inheritance

问题 here is my code: class package { protected: string name; string city; string state; int zip; double weight; double costPerOunce; public: package::package(string Name, string City, string State, int Zip, double Weight, double CostPerOunce): name(Name), city(City), state(State), zip(Zip), weight(Weight), costPerOunce(CostPerOunce) { } double calculateCost() { return (weight * costPerOunce); } }; class twoDayPackage: public package { protected: double flatFee; public: twoDayPackage:

问题 Just curious, is it ever possible to inherit from a template class and in constructor of the derived class, call constructor of the base class which is also templated and has no arguments to deduce its types from? template<typename T> struct Base { template<typename D> Base() { // no argument of type D to infer from static_assert(std::is_same<T,D>::value, ""); } }; struct Derived : Base<int> { Derived() : Base<int>::Base<int>() {} // is there a way to write it correctly? }; I can replace

问题 I've found this all over the place in this code: public enum Blah: int { blah = 0, blahblah = 1 } Why would it need to inherit from int? Does it ever need to? 回答1: According to the documentation: Every enumeration type has an underlying type, which can be any integral type except char. The default underlying type of the enumeration elements is int. So, no, you don't need to use int. It would work with any integral type. If you don't specify any it would use int as default and it's this type

问题 I have three tables: organization , organization_teams and org_users . Here organization_teams is inherited from organization . So suppose if a record is added in organizations_teams it will get the organization table id as value for id column in organization_teams . org_users has foreign key on id column of organization . Now when I try to insert data in org_users it giving me error as below insert or update on table "org_users" violates foreign key constraint "org_users_organizations"

问题 Are there rules of thumb that help determine which to use in what case? Should I prefer one over the other most times? Thanks! 回答1: Extension methods are useful, but they are harder to discover through the IDE than regular methods, since they are not attached to the original class and there are no clues as to where the code for them might reside. There are some best practice suggestions as to where to put them and how to name them, but these are only guidelines and there is no guarantee that

问题 Are there rules of thumb that help determine which to use in what case? Should I prefer one over the other most times? Thanks! 回答1: Extension methods are useful, but they are harder to discover through the IDE than regular methods, since they are not attached to the original class and there are no clues as to where the code for them might reside. There are some best practice suggestions as to where to put them and how to name them, but these are only guidelines and there is no guarantee that

问题 I am extending a base class (one which I do not control) in Swift. I want to provide a class function for creating an instance typed to a subclass. A generic function is required. However, an implementation like the one below does not return the expected subclass type. class Calculator { func showKind() { println("regular") } } class ScientificCalculator: Calculator { let model: String = "HP-15C" override func showKind() { println("scientific") } } extension Calculator { class func create<T

问题 I am extending a base class (one which I do not control) in Swift. I want to provide a class function for creating an instance typed to a subclass. A generic function is required. However, an implementation like the one below does not return the expected subclass type. class Calculator { func showKind() { println("regular") } } class ScientificCalculator: Calculator { let model: String = "HP-15C" override func showKind() { println("scientific") } } extension Calculator { class func create<T

问题 I have classes like: Person { Name Address } Employee : Person { Compensation - object } Visitor : Person { } If I write linq: var persons = Context.Persons .Include("Compensation"); I get error: A specified Include path is not valid. The EntityType 'Person' does not declare a navigation property with the name 'Compensation'. It works ok if I do: var persons = Context.Persons .OfType<Employee>() .Include("Compensation"); But I would like to get Employees and visitors in the same query. Looks

问题 I have classes like: Person { Name Address } Employee : Person { Compensation - object } Visitor : Person { } If I write linq: var persons = Context.Persons .Include("Compensation"); I get error: A specified Include path is not valid. The EntityType 'Person' does not declare a navigation property with the name 'Compensation'. It works ok if I do: var persons = Context.Persons .OfType<Employee>() .Include("Compensation"); But I would like to get Employees and visitors in the same query. Looks