abstract-class

问题 I have this models: class BillHeader(models.Model): billno = models.IntegerField(primary_key=True, blank=True) class BillData(models.Model): price = models.DecimalField(_('Price'), max_digits=12, decimal_places=2) amount = models.DecimalField(_('Amount'), max_digits=6, decimal_places=2) [... rest of the model ...] class Meta: abstract = True class BillFooter(models.Model): total = models.DecimalField(_('Total'), max_digits=12, decimal_places=2) [... rest of the model ...] class Meta: abstract

问题 I have a class that inherits a base class to which another class has relationships. Example: Base class: Animal Subclass 1: Dog Subclass 2: Cat Related one-to-many table: Vaccinations A dog can have multiple vaccinations. This is implemented as a List<Vaccination>. A cat can have multiple vaccinations. A vaccination record can only have one animal associated with it. A vaccination doesn't know if it's associated with a dog or a cat. (Dogs and cats use non-colliding GUIDs.) There is no Animal

问题 I have a class that inherits a base class to which another class has relationships. Example: Base class: Animal Subclass 1: Dog Subclass 2: Cat Related one-to-many table: Vaccinations A dog can have multiple vaccinations. This is implemented as a List<Vaccination>. A cat can have multiple vaccinations. A vaccination record can only have one animal associated with it. A vaccination doesn't know if it's associated with a dog or a cat. (Dogs and cats use non-colliding GUIDs.) There is no Animal

问题 This question already has answers here : Closed 8 years ago . Possible Duplicate: Why can’t I create an abstract constructor on an abstract C# class? How can I write one abstract class that tells that is mandatory for the child class to have one constructor? Something like this: public abstract class FatherClass { public **<ChildConstructor>**(string val1, string val2) { } // Someother code.... } public class ChildClass1: FatherClass { public ChildClass1(string val1, string val2) { // DO

问题 Consider the Container class, which basically stores a vector of unique_ptr s of Box objects and can perform some computations on them. class Container { private: std::vector<std::unique_ptr<Box> > boxes_; public: Container(std::vector<std::unique_ptr<Box> > &&boxes): boxes_(std::move(boxes)){} double TotalVolume() { /* Iterate over this->boxes_ and sum */ } }; Here, Box is an abstract class that has a pure virtual method such as double Box::Volume() . Now, suppose that I instantiate a

问题 Consider the Container class, which basically stores a vector of unique_ptr s of Box objects and can perform some computations on them. class Container { private: std::vector<std::unique_ptr<Box> > boxes_; public: Container(std::vector<std::unique_ptr<Box> > &&boxes): boxes_(std::move(boxes)){} double TotalVolume() { /* Iterate over this->boxes_ and sum */ } }; Here, Box is an abstract class that has a pure virtual method such as double Box::Volume() . Now, suppose that I instantiate a

问题 So while trying to understand abstract classes, there is still one thing I am confused on. When do you ever want to declare an object type of its abstract class. For example public abstract class GameObject { public abstract void draw(); public static void main(String[] args) { GameObject player = new Player(); Menu menu = new Menu(); } } public class Player extends GameObject { override public void draw() { // Something } } public class Menu extends GameObject { override public void draw() {

问题 So while trying to understand abstract classes, there is still one thing I am confused on. When do you ever want to declare an object type of its abstract class. For example public abstract class GameObject { public abstract void draw(); public static void main(String[] args) { GameObject player = new Player(); Menu menu = new Menu(); } } public class Player extends GameObject { override public void draw() { // Something } } public class Menu extends GameObject { override public void draw() {

问题 This question already has answers here : What is object slicing? (18 answers) Closed 3 years ago . Hey guys here is some code I am going to run, issue is it doesn't work the way I intend to. I am unable to figure out what's wrong with it. I am c++ noob please help. #include <iostream> #include <cmath> #include <stdexcept> using namespace std; /** * Super class */ class Shape { protected: int _dimensions; public: Shape() : _dimensions{0} {}; Shape(int dims) : _dimensions{dims} {}; virtual

问题 This question already has answers here : What is object slicing? (18 answers) Closed 3 years ago . Hey guys here is some code I am going to run, issue is it doesn't work the way I intend to. I am unable to figure out what's wrong with it. I am c++ noob please help. #include <iostream> #include <cmath> #include <stdexcept> using namespace std; /** * Super class */ class Shape { protected: int _dimensions; public: Shape() : _dimensions{0} {}; Shape(int dims) : _dimensions{dims} {}; virtual