constructor

问题 The following code throws std::bad_cast struct Foo { void foo () {} }; struct Bar { Bar () { dynamic_cast <Foo &> (*this) .foo (); } virtual ~ Bar () {} }; struct Baz : public Foo, public Bar { }; int main () { Baz b; } I remember once reading how dynamic_cast has implementation performance trade-offs because "it traverses the full inheritence lattice" in order to evaluate correctly. What the compiler needs to do here is first cast up and then down again. Is it possible to make the above work

问题 This question is very similar to this one Why can't I dynamic_cast "sideways" during multiple inheritence?, except that the cast does work - just not inside in the constructor. Header: class A { public: virtual ~A() {} void printA(); }; class B { public: B(); virtual ~B() {} void printB(); private: std::string message_; }; class C : public A, public B { public: C() {} virtual ~C() {} }; Source: void A::printA() { cout << "A" << endl; } B::B() { A* a = dynamic_cast< A* >( this ); if ( a ) {

问题 public class UserAction { private final UUID uuid; private String userId; /* more fields, setters and getters here */ public UserAction(){ this.uuid = UUID.fromString(new com.eaio.uuid.UUID().toString()); } public UserAction(UUID uuid){ this.uuid = uuid; } @Override public boolean equals(Object obj) { if (obj == null) { return false; } if (getClass() != obj.getClass()) { return false; } final UserAction other = (UserAction) obj; if (this.uuid != other.uuid && (this.uuid == null || !this.uuid

问题 I've been trying to call Entity Manager in a constructor: function __construct() { $this->getDoctrine()->getEntityManager(); ... but, as I've seen in this answer: Stackoverflow question, it can't be done. So I wonder if there is a way to achieve it, as I have to call it often, and want to do some stuff in the constructor after getting the repository. Edit: I've tried with @MKhalidJunaid answer: //src/MSD/HomeBundle/Resources/config/services.yml services: imageTransController.custom.service:

问题 Why throwing exception in constructor results in a null reference? For example, if we run the codes below the value of teacher is null, while st.teacher is not (a Teacher object is created). Why? using System; namespace ConsoleApplication1 { class Program { static void Main( string[] args ) { Test(); } private static void Test() { Teacher teacher = null; Student st = new Student(); try { teacher = new Teacher( "", st ); } catch ( Exception e ) { Console.WriteLine( e.Message ); } Console

问题 In new laravel I can't get session in constructor. Why? public function __construct() { dd(Session::all()); //this is empty array } and then below public function index() { dd(Session::all()); //here works } In old laravel i remember there was not this problem. something changed? 回答1: You can't do it by default with Laravel 5.3. But when you edit you Kernel.php and change protected $middleware = []; to the following it wil work. protected $middleware = [ \Illuminate\Foundation\Http\Middleware

问题 After debugging a particularly tricky issue in VB.NET involving the order in which instance variables are initialized, I discovered that there is a breaking discrepancy between the behavior that I expected from C# and the actual behavior in VB.NET. Nota bene: This question concerns a slight discrepancy in the behaviors of VB.NET and C#. If you're a language bigot that is unable to provide an answer other than "that's why you should use C#, noob" , there is nothing for you to see here; kindly

问题 Background: I have a MainTest class that has many buttons, each of which instantiate a class that I am coding/testing. I want the code/test cycle for these classes to be quick, and see the effect of my changes quickly, a few times a minute. MainTest which is stable takes about 20 seconds to load, which would not be a problem had I not needed to reload it for each change in the classes it instantiates. I want to load MainTest once, and when it instantiates another class, let's call it

问题 I've not been able to find a conclusive answer to this so far. When is it safe to call this-> from within an object. And in particular from inside the constructor and destructor. And also, when using public inheritance. Is it safe to use up and downcasting on the result of the this call? So for example: class foo { foo(): a(), b(this->a)//case 1 { this-> a = 5; //case 2 } int a; int b; }; class bar: public baz { bar(): baz(this)//case 3 - assuming baz has a valid constructor { } } And finally

问题 In Java, Why am I getting this error: Error: The constructor WeightIn() is undefined Java Code: public class WeightIn{ private double weight; private double height; public WeightIn (double weightIn, double heightIn){ weight = weightIn; height = heightIn; } public void setWeight(double weightIn){ weight = weightIn; } public void setHeight(double heightIn){ height = heightIn; } } public class WeightInApp{ public static void main (String [] args){ WeightIn weight1 = new WeightIn(); //Error