C++ :: Overriding Virtual Operator Of Parent Class
Mar 20, 2013
Below is simplified code consists of two classes, namely Parent and Child.
Child is inherited from Parent.
All member functions of class Parent are declared virtual, and they have been overridden in the class Child.
Code 1:
#include <cstdlib>
#include <iostream>
using namespace std;
#define QUANTITY 5
class Parent {
[Code] ....
The output of the code:
Child::showID() -- ID is 1804289383
Child::showID() -- ID is 846930886
Child::showID() -- ID is 1681692777
Child::showID() -- ID is 1714636915
Child::showID() -- ID is 1957747793
Parent::operator=() invoked.
Child::showID() -- ID is 1804289383
Child::showID() -- ID is 846930886
Child::showID() -- ID is 1714636915
Child::showID() -- ID is 1714636915
Child::showID() -- ID is 1957747793
Question:
Why is Parent::operator= invoked instead of Child::operator= ..?
Isn't it already declared virtual and hence would be overridden..?
I need to invoke Child::operator= instead. How to achieve this?
The compiler creates virtual table for the base class and also for the derived class whether we override it or not.
That means each class has separate virtual table. when we get the size of the each class with out any data members... the size of base is -- 4 bytes(64 bit) and the size of derived is -- 1
The size of base class 4 is correct since it creates the virtual pointer internally and its size is member data + virtual pointer, but it in this case I have included any data members so it has given 4 byts.
But why in case of derived is 1 byte, since it the derived class has overridden the virtual function from base, this will also contains the virtual pointer which will be pointing to derived class Vtable, it the size of the class suppose to be 4 instead of 1 byte.
I've got the following code with output. I can't figure out myself why it's what printed out there. I believe, it has something to deal with overloading/overriding/virtual functions implementations in C++:
class Base{ public: virtual void f(int); virtual void f(double); }
[Code].....
Thus here're my conclusions: 1) in line d.f(1.0); for some reason compiler preferred casting double->int of the argument and then call to 'Derived::f(int)'.
2)in line pb->f(1.0); for some reason compiler preferred call to 'Base::f(double);'. 'Base' is static type of pb, but the dynamic type is 'Derived'.
I believe the answer has to deal with the fact whether virtual table contains in addition to functions' names also the types of arguments they accept. AFAIK, vTable doesn't include such info.
class A //parent { public: virtual void DoSomething() = 0; };
class B : public A //child { public: void DoSomething(string s) override; }
Where the child member function overrides and changes the parents member function.
I need to pass an array of key states to the Controller class' Update() function but don't want to send it to every class derived from Object (like Controller).
Is this possible or do I have to overload the original Update() member function (but I would need to define the method in Object then (i.e remove the pure virtual function (=0)))
I create two memory manager called CMemManager1 and CMemMangaer2, using different algorithms to allocate buffer. Now I want to control which memory manager to be used, when calling new.
I try to add a parameter bUseWhichMemManager to the constructor, but in overrided new function, there are no way to access the parameter. Is there a way to pass more parameters to new operator, such as:
I am currently having an issue with a piece of code that I am writing in which I need to use a vector of a child class as a parameter in a function in the parent class. Below is an example of my code:
#include "child.h" #include <vector> class parent { parent(); function(std::vector<child> children); // rest of class here }
When I do this my program doesn't compile. However if I try to forward declare, as shown in the following example, it once again refuses to compile:
#include <vector> class child; class parent{ parent(); function(std::vector<child> children); // rest of class here }
This time, it refuses to compile because it needs to know the full size of the class child in order to create the vector. How to being able to access the child is essential for my program, so what should I do?
I am doing C++ data structures exercises, and I am still learning some of the more basic concepts. I have a parent class:
template<class T> class linkedListType { public: protected: int count; nodeType<T> *first; nodeType<T> *last; private: };
Derived class:
#include "linkedListType.h" template<class T> class orderedLinkedList: public linkedListType<T> { public: void mergeList(orderedLinkedList<T> &list1, orderedLinkedList<t> &list2) { first = list1.first; ... } private: };
There is more code in my mergeList() function, but I included the line that is giving me problems. The error that my CodeBlocks compiler is giving me is that 'first' was not declared in this scope.
Strangely enough, if I change it to this->first, the error disappears.
1. Why does it not recognise 'first'? 2. Why would this->first work at all? Is the 'this' object a smart pointer?
In my program, I create controls by deriving base objects of them I've made. These controls are then are attached within the OnCreate() function via a method I've created. For example:
Code: class tChat: public TextBox { public: void OnKeyDown(UINT &KeyCode) { if (KeyCode == VK_RETURN) { MessageBox(NULL, "Pressed enter!", NULL, 0); [Code] ....
The use of AddControl() feels quite redundant and is only their to parse a pointer to txtChat's Parent. I'm trying to see if it's possible to remove this line and automatically associate txtChat to fMain.
The user can then derive the Form and Controls and use their virtual OnEVENT functions to handle all the messages they expose.
So far my first concept is using the order-of-creation based on base-class constructor's being fired to determine which object is associated with what.
