constructor

问题 The C++ standard says that invoking a pure virtual function from a constructor or destructor is forbidden. What is the reason for this? Why should the standard place a restriction like this? 回答1: At the point a class destructor is run, all subclass destructors have already been run. It would not be valid to call a virtual method defined by a subclass, for which its destructor has already run. A similar restriction exists around calling virtual methods in constructors. You can't call a virtual

问题 So I'm learning java. I'm one month in and I just learned about constructors. But I don't see the whole purpose of creating one. Why and when would I ever want to use one? I get the whole idea that it does not have a main method and you can call a constructor from your main class. Anyone can enlighten me on this topic, it would help me a great deal. 回答1: Constructors are what you use to initialize/set up the instances of your classes. If you have an object that needs some processing before it

问题 So I'm learning java. I'm one month in and I just learned about constructors. But I don't see the whole purpose of creating one. Why and when would I ever want to use one? I get the whole idea that it does not have a main method and you can call a constructor from your main class. Anyone can enlighten me on this topic, it would help me a great deal. 回答1: Constructors are what you use to initialize/set up the instances of your classes. If you have an object that needs some processing before it

问题 Consider the following sample code below: #include <iostream> using namespace std; class base { public: base() { cout << "ctor in base class\n"; } }; class derived1 : public base { public: derived1() { cout <<"ctor in derived class\n"; } }; int main() { derived1 d1obj; return 0; } Questions When d1obj is created, the constructors are invoked in the order of derivation : base class constructor is called first and then the derived class constructor. Is this done because of the following reason

问题 Consider the following sample code below: #include <iostream> using namespace std; class base { public: base() { cout << "ctor in base class\n"; } }; class derived1 : public base { public: derived1() { cout <<"ctor in derived class\n"; } }; int main() { derived1 d1obj; return 0; } Questions When d1obj is created, the constructors are invoked in the order of derivation : base class constructor is called first and then the derived class constructor. Is this done because of the following reason

问题 I've already tried to ask this question but I wasn't clear enough. So here is one more try. And I am very sorry for my English ;) Let's see the code: #include <iostream> #include <memory> using namespace std; struct A { unique_ptr<int> ref; void printRef() { if (ref.get()) cout<<"i="<<*ref<<endl; else cout<<"i=NULL"<<endl; } A(const int i) : ref(new int(i)) { cout<<"Constructor with "; printRef(); } ~A() { cout<<"Destructor with"; printRef(); } }; int main() { A a[2] = { 0, 1 }; return 0; }

问题 I am trying to understand how default constructor (provided by the compiler if you do not write one) versus your own default constructor works. So for example I wrote this simple class: class A { private: int x; public: A() { std::cout << "Default constructor called for A\n"; } A(int x) { std::cout << "Argument constructor called for A\n"; this->x = x; } }; int main (int argc, char const *argv[]) { A m; A p(0); A n(); return 0; } The output is : Default constructor called for A Argument

问题 I am trying to understand how default constructor (provided by the compiler if you do not write one) versus your own default constructor works. So for example I wrote this simple class: class A { private: int x; public: A() { std::cout << "Default constructor called for A\n"; } A(int x) { std::cout << "Argument constructor called for A\n"; this->x = x; } }; int main (int argc, char const *argv[]) { A m; A p(0); A n(); return 0; } The output is : Default constructor called for A Argument

问题 I have troubles with testing Model in CakePHP 2.0 and it seems the problem is on the model's constructor. public function __construct(){ parent::__construct(); $this->_pagi_cuantos = 2; } Even if I delete all its content, I still getting problems trying to run the test. Mark Story told me: if you have a constructor make sure you're overriding the constructor correctly. Failing to do so will cause errors like this. What do I wrong? 回答1: why don't you look into the core code its open source

问题 I know that static blocks are initialized at the time the class is loaded and since the class is loaded only once in a program,they are initialized only once. IIB (Instance initialization blocks) are initialized every time an instance of the class is made, and the same for constructors: they are executed during object creation. I don't understand why in the below program IIB is executed in prior to Constructors. Code- public class Hello { public static void main(String args[]) { C obj = new C