instance

问题 Let's say I have class SuperClass { public: int a; } and class SubClass : SuperClass { public: int b; } and I took a pointer to an instance of the SubClass SubClass *subPointer and addressed that pointer to a SuperClass pointer SuperClass *superPointer = subPointer . Now of course I can always cast the superPointer object to a pointer of SubClass because the only thing it stores is an adress. But how would I know if the object superPointer is pointing to an instance of SubClass or is just a

问题 I have created a custom class in xcode: PaperPack and defined 2 instant variables: title and author - Then I alloc 2 instances of the class as below: PaperPack *pack1 = [[PaperPack alloc] init]; pack1.title = @"Title 1"; pack1.author = @"Author"; PaperPack *pack2 = [[PaperPack alloc] init]; pack1.title = @"Title 2"; pack1.author = @"Author"; Then how do I count and return number of instances I have created with that class? 回答1: No you can not get directly. Whenever you create in instance add

问题 I just came across the following paragraph in the AppEngine documentation for Query Cursors: An interesting application of cursors is to monitor entities for unseen changes. If the app sets a timestamp property with the current date and time every time an entity changes, the app can use a query sorted by the timestamp property, ascending, with a Datastore cursor to check when entities are moved to the end of the result list. If an entity's timestamp is updated, the query with the cursor

问题 I have a problem in understanding of the notion of static variables in Python classes. According to Static class variables in Python, whenever we define a variable outside a method and inside a python class, this variable is static. This means this variable can be accessed without any need to instantiate an object from a class and can be accessed by the class name directly. For example: class my_class: i=12 def __init__(self,j): self.j=j instance=my_class(10) my_class.i: >12 instance.i: >12

问题 On the stage, I have 3 MovieClips who have the same instance name, which is zeroMC but all three are an instance of different MovieClips. The first zeroMC is an instance of blank1,the second zeroMC is an instance of blank2 and the third zeroMC is an instance of blank3. I want to make all three movieclips gotoAndStop at 2, but when I do zeroMC.gotoAndStop(2); only one goes to and stops at 2. I also tried var containers = [zeroMC, zeroMC, zeroMC]; for (var i:int = 0; i<containers.length; i++) {

问题 I'm new to PostgreSQL and I'm struggling to understand how to use a reference to an 'instance' (or row) from a table as a value within a row of another table. Here is my desired result: class User{ int age; Post[] posts; } class Post{ int postId; ... } // Sql script sqlMain{ User k = new User(20); k.addPost(10, ...); } As you can see, I want an (dynamic prefereably, like an ArrayList) array of posts as an attribute of a user. So far I have the following Script: CREATE TABLE Post( postId INT )

问题 I have a project that has 3 View/ViewModels (Until now of course!). So I have a ListView in MainView to show each one of them, and could be selected by the user. So I used something like this: class MainViewModel :INotifyPropertyChanged { public ObservableCollection<BaseViewModel> obv { get { return this._obv; } } public MainViewModel() { pvm = new PViewModel(); lvm = new LViewModel(); svm = new SViewModel(); cvm = new CViewModel(); ivm = InstanceViewModel.Instance; obv.Add(pvm); obv.Add(lvm)

问题 Is there a keyword to refer to the current class type similar to how this refers to the current instance? My intention is to avoid having to type the full class name when refering to static members or Enums but I want to prefex it with something like "this" for readability like I do for instance members. The reason being some of the class names are pretty huge and cause excessive wrapping. 回答1: I don't think there's a keyword, but maybe as good: this.GetType(); GetType is one of the few

问题 In the middle of Mozilla documentation page it switches (without clear enough explanation) examples from WorkerBee.prototype = Object.create(Employee.prototype); to WorkerBee.prototype = new Employee; Second form new Employee will initialize prototype of WorkBee with properties that are supposed to exist only in instances of WorkBee. Why is second form being used in so many examples of JavaScript inheritance ? Isn't it undesired to give different status to inherited properties vs own

问题 In the middle of Mozilla documentation page it switches (without clear enough explanation) examples from WorkerBee.prototype = Object.create(Employee.prototype); to WorkerBee.prototype = new Employee; Second form new Employee will initialize prototype of WorkBee with properties that are supposed to exist only in instances of WorkBee. Why is second form being used in so many examples of JavaScript inheritance ? Isn't it undesired to give different status to inherited properties vs own