to the same object in memory; it is not like the is operator in C#. From the docs: The operators is and is not test for object identity: x is y is true if and only if x and y are the same object. … Read more
The g of the derived class will be called. If you want to call the function in the base, call Base::g(); instead. If you want to call the derived, but still want to have the base version be called, arrange that the derived version of g calls the base version in its first statement: virtual … Read more
Putting functions inside pure data structures in a prototype-like fashion – this seems like it works but doesn’t it also violate the idea of defining pure functions separately from data? If virtual method dispatch is the way you want to approach the problem, this is a perfectly reasonable approach. As for separating functions from data, … Read more