How to Programming with Generics

c++,templates,generics,constraints

It can't be done, but…let's suppose for a moment that we could figure out a way for the following statement to compile in the way that you want: var myObject = MyMethod(); Then what is the code using myObject going to look like? In this hypothetical scenario, the variable myObject...

C++ Templates with multiple constraints

you usually make constraints on C++ template with std::enable_if here is the trick - if your template looks like this: template <class T> <return type> <function name> (args...) you take the return type and corporate it with enable_if like this : template <class T> typename std::enable_if<XXX,<return type>>::type <function name> (args...)...

Cannot invoke method with argument list of type KeyType in Swift

c#,generics,castle-windsor

I think that best solution for your case would be changing class declaration to: class EventDispatcher<U: EventDispatcherProtocol> { typealias KeyType = U.T And it will also simplify creation of the EventDispatcher with skipping the redundant type declarations: var dispatcher = EventDispatcher<CustomListener<CustomEvent>>() EDIT: Since the code was altered multiple times while...

Resolving Interface with generic type constraint with Castle Windsor

In Castle Windsor you can use such code : public void Install(IWindsorContainer container, IConfigurationStore store) { container.Register( Component.For(typeof(IRepository<>)).ImplementedBy(typeof(Repository<>)) } ); public class Repository<T> : IRepository<T> where T : class, IEntity { ... } Therefore I find registering and resolving generics pretty simple for registering and resolving generics with interfaces. There...

Why can a raw type reference refer to a generic instance? [duplicate]

java,generics,reference,generic-collections,raw-types

Map<Object,Object> map2 = new HashMap<String,String>(); As per my understanding Map map1 is same as Map<Object,Object> map1 No. A Map is not the same as a Map<Object,Object>. A reference of type HashMap<T,T> is a subtype of a reference of type Map. In other words, a reference of type Map can...

How to compose two lambdas of type “delegate” in c#

c#,.net,generics,lambda

Does this what you want?: Func<int, int> a = p0 => p0 << 1; Func<int, int, int> b = (p0, p1) => p0 + p1; Delegate da = a; Delegate db = b; var inner = da.Method.GetParameters().Length < db.Method.GetParameters().Length ? da : db; var outer = inner == da ?...

Reflection on IQueryable OfType<>

c#,entity-framework,generics,reflection

Try var obj = methodinfo.Invoke(null, new[] { employees }); The OfType is static, so null obj (the first parameter of Invoke, that is the instance of the object to use for the method)!...

Scala generic cast from String function

scala,generics,casting

I don't think there is a way to write the function as such with Manifest, ClassTag or a TypeTag as they can only tell you the type at runtime. For something like the pseudocode function to work, the return type must be known at compile time. However, could implement the...

Generics: Where T is TypeA OR TypeB

c#,generics,extension-methods,ienumerable

That is not possible and it wouldn't really make sense. You could never rely on T having certain members available because it could be A or B. All members are optional then. It certainly is conceivable to have this feature but it goes against the spirit of generics. This would...

Creating an extension method to wrap types as IEnumerables

Here is another attempt, inspired from Eric Lippert's excellent post at: http://stackoverflow.com/a/1451184/4955425. You can control the overloading resolution by placing your 2 extension methods at different levels in the namespace hierarchy. namespace MyExtensions { public static class HighPrecendenceExtensions { public static IEnumerable<TSource> WrapAsEnumerable<TSource>(this IEnumerable<TSource> source) { return source; } }...

Test if a list contains element of specific type in Swift

ios,swift,generics

You were so close! Use your second (nongeneric) implementation, and: if entity.dynamicType === type { ...

Scala generics: specializing a method

scala,generics

This may not be the answer you are looking for, as I haven't done much Haskell, but it's a possibility: You can define a trait that can only be mixed into specific instances of a tree: trait StringFooFunctionality { this: Tree[String] => // Selftype, can only be mixed in to...

Understanding a captured type in Java (symbol '?')

The capture of a wildcard type is a type that is used by the compiler represent the type of a specific instance of the wildcard type, in one specific place. Example: Take for example a method with two wildcard parameters, void m(Ex<?> e1, Ex<?> e2). The declared types of e1...

