constructor

问题 Temp1 t1=new Temp2(); Here Temp1 is superclass of Temp2. The code works perfectly fine and t1 do acts as a reference variable for Temp1 but how can Temp2() works as constructor for Temp1? 回答1: This is the basis for polymorphism: Imagine you have several child classes that inherit from you parent class. You want to use all these child classes through the interface / methods defined on your parent class, without worrying about the implementation details in each child class (each might do

问题 I migrated a project from Qt4 to Qt5, and it compiles and everything but the application crashes before it even reaches the main function. I know there is a null value that fucks up something at some point, maybe a file that cant find or something, but there are so many .cpps and .h and libraries that its pretty hard to locate the source of the error plus I cant set any breakpoints. I have a lot of debugging data so maybe any of you can guide me in the right direction. I dont know what I

问题 We can call a constructor using qualified-name, although the constructor doesn't have a name. Indeed 3.4.3.2/2: In a lookup in which function names are not ignored and the nested-name-specifier nominates a class C: — if the name specified after the nested-name-specifier, when looked up in C, is the injected-class-name of C (Clause 9), or [...] the name is instead considered to name the constructor of class C. Consider the following example: #include <iostream> using std::cout; using std::endl

问题 I'm having problems with this assignment I have to do for my java programming class. What I have to do is perform a chroma key technique on an image with a solid background. More info can be found here. The method I created works when I put the two object parameters for the method. But, when I put the parameters in the constructor and then try to use the method, I get a compiler error. I have my class down below (I have to use two different classes, one for methods, one for testing). Any help

问题 Excuse my terminology if it's off. I don't understand the difference between: function Person() {}; Person.walk = function() {}; and... function Person() {}; Person.prototype.walk = function() {}; It seems that the second way is the convention for constructors, but I don't understand the difference and why it is done that way. Thanks! 回答1: In the first case: function Person() {}; Person.walk = function() {}; You will be able to call the function only with: Person.walk(); If you create an

问题 hey i saw this topic here: Can I call methods in constructor in Java? My Question is smilar, but a little bit different. I want to know whats happening, if i call from a constructor from class A something outside of that from class B with this from A. In my example below A is JobImpl , and B is that queue in Scheduler. class JobImpl implements Job { public JobImpl(Scheduler s, JobHandler runJob, boolean advancedWaitOnCancel, boolean oneShot) { this.s = s; this.runJob = runJob; this

问题 Everything went fine while compiling in debug mode. But while compiling in Release configuration, following error occurs : Error:(28) Error: This class should provide a default constructor (a public constructor with no arguments) (myclassname.HelpDialog) [Instantiatable] This is the code public class HelpDialog extends Dialog { Activity mActivity; Button btn_go_back; public HelpDialog(Activity mactivity) { super(); this.mActivity = mactivity; } @Override protected void onCreate(Bundle

问题 This is how i am routing to next page , '/ot1': (context) => CustomListView(), and it was working fine onTAP onTap: (){ Navigator.pushNamed(context, '/ot1'); }, but when i created constructor in class CustomListView and passed field i got this error that one argument is missing in this line '/ot1': (context) => CustomListView(), this is the code of my class CustomListView share below class CustomListView extends StatelessWidget { final List<Spacecraft> spacecrafts; CustomListView(this

问题 This question already has answers here : Default constructor with empty brackets (9 answers) Closed 4 years ago . I'm pretty new to c++, i'm now trying to learn all the basics, I know when default constructors are called, but when i tried different syntax it doesn't work like i expected. Look at the following code: class a; class b(); class c(NULL); 'class' is a class i created with default constructor, for a and c everything works well, but for b it just won't recognize the variable as a

问题 This java program is easy and full of comment,so you can understand it fast.however,why in construct staff[1],the program first go to the statement: this("Employee #" + nextId, s); then go to the object initialization block,and then go back to the statement,how confusion.why not it first use the object initialization block import java.util.*; public class ConstructorTest { public static void main(String[] args) { // fill the staff array with three Employee objects Employee[] staff = new