If I create a copy of a class (i.e. a Form-derived object), first the Form's constructor is fired, and then the constructor's of any class-based member-variables are fired. Is this a safe assumption? I imagine the only time this could be affected is by another thread creating the object of a Form or Control derivative?
If this assumption is true, I could save the 'this' pointer from the FormBase constructor, and then associate it with each Control via the base Control class' constructor? Then to ensure thread-safety, I could map the current FormBase pointer to the local thread id to ensure no conflict if multiply threads are creating forms at the same time?
I've created some mock-up code before trying to implement this into my main code. The following keeps track of the current Form being created by using a ThreadId-based map. When a control is created it gets the FormBase pointer based of it's ThreadId calling. The control then calls an Attach() function of it's parent Form using the pointer it just got, and parses a pointer to the control. The Form then adds the control's pointer to a list. When the Form eventually parses WM_CREATE, it automatically pulls the controls from the list and fires their virtual Create() functions to build them.
Mock-up:
Code: #include <Windows.h> #include <string> #include <map> #include <list> class FormBase;// Forward declaration class FormMap;// Forward declaration class Object {};// Base Object
[Code] ....
Is this plausible to use? I imagine C++ does not have many "guarantees" about how it creates objects and when. But I thought it would be safe that it would never create member-variables before the class of them is first created?
I am making a very basic parent/child class based program that shows polymorphism. It does not compile due to a few syntax errors reading "function call missing argument list. Lines 76 and 77, 81 and 82, and 86 and 87.
#include<iostream> using namespace std; class people { public: virtual void height(double h) = 0; virtual void weight(double w) = 0;
I am trying to create a platformer and is stuck on a problem regarding my virtual class Entity. I wish to use it to create stuff like the Player and Enemy class(es). But how to do the parameter for my collision check function. Below is my Entity- and player class.
There might be a better way to check CC with a lot of different objects, this is my first attempt.
This is the error I am getting: "error C2664: 'bool Player::CollisionCheck(Hostile)' : cannot convert argument 1 from 'Player' to 'Hostile'"
and in Hostile I would (I guess) use bool CollisionCheck(Player p);
But if I try for example to use Player in the CC in player.h it will complain that the function doesn't have an overload for that. Hostile is just a example class name right now, it isn't implemented yet. I am trying to use Player, but if possible wish to be able to have a different class depending on what kind of entity it is. The entity will probably also be the players projectiles and so on.
I have this header file called Shape.h containing these function declarations. and a Shape.cpp which contains the body of the function. I am not showing it since it is not needed.
//This is from Shapes.h header file #ifndef SHAPES_H #define SHAPES_H #include <iostream>
[Code]....
I have this unfinished Main.cpp because the third line "JuanSanchez::Circle *pCar = new Circle; " is giving me a compiler error "error C2061: syntax error : identifier 'Circle' "
#include "Shapes.h" int main() { const int arrayIndex = 4; JuanSanchez::Shape *myShape[arrayIndex]; JuanSanchez::Circle *pCar = new Circle; }
How can I access the virtual base class? This is a practice exercise from c++ primer plus 6.
The problem is that the name becomes No Name instead of the name specified when creating the gunslinger, I don't know how I can call the virtual base class explicitly
Output,
#ifndef PERSON_H_ #define PERSON_H_ #include <string> #include <iostream> #include <cstdlib> using std::string; class person
I have a 'Graph' class, which has derived classes for Adjacency Matrix and Adjacency List representations.
How do I provide iterators for traversing vertices and edges, when the iterator classes would have different implementations for the different derived classes ?
The following way is the only one I can think of, but seems quite cumbersome.
Or is there a pattern for doing this that I'm not aware of ? Would composition be a better idea here compared to polymorphism ? I mean, I can think like..a Graph can 'have' several representation 'objects' within it.
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Basically object a contains a virtual table pointer which is of size 4 bytes. Since class A should have a virtual table which contains three pointers pointing to foo, foo2,foo3 separately. So the virtual table should be of size 12 bytes. I wonder where is virtual table located in memory?
I develop add-ons for MS Flight Simulator and I use the poorly documented SDK for this. The SDK provides a .h file in which an interface class is defined (with pure virtual methods only), something like this:
In my code, I use this interface like this: Code: IPanelCCallback* pCallBack = panel_get_registered_c_callback("fs9gps"); ... SINT32 id; pCallBack->ConvertStringToProperty(propertyName, &id);
Everything works fine, but I don't understand why... I thought the linker would stop with an "undefined symbol" error because the IPanelCCallback methods, such as ConvertStringToProperty, are declared as pure virtual but defined nowhere, and I don't use any library for linking. With such an interface class, I thought I would have to defined a subclass of IPanelCCallback and define the ConvertStringToProperty method.
Is it usual to rely completly on the new operator in constructors/copy constructors. What if new trows an exception? The application ends and that's it? The new operator can be placed where it can't be catch like in constructor initialization list. What kind of practice I should adopt when using "new" in those cases?
The sample code below is taken from here... [URL] ....
class MemoryBlock { public:
// Simple constructor that initializes the resource. explicit MemoryBlock(size_t length) : _length(length) , _data(new int[length])
