constructor

问题 I've put this code in the compiler package com.employer.constractor; public class ConstractorDemo extends A{ public ConstractorDemo(){ System.out.print("Demo"); } public static void main(String[] args){ new ConstractorDemo(); } } class A { A(){ System.out.print("A"); } } And it gave "ADemo" why? I'll appreciate any detailed answer for this case and mention how compiler will deal exactly with that 回答1: The child constructor is invoked first. The first line in the child constructor will be a

问题 I am getting problems in initializing the nested class constructor. Here is my code: #include <iostream> using namespace std; class a { public: class b { public: b(char str[45]) { cout<<str; } }title; }document; int main() { document.title("Hello World"); //Error in this line return 0; } The error I get is: fun.cpp:21:30: error: no match for call to '(a::b)' 回答1: You probably want something like: class a { public: a():title(b("")) {} //.... }; This is because title is already a member of a ,

问题 I'm having issues using a private method inside a private class inside a public class using the reflections api. Here's a simplified code example: public class Outer { private class Inner { private Integer value; private Inner() { this(0); } private Inner(Integer y) { value = y; } private Integer someMethod(Integer x) { return x * x; } } } Again, I want to be able to instantiate an Outer class object then call someMethod from the private Inner class. I've been trying to do this with

问题 I have a Prim class that implements an IPrimitiveDecomposer interface for MEF and inherits Node base class. public class Node { public Node() { } } public interface IPrimitiveDecomposer { bool Match(Node node); } [Export(typeof(IPrimitiveDecomposer))] public class Prim : Node, IPrimitiveDecomposer { public bool Match(Node node) {return true;} } However, when I inherit a class that doesn't have a parameterless constructor, MEF's ComposeParts() method cannot import the Prim object. I added the

问题 Here is a simple test case that compiles without any warning. Looks like a common mistake but clang, gcc and visual studio doesn't emit warning in this case. Why? class Image { private: int width, height; int* array; public: Image(int _width, int _height); void crashTest(); }; Image::Image(int _width, int _height) { array = new int[width * height]; // ^^^^^ ^^^^^^ this is wrong // I expect a warning here e.g.: 'width is uninitialized here' width = _width; height = _height; } void Image:

问题 Here is a simple test case that compiles without any warning. Looks like a common mistake but clang, gcc and visual studio doesn't emit warning in this case. Why? class Image { private: int width, height; int* array; public: Image(int _width, int _height); void crashTest(); }; Image::Image(int _width, int _height) { array = new int[width * height]; // ^^^^^ ^^^^^^ this is wrong // I expect a warning here e.g.: 'width is uninitialized here' width = _width; height = _height; } void Image:

问题 As an example: public class Foo { private Foo() {} } public class Bar extends Foo { private Bar() {} static public doSomething() { } } That's a compilation error right there. A class needs to, at least, implicitly call its superclass's default constructor, which in this case is isn't visible in Foo . Can I call Object 's constructor from Bar instead? 回答1: You can't. You need to make Foo's constructor package private at the very least (Though I'd probably just make it protected. (Edit -

问题 Is there a way to assign instance variables declared in a super class, from a constructor in a sub class? I have gotten used to using BUILD() as constructor, but I am wondering if this is a good idea. I.e: use v6; class File { has $!filename; } class XmlFile is File { submethod BUILD(:$!filename) { } } my XmlFile $XF = XmlFile.new(filename => "test.xml"); The code above doesn’t work, prompting an error: "Attribute $!filename not declared in class XmlFile". Is it a matter of using the right

问题 I am trying to write a constructor for a derived type of an abstract one to solve this other question, but it seems that it's not working, or better, it isn't called at all. The aim is to have a runtime polymorphism setting the correct number of legs of an animal. These are the two modules: animal module animal_module implicit none type, abstract :: animal private integer, public :: nlegs = -1 contains procedure :: legs end type animal contains function legs(this) result(n) class(animal),

问题 I'm reading clause 12.7 of N3797. The following example is given: struct X { int i; }; struct Y : X { Y(); }; // non-trivial struct A { int a; }; struct B : public A { int j; Y y; }; // non-trivial extern B bobj; B* pb = &bobj; //1 int* p1 = &bobj.a; //2 undefined, refers to base class member int* p2 = &bobj.y.i; //3 undefined, refers to member’s member A* pa = &bobj; B bobj; extern X xobj; int* p3 = &xobj.i; X xobj; This example must reflect the rule: For an object with a non-trivial