Java Generics missunderstanding

You got it the wrong way around. Each T is a Bar, but not each Baris a T. T is more specialized than Bar (each dachshound is a dog, but not each dog is a dachshound). This means, that return Optional.ofNullable(array[index]); tries to match a Bar on a T, which...

Generic method only compiles with one argument

generics,sharepoint-2010,.net-3.5

The problem is that when you call addShape(someColl, new Circle()); there are two different definitions of T ? extends Shape from Collection<? extends Shape> someColl Circle from the second parameter The other problem with that call is that T needs to be a concrete type for the second parameter, i.e....

I am getting a The type or namespace name 'listcolumns' could not be found (are you missing a using directive or an assembly reference?)

C# is a case-sensitive language. You defined a class ListColumns, but you're referring to it as listcolumns. That won't work....

How can I pass N number of generic arguments to a typedef function pointer?

c++,generics,typedef

In C++11, you can do something like this template<typename T> using fun_ptr = void (*)(T); And for the second case, template<typename... T> using fun_ptr = void (*)(T ...); ...

Java Generic Map with Generic Type as key/ values

java,generics,collections

You've got two options: Introduce V as a type parameter to the class. public class Test<V> Introduce V as a type parameter to that function. public <V> void genericTest(Map<String, V> params) ...

Java generics: Bound mismatch: The type is not a valid substitute for the bounded parameter of the type

java,eclipse,generics

The error message comes from the fact that R is not within its bounds. Your ViewModelHelper class extends AbstractViewModel<IRefreshPostViewCallback>, no matter what R1 really is. In the class ViewModelHelper, change the type argument in the extends clause of AbstractViewModel to R1, instead of IRefreshPostViewCallback. public abstract class RefreshPostViewModel<R1 extends IRefreshPostViewCallback>...

Scala: generic method using implicit evidence doesn't compile

scala,generics,types,implicit-conversion

You've just told the compiler that types passed to your class as T and S should be equal - you just require an evidence of their equality, which can be used to infer actual T and S correctly when you pass actual types (but not inside generic class itself). It...

Comparing Generics that are comparable in Java

java,generics,compare,comparable

I know the data value will be a object-wrapped primitive or a String, so they are comparable. Then you can tell the compiler about it: public class TreeNode<T extends Comparable<T>> If you do that, you will get access to the compareTo method defined in Comparable....

C# generics, cross referencing classes for state pattern

You can abuse interfaces and variance to achieve that: public interface IState<out TObject, in TState> where TObject : IStatefulObject<TObject, TState> where TState : IState<TObject, TState> { } public interface IStatefulObject<in TObject, out TState> where TObject : IStatefulObject<TObject, TState> where TState : IState<TObject, TState> { } public abstract class AbstractState<TObject> :...

How to create object/singleton of generic type in Scala?

scala,generics,singleton

Let me detalize Alexey's answer. Here is full implementation with some code style improvements: First define your trait with aknowledgment of it's covariance: trait Tree[+T] { def contains[U >: T : Ordering](num: U): Boolean def inc[U >: T : Ordering](num: U): Tree[U] } Next define your subtype-of-all-trees object case object...

Writing a generic method and the compiler expects T to be a known type [duplicate]

Your method signature needs a type parameter: public static Stack<T> ToStack<T>(this IEnumerable<T> source) As a sidenote, this won't compile once you fix that. You need to call the Count() method on IEnumerable: var returnStack = new Stack<T>(reversed.Count()); Also as @Servy points out in the comments, the check for whether this...

Ninject generic type xml binding

c#,xml,generics,ninject

You want to bind open generic types, so this type definition should do the trick: <bind service="Base.IJsonProvider`1, Base" to="Base.JsonProvider`1, Base" name ="Config"/> ...

Generic argument 'extends' multiple classes

c#,.net,generics,type-constraints

Here, you're defining two type-parameters: T extends Renderable Box Box is the alias of the second method-scoped type-parameter and if you have another one with the same name (class-scoped), the method-scoped one will hide it. That's why Eclipse throws a warning. If you want T to extend both Renderable and...

