constructor

问题 As part of an assignment we have been asked to create a Vector3D class which uses memory allocated on the Heap. I have a Vector3DHeap class with the following constructor. Vector3DHeap::Vector3DHeap(float& x, float& y, float& z) { this->x = &x; this->y = &y; this->z = &z; } If I want to get a unit vector, I was expecting the be able to do the following. This gives the error message "No instance of constructor matches the argument list, argument types are (float, float, float). Vector3DHeap*

问题 Bjarne wrote:- For a type T, T() is the notation for the default value , as defined by the default constructor . What happen when we don't declare default constructor ? For example using namespace std; class date{ int n,m; public: int day(){return n;} int month(){return m;} };//no default constructor int main() { date any =date(); cout<<any.month()<<endl; cout<<any.day()<<endl; return 0; } Output of this program is 0 and 0 every time i run my program. I haven't declare any default constructor

问题 Consider this class: #include <iostream> struct foo { int a = 42; int b = bar(); int bar() { return a; } }; int main(){ foo f; std::cout << f.a << " " << f.b; } It prints the expected 42 42 . Is it allowed by the standard to call a member function in a default member initializer? The following I would expect to be undefined: struct broken { int a = bar(); int b = 42; int bar() { return b; } }; Unfortunately it does compile without warnings. 回答1: As you found, this is legal, but brittle and

问题 I'm trying to create a specialized constructor for std::string arguments, but the other one is always used when I call it with a string argument. struct Literal : Expression { template <typename V> Literal(V val) { value = val; } }; template <> Literal::Literal(std::string const& val) { value = val.c_str(); } It doesn't matter if both are defined inside the class, both outside the class, or like in the posted example only the specialization is defined outside the class: When called with std:

问题 I was looking at other questions on SO, but I didn't really see an explanation for my question. I read that calling a constructor from another constructor (using the this keyword) was valid , but I didn't understand why it was valid. Previously, I thought that only one constructor could act per object. Constructor chaining seems to break this logic, in that while calling one constructor, it runs another in conjunction to the original, targeted constructor. Why does constructor chaining work?

问题 I was looking at other questions on SO, but I didn't really see an explanation for my question. I read that calling a constructor from another constructor (using the this keyword) was valid , but I didn't understand why it was valid. Previously, I thought that only one constructor could act per object. Constructor chaining seems to break this logic, in that while calling one constructor, it runs another in conjunction to the original, targeted constructor. Why does constructor chaining work?

问题 I'm trying to add a lazy-initialization function to my class. I'm not very proficient with C++. Can someone please tell me how I achieve it. My class has a private member defined as: std::unique_ptr<Animal> animal; Here's the original constructor that takes one parameter: MyClass::MyClass(string file) : animal(new Animal(file)) {} I just added a parameter-less constructor and an Init() function. Here's the Init function I just added: void MyClass::Init(string file) { this->animal = ???; }

问题 I'm trying to add a lazy-initialization function to my class. I'm not very proficient with C++. Can someone please tell me how I achieve it. My class has a private member defined as: std::unique_ptr<Animal> animal; Here's the original constructor that takes one parameter: MyClass::MyClass(string file) : animal(new Animal(file)) {} I just added a parameter-less constructor and an Init() function. Here's the Init function I just added: void MyClass::Init(string file) { this->animal = ???; }

问题 This is not about "When VTABLE is created?". Rather, when the VPTR should be initialized? Is it at the beginning/end of the constructor or before/after the constructor? A::A () : i(0), j(0) -->> here ? { -->> here ? //... -->> here ? } 回答1: The machinery for virtual calls (usually a v-table, but doesn't need to be) is set up during the ctor-initializer , after construction of base subobjects and before construction of members. Section [class.base.init] decrees: Member functions (including

问题 I have some niggling questions about factory constructors example mentioned here (https://www.dartlang.org/guides/language/language-tour#factory-constructors). I am aware of only three types of constructors on a basic level - default, named and parameterised. Why should I use factory at all for this example? Is that a named constructor which is being used? and why? 回答1: tl;dr Use a factory in situations where you don't necessarily want to return a new instance of the class itself. Use cases: