overriding

问题 A virtual function's return type should be the same type that is in base class, or covariant. But why do we have this restriction? 回答1: Because of the nonsense that would ensue: struct foo { virtual int get() const { return 0; } }; struct bar : foo { std::string get() const { return "this certainly isn't an int"; } }; int main() { bar b; foo* f = &b; int result = f->get(); // int, right? ...right? } It isn't sensible to have a derived class return something completely unrelated. 回答2: Because

问题 While reading on Covariant Overriding, i find out very strange fact, covariant method overriding is implemented using a bridging technique. it also said that this feature is implemented in java5 and above.(i think it is because generics introduced from java5) How it happens.Please help me with example. 回答1: Consider an example: public interface Shape<T extends Shape<T>> { T getType(); void setType(T type); } public class Circle implements Shape<Circle> { Circle getType() { } void setType

问题 I'm trying to understand GAS's behavior of .code16. From the manual, it seems in 16-bit section, for 32-bit operands or instructions, a 66H operand override prefix will be produced for the instruction encoding. Does that mean .code16 movw %eax, %ebx is legal in such mode? Then the code cannot run on 16-bit processor? 回答1: These are legal instructions for 80386+. Starting with the 80386 we can use operandsize- and addresssize- override prefixes. Those prefixes can be used in combination with

问题 Say you have a data-structure (borrowed from this question): data Greek = Alpha | Beta | Gamma | Delta | Eta | Number Int Now one can make it an instance of Show by appending deriving Show on that instruction. Say however we wish to show Number Int as: instance Show Greek where show (Number x) = show x -- ... The problem is that one must specify all other parts of the Greek data as well like: show Alpha = "Alpha" show Beta = "Beta" For this small example that's of course doable. But if the

问题 I have two question on this code public class Override { private void f() { System.out.println("private f()"); } public static void main(String[] args) { Override po = new Derived(); po.f(); } } class Derived extends Override { public void f() { System.out.println("public f()"); } } /* * Output: private f() */// :~ 1) How is function f is visible on the reference of Override po; 2) Why is output "private f()" 回答1: The main method is inside class Override , so ofcourse it can see the private

问题 I encounter a problems about override virtual functions, in fact,it is about hessian (a web service protocol). it has a base class Object, and some derived classes : Long,Int,String,...,all derived classes has a no-virtual function "value" class Object { ... }; class Long :public Object { ... public: typedef long long basic_type; basic_type value(){return value_;} private: basic_type value_; ... }; class Int :public Object { ... public: typedef int basic_type; basic_type value(){return value_

问题 This question already has answers here : Closed 7 years ago . Possible Duplicate: Why is super.super.method(); not allowed in Java? Let's assume I have 3 classes A , B and C , each one extending the previous one. How do I call the code in A.myMethod() from C.myMethod() if B also implements myMethod ? class A { public void myMethod() { // some stuff for A } } class B extends A { public void myMethod() { // some stuff for B //and than calling A stuff super.myMethod(); } } class C extends B {

问题 I'm using a time picker to let the user enter his desired time to do a specific task, I'm using the DialogFragment class that's available in the support library for backward compatibility with older Android versions. Here is my code to create the TimePickerFragment class, created in a seperate file, taken from: http://developer.android.com/guide/topics/ui/controls/pickers.html : package com.calls.only; import java.util.Calendar; import android.app.Dialog; import android.app.TimePickerDialog;

问题 Best practice to Override a class method of the gem in rails Application ? . I need to override the behaviour of the find method of a gem. following is the code in the gem module Youtube class display attr_accessor :base def find(id, options = {}) detailed = convert_to_number(options.delete(:detailed)) options[:detailed] = detailed unless detailed.nil? base.send :get, "/get_youtube", options.merge(:youtube_id => id) end end end How do i override the above find method in my YoutubeSearch

问题 Can anybody tell the working of overriding and hiding in terms of memory and references. class A { public virtual void Test1() { //Impl 1} public virtual void Test2() { //Impl 2} } class B : A { public override void Test1() { //Impl 3} public new void Test2() { Impl 4} } static Main() { A aa=new B() //This will give memory to B aa.Test1(); //What happens in terms of memory when this executes aa.Test2(); //-----------------------SAME------------------------ } Here memory is with class B but in