Pointer of generic type

c#,pointers,generics

To quote the C# 5.0 specification §18.2 Pointer Types Unlike references (values of reference types), pointers are not tracked by the garbage collector—the garbage collector has no knowledge of pointers and the data to which they point. For this reason a pointer is not permitted to point to a reference...

Generic constraint of X AND Y

Sounds like you're just missing the ability to specify both a class and interfaces as a comma-separated list in the constraints: static void INeedToDoBoth<T>(T dependency) where T : Dependency, IOptionalDependency Note that the class constraint must come first here. See the MSDN page for type parameter constraints or the C#...

How to refactor repeated code using reflection

c#,linq,generics,azure,reflection

Answering on original question how to rewrite this in generic way with reflection, so that method should not require changes for every new social network. If MySpace gonna be handled, you need just to add it to handlerMapping, HandleAnySocialNetworkByMapping() not changing (see code below). class Program { private readonly static...

Java generic class that contains an instance of implementation of generic interface

java,generics,interface,architecture

It looks like what you want is : public class B<S,T extends A<S>> { private T t; public S getTsSomething(){ return t.f1(); } } ...

Vector : References to generic type Vector should be parameterized

jsp,generics,vector

I don't think so these warnings would hang your IDE, these are harmless. And also it is always a best practice to specify the type for generics like Vectar<Object> or Vectar<String> or List<String> or ArrayList<String> etc and not use raw types. Please read from updated sources and books. It is...

Grabbing, storing and using all interfaces in the solution, when the interfaces are

Make the method generic. void TransformBiscuit<T>(List<T> biscuitBarrelBiscuits) This is assuming that a single instance of BiscuitTransformer will be able to transform any top of biscuits that you would pass in, of course....

How to tell Java that two wildcard types are the same?

java,generics,types

You need to use a type parameter to say that two 'unknown' types are the same. I thought that maybe you should change your method signature from: static <T> void sort(List<T> data, final Key<T, ?> key[], final int dir[]) to static <T,U> void sort(List<T> data, final Key<T,U> key[], final int...

Changing Type at Runtime with GenericTypeArgument

generics,inheritance,anonymous-function,nim

I solved like this; IList targetList = (IList)listPropertyInfo.GetValue(item); Type foo = targetList.GetType().GenericTypeArguments.Single(); Type unboundGenericType = typeof(READ<>); Type boundGenericType = unboundGenericType.MakeGenericType(foo); MethodInfo doSomethingMethod = boundGenericType.GetMethod("trigger"); object instance = Activator.CreateInstance(boundGenericType); doSomethingMethod.Invoke(instance, new object[] { targetList, f, properties }); ...

Define generic function type

The problem is that type class constraints for type parameters currently do not match subtypes; I do not know if that is a bug or intentional. For now, simply remove the : Base constraint from the type parameter T.

Implicit Reference Conversion Compliation Error with Generic Base class

c#,generics,covariance

Introducing an interface will allow you to take advantage of Covariance in the generic declaration. This will allow Building to be treated as Building which is ultimately what you are wanting to achieve. Note that only interfaces support this via the 'out' modifier: public class Person { public Person() {...

How to provide a default typeclass for generic types in Scala?

scala,generics,types,typeclass,type-parameter

You can provide an implicit function with the required type: implicit def SeqMonoid[T]: Monoid[Seq[T]] = new Monoid[Seq[T]] { override def op(a: Seq[T], b: Seq[T]): Seq[T] = a ++ b override def id: Seq[T] = Nil } ...

Why is my Rigidbody != null immediately after seeing that it is null?

c#,generics,unity3d,null

From other research (Equals(item, null) or item == null and Unity Forums) and to elaborate on usr's answer: I needed to guarantee that a Component was being passed through in my CheckIfNull header. The updated code looks like this: using UnityEngine; using System.Collections; public class NullChecker : MonoBehaviour { void...

How to code a generic Swift class which stores a Generator of the same type

ios,objective-c,xcode,generics

You have to use the generator type as the type placeholder G, and refer to its element type as G.Element: class MyGenericClass<G : GeneratorType> { var source : G var itemsProcessed : [ G.Element ] = [] init(source: G) { self.source = source } func getValue() -> G.Element? { let...

