instantiation

问题 Let's say i acquire the name of a class that i made as a String . How can i Instantiate the class with the name contained in that string? I I know it will be derived from a certain parent class, but the actual class will vary. 回答1: var instance : MyClass = Type.createInstance(Type.resolveClass("path.to.MyClass"), []); Few notes: resolveClass() takes the full path (packages included) of the classe you need createInstance() takes as the second argument an array of values that are applied to the

问题 I got the errors compiling this code with g++ 4.6 and 4.8. g++ 4.2 and 4.4 is OK. Is it a bug or some new language feature? template <typename T> struct A { typedef typename T::value_type type; }; template <typename U> struct B { void bar () { } void foo () { // OK this->bar (); // OK (*this).bar (); // Error in g++ 4.6-4.8 // leads to full instantiating of template arg "U" (&*this)->bar (); } }; int main () { B< A<void> > b; b.foo (); return 0; } g++ inst.cc inst.cc: In instantiation of

问题 I want to instantiate a user control programmatically in a DLL to save it afterwards as PNG file. This is generally no problem with PngBitmapEncoder and RenderTargetBitmap. This are my questions: How do I instantiate the control? Simply with the new-operator? Do I have to instantiate it in an seperate thread? How do I force the control to update all its children and to render itself again? This is my code to instantiate the user control and save it as PNG-file (LetterFrequency is the user

问题 I (vaguely) know that a template is not instantiated if it is not used . For example, the following code will compile fine even though T::type doesn't make sense when T = int . template<typename T> struct A { void f() { using type = typename T::type; } }; A<int> a; //ok It compiles because f() is not used , so it is not instantiated — thus the validity of T::type remains unchecked. It doesn't matter if some other member function g() calls f() . template<typename T> struct A { void f() { using

问题 Is it possible to dynamically instantiate a class using a variable? For example is something like this possible in PHP? class foo { public $something; } $class_name = "foo"; $f = new $class_name(); 回答1: That should work, yes. You can also do: $f = new $class($arg1,$arg2); 回答2: Yes, this code will work fine. 回答3: In PHP 5 can I instantiate a class dynamically? Yes you can, your code should work fine. 回答4: Yes of course you can instantiate using dynamic names; 来源： https://stackoverflow.com

问题 I could instantiate class from string in ObjC.For example,I have defined new DBCell class through subclassing UITableViewCell,and I instantiate class from name with these codes: DBCell *cell=[tableView dequeueReusableCellWithIdentifier:cellClassname]; if (cell==nil) { Class cellClass=NSClassFromString(cellClassname); cell=[cellClass alloc]; cell=[cell initWithStyle:cellStyle reuseIdentifier:cellClassname]; } Now I need migrate codes to Swift,I have redefined DBCell class in Swift: class

问题 I need to perform a datamigration of a model Answer in app Question . In that script there is a dependency such that I need to create an instance of a model Chapter which is in the app Journal . So, I coded it as follows: def forwards(self, orm): for answer_object in orm.Answer.objects.all(): #This Works. blog, is_created = orm['blog.Post'].objects.get_or_create(title=answer_object.answer[:100]) blog.save() #This DOES NOT work chapter, is_created = orm['journal.Chapter'].objects.get_or_create

问题 My full code is too long, but here is a snippet that will reflect the essence of my problem: class BPCFGParser { public: ... ... class Edge { ... ... }; class ActiveEquivClass { ... ... }; class PassiveEquivClass { ... ... }; struct EqActiveEquivClass { ... ... }; struct EqPassiveEquivClass { ... ... }; unordered_map<ActiveEquivClass, Edge *, hash<ActiveEquivClass>, EqActiveEquivClass> discovered_active_edges; unordered_map<PassiveEquivClass, Edge *, hash<PassiveEquivClass>,

问题 I have a class an abstract class Room which has subclasses Family and Standard , I have created room = new ArrayList<Room>(); within a Hostel class. I have a method to add a room to the ArrayList; public String addRoom(String roomNumber, boolean ensuite) { if (roomNumber.equals("")) return "Error - Empty name field\n"; else room.add( new Room(roomNumber,ensuite) ); return "RoomNumber: " + roomNumber + " Ensuite: " + ensuite + " Has been added to Hostel " + hostelName; } However I get the

问题 #include <map> #include <iostream> template <typename T> class A { static std::map<int, int> data; public: A() { std::cout << data.size() << std::endl; data[3] = 4; } }; template <typename T> std::map<int, int> A<T>::data; //std::map<int, int> A<char>::data; A<char> a; int main() { return 0; } What is wrong with this? Without explicit instantiation it breaks at data[3] = 4; Explicit instantiation solves the problem but the program breaks after std::cout << data.size() << std::endl; what means