constructor

问题 Newbie Java question. Say I have: public class Car{ ... } public class Truck extends Car{ ... } Suppose I already have a Car object, how do I create a new Truck object from this Car object, so that all the values of the Car object is copied into my new Truck object? Ideally I could do something like this: Car c = new Car(); /* ... c gets populated */ Truck t = new Truck(c); /* would like t to have all of c's values */ Would I have to write my own copy constructor? This would have to be

问题 This question already has answers here : Default constructor with empty brackets (9 answers) Closed last year . #include <iostream> using namespace std; class CTest { int x; public: CTest() { x = 3; cout << "A"; } }; int main () { CTest t1; CTest t2(); return 0; } CTest t1 prints "A" of course. But it seems like nothing happens at t2(), but the code runs well. So do we use those parentheses without argument? Or why can we use it this way? 回答1: This is a quirk of the C++ syntax. The line CTest

问题 I'm reading through some code. In the constructor it has super() but the class implements interface which of course doesn't have a constructor. So which super() it is referring to? public class BoundingBox implements IBoundingVolume { public BoundingBox() { super(); mTransformedMin = new Number3D(); mTransformedMax = new Number3D(); mTmpMin = new Number3D(); mTmpMax = new Number3D(); mPoints = new Number3D[8]; mTmp = new Number3D[8]; mMin = new Number3D(); mMax = new Number3D(); for(int i=0;

问题 Suppose I'm writing a class template C<T> that holds a T value, so C<T> can be copyable only if T is copyable. Normally, when a template might or might not support a certain operation, you just define the operation, and it's up to your callers to avoid calling it when it's not safe: template <typename T> class C { private: T t; public: C(const C& rhs); C(C&& rhs); // other stuff }; However, this creates problems in the case of a copy constructor, because is_copy_constructible<C<T>> will be

问题 As I sit in the C++ Standards committee meetings, they are discussing the pros and cons of dropping Inheriting Constructors since no compiler vendor has implemented it yet (the sense being users haven't been asking for it). Let me quickly remind everyone what inheriting constructors are: struct B { B(int); }; struct D : B { using B::B; }; Some vendors are proposing that with r-value references and variadic templates (perfect forwarding constructors), it would be trivial to provide a

问题 How can I declare a class type, so that I ensure the object is a constructor of a general class? In the following example, I want to know which type should I give to AnimalClass so that it could either be Penguin or Lion : class Animal { constructor() { console.log("Animal"); } } class Penguin extends Animal { constructor() { super(); console.log("Penguin"); } } class Lion extends Animal { constructor() { super(); console.log("Lion"); } } class Zoo { AnimalClass: class // AnimalClass could be

问题 It seems that a vector will check if the move constructor is labeled as noexcept before deciding on whether to move or copy elements when reallocating. Is the default move constructor defined as noexcept? I saw the following documentation but it didn't specify this.http://en.cppreference.com/w/cpp/language/move_constructor Implicitly-declared move constructor If no user-defined move constructors are provided for a class type (struct, class, or union), and all of the following is true: there

问题 suppose I have the following class: class MyInteger { private: int n_; public: MyInteger(int n) : n_(n) {}; // MORE STUFF }; And suppose this class don't have a default trivial constructor MyInteger() . I must always supply an int to initialize it for some reason. And then suppose that somewhere in my code I need a vector<MyInteger> . How do I initialize each MyInteger component in this vector<> ? I have two situations (probably the solution is the same, but I'll state them anyway), a normal

问题 I just had a rather unpleasant experience in our production environment, causing OutOfMemoryErrors: heapspace.. I traced the issue to my use of ArrayList::new in a function. To verify that this is actually performing worse than normal creation via a declared constructor ( t -> new ArrayList<>() ), I wrote the following small method: public class TestMain { public static void main(String[] args) { boolean newMethod = false; Map<Integer,List<Integer>> map = new HashMap<>(); int index = 0; while

问题 This question already has answers here : Calling virtual functions inside constructors (13 answers) Closed 3 years ago . class Base { public: Base(){Foo();} ~Base(){Foo();} virtual void Foo(){std::cout<<"base";} }; class Derived: public Base { public: Derived(){Foo();} ~Derived(){Foo();} void Foo(){std::cout<<"derived";} }; //main { Derived d; } Any idea why this code prints out "base" and "derived"? I understand the advice is not to put virtual function calls inside constructor or