How can I find out the Objective-C generics type?

The lightweight generics introduced in Xcode 7 are just compile time hints to help the compiler raise warnings, but at run time you get the same old behavior with your variable being just NSArrays of ids. Source: WWDC '15 "Swift and Objective-C Interoperability" session See the transcript of the talk:...

Java Generics - Redundant Type Initialization [duplicate]

java,generics,equality,string-comparison,method-signature

This one: Box<Integer> integerBox = new Box(); assigns a raw Box to integerBox, bypassing generic type checks. While that might look fine in this case, it isn't so great when you have something like List<String> strings = Arrays.asList("a", "b", "c"); ... List<Integer> integers = new ArrayList(strings); If you do List<Integer>...

Why does the Java equals(Object O) method not have a variant which can take a specific object type (e.g. String, Integer, etc) as input?

Because equals() is declared in Object and compareTo(T foo) is defined in Comparable<T>. Before generics the issue was the same with Comparable, compareTo taking an Object argument, but since there's no "Equalable" interface there was no place to stick the generic parameter....

Restrict generic T to specific types

You can't restrict generics in that way, you can only choose a single class as a constraint. You must either make 5 overloads or find a interface all 5 of those things share and use that. Which option you choose will depend on what Validate doe. Here is how you...

How to create generic convenience initializer in Swift?

c#,database,generics,inheritance,abstract-class

As matt already explained, you cannot define an initializer which is restricted to types implementing a protocol. Alternatively, you could define a global function instead: public protocol Nameable { static func entityName() -> String } func createInstance<T : NSManagedObject where T: Nameable>(type : T.Type, context : NSManagedObjectContext) -> T {...

Creating a generic / abstract “DBContext” Class for shared functionality among different DBs

The key here is to step back and think about the problem from another angle. You are duplicating lots of code because you are creating instances of the database and command classes within the method. So inject them instead: public class SomeDBClass { static DataTable exec_DT(DBConnection conn, DBCommand cmd) {...

Cannot implicitly convert type from System.Collection.Generic.IEnumerable.MyClass to MyClass

c#,generics,neo4j,neo4jclient

You have several possibilites: change return type of getPath to IEnumerable... Use only first element of the query result: Add .FirstOrDefault() var res = pathsQuery.Results.FirstOrDefault(); Generally it is a good idea to set a breakpoint and put the mouse over var to inspect what type it is. Even better to...

Set only second argument type in generic method

c#,.net,generics,clr

C# Generics simply does not allow you to specify a subset of the type parameters. It's all or nothing. The way to work around this is to write a fluent interface. You break this operation up into a chain of methods. public class FirstPropertyWithTypeSelector<T> { private readonly IQueryable<T> _source; public...

Using a Type Variable in a Generic

java,android,generics,android-studio,java-generics

Swift's static typing means the type of a variable must be known at compile time. In the context of a generic function func foo<T>() { ... }, T looks like a variable, but its type is actually known at compile time based on where the function is called from. The...

Instantiate generic c# List when the reflected type itself is a List

c#,asp.net,list,generics,reflection

Have a look at this example, it uses the Type.GetGenericArguments Method to retrieve the lists inner type. Then proceed with reflection as usual. static void Main(string[] args) { Type modelType = GetMyType(); var myList = Activator.CreateInstance(modelType); var listInnerType = modelType.GetGenericArguments()[0]; var listInnerTypeObject = Activator.CreateInstance(listInnerType); var addMethod = modelType.GetMethod("Add"); addMethod.Invoke(myList, new[]...

Register return Type

c#,generics,return-type

If you know the type at startup, you could just derive the class: public class UserLogin : GenericLogin<ABC01_REGISTERED_USER> { } Then use that class all along. Else, you have to supply the type name every time, since else it can't know that you want to use that type every time....

Why can't we capture wildcards for the method with two parameters?

java,generics,wildcard

In this method private <T> int comp(Comparable<T> upper, Comparable<T> lower){ return upper.compareTo((T) lower); } both of the parameters share the same type-parameter. Meanwhile, this is not true for the other method: private int compare (Comparable<?> upper, Comparable<?> lower){ return comp(upper, lower); } Here, the compiler has no evidence that the...

