In C++ I can make interface and class declarations. However, in contrast to C# the interface here is very likely to be a class so why is it there ?
interface Interface {
};
And
class Class {
};
As has been mentioned in comments, standard C++ does not support an "interface" construct directly. This is an extension from some vendors.
Other languages do (e.g Java).
The equivalent in standard C++ is an abstract base class, which does not have data members.
class SomeInterface
{
public:
virtual ~SomeInterface() = 0;
virtual void SomeAction(int flag) = 0;
};
Inheriting from such a class forces the derived class to override/implement all virtual functions, otherwise they cannot be instantiated.
Generally speaking, an abstract base class without data members can often get away without having non-trivial constructors (since the class has no members to initialise).
Unlike other languages that do support an interface construct, the abstract base class permits;
Having data members in the base class. (In which case constructors, destructors, assignment operators, etc may need to be defined to manage those members)
Having some members pure virtual and some not
Providing implementations of pure virtual methods (which must be explicitly overridden by derived class, but the base class version can be called)
Related
What is the difference between a Plain Class Vs Abstract Class Vs Interfaces. Kindly, explain the same using snippets and demo.
Class:
It is a class without any abstract members. You can instantiate normal Class where as you cant instantiate Interface and Abstract class; you can just inherit from Abstract and Interface. Inherit from it (unless the class is sealed), use its methods, override those methods, etc.
Abstract Class:
May/mayn't contain Implementations; At least one member will not be implemented. A Class may inherit from a single Base class; multiple inheritance not allowed. Members have access modifiers May contain fields, properties, constructors, destructors, methods, events and indexers.
Interface:
Does not contain Implementations Interface can inherit from a number of interfaces(Multiple inheritance supported) Members are automatically public May contain properties, methods, events and indexers
I'm using the Mixin pattern as illustrated below. Why does Typescript require you to provide stand-in properties for private properties of the mixin class in the target class (A)? It perfectly makes sense for the public properties, but for private properties it unnecessarily crufts up the target class with details of the internal implementation of the mixin class by requiring them to be stubbed-out in the target class. Seems like the Typescript transpiler should be able to not require this.
class Mixin {
private foo:string;
}
class A implements Mixin {
// stand-in properties, typescript requires even
// private properties to be stubbed-out
foo:string;
}
Private members contribute to the structure of a type in TypeScript, so if you don't implement them you are not compatible with the type. This actually makes it impossible to match a type structurally in TypeScript if it has a private member, because you are either:
a. Failing to provide the type
or
b. Providing a separate implementation of the private member
So you can only extend a type with a private member, not implement it.
With this in mind, you are better off not using private members with mixins. Provide the ghost-members in the implementation class and keep your fingers crossed that if mixins gain some traction the ghosting will become unnecessary (see TypeScript mixins part one).
I just start learning what is Dependency Injection and InversionOfControll is. But I cant get one thing. The interface injection is onle when I define some interface where describe method what need to be realized. And that method gets instance of some class as parameter, and then In class what implements interface just describe body of this method ?
An interface is only a contract that defines what public members a class should implement. It does not control the actual implementation - you need a concrete class to do that.
// This is only a contract that defines what members
// all concrete types must implement.
public interface ISomeType
{
void DoSomething();
}
// This class implements the interface. Therefore, it must
// have all of the methods the contract specifies. In some
// languages, this can be done implicitly just by adding the
// member, but it usually must be public.
public class SomeType : ISomeType
{
public void DoSomething()
{
Console.WriteLine("Hello World");
}
}
When you make a class implement an interface it implicitly means that instances of the class can be cast to the interface type.
ISomeType x = new SomeType();
Dependency Injection takes advantage of this behavior. You typically define both the interface type and the concrete implementation together in a mapping.
container.For<ISomeType>().Use<SomeType>();
Then when a service is declared to take ISomeType as a constructor argument, the map is used to determine which concrete type to create an instance of.
public class SomeService : ISomeService
{
private readonly ISomeType someType;
public SomeService(ISomeType someType)
{
if (someType == null) throw new ArgumentNullException("someType");
this.someType = someType;
}
}
The recommended way is to allow the DI container to do this implicitly when your entire object graph is composed (in the Composition Root), but it is possible also to do it explicitly (and it makes a better example):
ISomeService = container.GetInstance<ISomeService>();
Assuming that the container was configured to map ISomeService to SomeService (like I showed before with ISomeType), this one line of code will create an instance of SomeService and automatically inject an instance of SomeType into its constructor.
It is difficult to see the point in a simple example, though. Dependency Injection is meant for complex applications with many types. It simplifies things when the application is complex, but when the application is simple it has a tendency to make things more complex.
I have a library here with some Java classes. One class has some protected static methods, which I realize is sorta an OOP no-no but I can't change its code. Assuming I have a Scala class that subclasses the aforementioned Java class, how can I call its protected static members?
See Frequently Asked Questions - Java Interoperability:
This is a known limitation of Scala:
there is no notion of 'static' members
in Scala. Instead, Scala treats
static members of class Y as members
of the singleton object Y (the
companion object of class Y). When
inheriting from this class, one can
access only protected members of class
Y but cannot access protected members
of object Y.
There's no way Scala can simulate
static protected without impairing the
integrity of Scala's object model in a
fundamental way, so this is not going
to change. To work around this
limitation, one has to create an
implementation of the enclosing class
with Java code which encapsulates all
accesses to the protected static inner
class.
See ticket #1806 for more
information and a concrete example of
the limitation and its workaround.
Whenever I create an abstract class I tend to create an interface to go along with it and have other code refer to the interface and not the abstract class. Usually when I don't create an interface to start with I regret it (such as having to override all implimented methods to stub the class for unit testing or later down the line new classes don't need any of the implimentation and override everything also finding themselves unable to extend any other class).
At first I tried to distinguish when to use an interface and when to use an abstract class by considering is-a vs able-to but I still would end up suffering later down the line for not making an interface to start with.
So the question is when is it a good idea to only have an abstract class and no interface at all?
When you wish to "give" some base class functionality to derived classes but when this functionality is not sufficient to instantiate a usable class, then go for abstract classes.
When you wish that some classes completely implement a set of methods (a public contract), then it is a convenient to define such contract with interfaces and enforce them onto classes by making them inherit this interface.
In short:
With abstract classes you give some common base functionality to derived classes. No further actions are necessary unless abstract class has some stubs (which have to be implemented down there).
With interfaces you require derived classes to implement a set of functions and you do not pass along any implementation.
So the question is when is it a good idea to only have an abstract class and no interface at all?
When you do not wish to enforce any public contract (a set of methods/properties defined by an interface).
Also when you do not plan to use certain coding techniques like casting object to an interface type (run-time polymorphism) or limit allowed input (some method argument will only accept object of types which implement certain interfaces).
Well, the main case it is useful to have only an abstract class without any interface is to mark a certain type. It is useful to be able to check if an object "is-a" something. These interface "mark" an objet to be of a certain type. Depending on the language you use, different design patterns apply ...
These sort of abstract classes exist in java. You can also use them in C++ with RTTI.
my2c