Java infinitely recursive self referential types

java,generics,recursion,types

You'll need a type parameter for the Collection element type, potentially a type parameter for the actual Collection type if you need it, and a type parameter for the values. class SubClass<E, K extends Collection<E>, V> implements Map<K, V> { ... } If you don't need the specific Collection type,...

Can't figure out how to instantiate generic class returned from function with condition like

You're using the raw type Class. Class c = GiftdServiceLoader.getClassForInterface(APIAsyncLoaderTask.class); All generatic types and generic type parameters in any method invoked on such a reference are erased. Parameterize it correctly Class<APIAsyncLoaderTask> c = MyServiceLoader.getClassForInterface(APIAsyncLoaderTask.class); Note that you should use only the interface type if you aren't sure of what you'll...

Java template function

This is one of the tricky areas of Generics. The only way to get this to work would be to take a Class argument, so the method knows what type of object to create. It can't know at the moment, because of Type Erasure. Alternatively (much simpler) is to always...

Creating a view of multiple Maps' entrySets

This is technically doable with the unsafe raw cast Set<Map.Entry<K, V>> unionSet = ImmutableSet.of(); for(Map<? extends K, ? extends V> map : chain) { unionSet = Sets.union((Set) map.entrySet(), unionSet); } ...and that is almost certainly the best way to provide a live view of the union of the entry sets....

Arrays, List and Generics in Java

java,function,generics,java-8,overloading

This isn't something you're going to want to do often (since generics and arrays don't mix well), but there are some important things to make note of: T holds the type information for the method. You may not be passing in a City at all times, and in the off...

Reversing a Generic Array using “in situ”

java,arrays,algorithm,generics,reverse

If you mean that you want to do it manually without using Collections.reverse (that code shouldn't even compile btw, reverse needs a List as parameter), just traverse the array from the beginning to the middle swapping elements at the opposite sides of the range: public E[] reverse() { for (int...

Deserializing Generic Types from a ClassLouder class with GSON

java,json,generics,reflection,gson

see here a default implementation of ParametrizedType: http://www.java2s.com/Code/Java/Generics/DefaultimplementationoflinkjavalangreflectParameterizedType.htm properly prepared, such a type could be used to represent a generic type, which may work with GSON. Without having tested it, your code might then look like this: File root = new File("./build/classes"); URLClassLoader classLoader = URLClassLoader.newInstance(new URL[] { root.toURI().toURL() });...

Generic method to perform a map-reduce operation. (Java-8)

The example you present in your question has got nothing to do with Java 8 and everything to do with how generics work in Java. Function<T, Integer> function and Function<T, Double> function will go through type-erasure when compiled and will be transformed to Function. The rule of thumb for method...

unchecked call to … as member of the raw type

java,generics,compiler-warnings,unchecked

Your types are raw - that is, your generic types are bonded to a type that itself has a type, but you haven't provided one, so it's raw. Change your type bounds to be typed. Try this: public abstract class AbstractPresenter<V extends AbstractView<V>, M> implements Presenter<V, M> and public abstract...

Is there a generic way to negate a decltype condition with SFINAE?

c++,templates,c++11,generics,sfinae

In essence, the equivalent of if ... else if with SFINAE. You can manually control overload resolution with an extra parameter: template<int I> struct rank : rank<I-1> { static_assert(I > 0, ""); }; template<> struct rank<0> {}; template <class A, class B> auto foo(A& a, B& b, rank<10>) ->...

Generic TypeCode Type Checking?

You can make your code generic with something like this: var size = Marshal.SizeOf(typeof(T)); var subBuffer = new byte[size]; Array.Copy(Buff, Peek, subBuffer, 0, size); var handle = GCHandle.Alloc(subBuffer, GCHandleType.Pinned); var ptr = handle.ToIntPtr(); var val = (T)Marshal.PtrToStructure(ptr, typeof(T)); ptr.Free(); Peek += size; Peek = ( Peek + ( Align -...

Confused about the following 'generics' code

<T> is not a return type. It's a declaration of the generic type parameter used by the method. Once it's declared, it can be used in the method signature without the <>;

Swift generic constructor