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
[code]....
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.
#include<iostream>
class A{
public:
[Code].....
if we don't provide the acces modifiers for base class and we need to manipulate the private data of base class in derived class. Is there anyway to acces the private data members? Here's a coding example
class A {
private :
int a;
};
class B : public class A {
public :
void displayA() { cout<<a<<endl; }
};
how i can acces the a of base class A in derived class B without acces modifiers.
I recall when I first started playing with C++ I was told that you should never use virtual functions unless you absolutely cannot think of a better way to do whatever you are attempting. This is something I have tried to stick to over the years - and indeed is probably why I have never used inheritance or polymorphism much in my own programmes.
However, I notice through a great deal of the code examples offered to questions here and even over on StackOverflow that commentators show no hesitation to recommend code that involves virtual functions. More so, I have even seen several instances here where - what I was taught as, but they may well have a different official name - 'pure virtual functions' (those with definitions inside a class of something like virtual int function_name(void)=0) are demonstrated and I was very clearly taught to avoid those like the plague.
I was wondering therefore has the official thinking changed since the middle nineties on when - and even whether - to use virtual functions in your programmes?
So I have 2 seperate base classes, (note that I removed the variables and functions that do not relate to the topic) Object.h
class Object{
public:
Object();
~Object();
[Code].....
The error I get is saying I am calling a function declared with one calling convention with a function pointer declared with a different calling convention and this makes perfect sense because for some reason, the function pointer is pointed at the virtual function Object::update but I can't figure out why and how to make it point at the virtual function Drawable::getImage.
Also, the virtual update function is called in a different place just before this and works correctly.
I would like to access (use) a (virtual) method declared in a base class via a pointer to an object belonging to a derived class, which overrides the base method. Is it possible? Up to now I have not been successful. I am including a program with a more detailed description.
// declare a base class T0 with virtual function P, and a derived
// class T1 which overrides P; declare two ptrs, to T0 and T1,
// and generate corresponding objects; calling P via the two
// ptrs gives expected result; however I would like to take
// ptr to T1 object & execute the base function (declared
// in T0); is it possible? I tried all possible casts but nothing
// works; no matter how I cast the pt1 ptr, I always end up executing
// the overriding function (declared in T1)
// actual output (debian 7, gcc 4.7.2-5):
[code]....
class Parent{
public:
virtual int width();
virtual int height();
int area(){return width()*height();};
[Code] ....
Say I have 3 classes:
class Player {
public:
virtual func1();
[code]....
Say in my main class, I have a function fight(Player p1, Player p2) and I would like to do something like this in the fight function, given that p1 is the human and p2 is the computer:
//function fight()
fight(Player p1, Player p2) {
p1.func2();
}
//using function fight()
fight(human, computer);
When I compile the program, I got this: error: ‘class Player’ has no member named 'func2()' What can I do to allow p1 to call func2 inside fight()? I'm not allowed to use pointers as the parameter for fight() and have to use the signature fight(Player p1, Player p2).
I have my main.cpp like this:
#include <iostream>
#include "curve1.h"
#include "curve2.h"
using namespace std;
int main() {
Curve1 curve1Obj;
Curve2 curve2Obj;
[Code]...
Base class Score has two derived classes Curve1 and Curve2. There are two curve() functions, one is in Curve1 and other in Curve2 classes. getSize() returns the value of iSize.
My base class header score.h looks like this:
#ifndef SCORE_H
#define SCORE_H
class Score {
private:
int *ipScore;
float fAverage;
int iSize;
[Code]...
You can see that I have used curve1Obj to enter scores, calculate average and output. So if I call getSize() function with cuve1Obj, it gives the right size that I took from user in enterScores() function. Also the result is same if I call getSize() in score.cpp definition file in any of the functions (obviously).
.....
The problem is when I call curve() function of Curve2 class in main (line 23) with the object curve2Obj, it creates a new set of ipScore, fAverage and iSize (i think?) with garbage values. So when I call getSize() in curve() definition in curve2.cpp, it outputs the garbage. .....
How can I cause it to return the old values that are set in curve1.cpp?
Here is my curve2.cpp
#include <iostream>
#include "curve2.h"
using namespace std;
void Curve2::curve() {
cout << "getSize() returns: " << getSize() << endl; // out comes the garbage
}
Can I use a function to simply put values from old to new variables? If yes then how?
In this book, item 3 is about never treat arrays polymorphically. In the latter part of this item, the author talks about the result of deleting an array of derived class objects through a base class pointer is undefined. What does it mean? I have an example here,
Code:
class B
{
public:
B():_y(1){}
virtual ~B() {
cout<<"~B()"<<endl;
[Code] ....
This sample code does exactly what I want. So does the author mean the way I did is undefined?
Please consider the following code :
#include <iostream>
using namespace std;
class superclass;
class subclass1;
class subclass2;
[Code] ....
As you can see I want to create a dynamically allocated storage of references to a parent class each of which can then point to a child class, how ever I do not know how to extract the child class out again from that array so i may access its variable b.
Let's say I have a Car object , and it contains inner Engine object.
Code:
struct Car{
Engine mEngine;
};
In order to initialize the engine object NOT by the default constructor (if it has any) , we use initialization semantics:
Code:
Car::Car:
mEngin(arg1,arg2,...)
{
other stuff here
}
Now it gets tricky: Let's say a Car objects has 10 inner objects, each object has about 5 variables in it . Car is a base class for , e.g. , Toyota class. you don't want the Car class to have a constructor with 50 arguments. Can the inner objects of Car be initialized from the base class , e.g. Toyota?
Code:
class Toyota:
Car(...),
mEngine(...),
mGear(..)
{
...
};
The other options are:
1) like said , create a Car constructor which gets 50 arguments, then initialize Car as whole from Toyota - the code becomes less readable and less intuitive
2) Car constructor which get built-objects as arguments and initialize the inner objects with copy constructor . the code gets more readable but then you create many excess objects .
So I have a base class, lets call it base. In base I have a virtual function called update(), update just couts "base" then I have a class derived from base called derived;
it has a function called update(), update just couts "derived" then I create a vector called Vec it's initialised like this:
std::vector<base> Vec;
then I add an element into it like this
Derived DerElement;
Vec.push_back(DerElement);
then when I type:
for (int i=0; i<Vec.size(); i++) {
Vec.at(i).Update();
}
It outputs:
Derived DerElement2;
DerElement2.Update();
and it outputs this:
#include <iostream>
#include <vector>
class Base {
public:
virtual void Update() {
[Code] .....
and this is it's output:
Base
Derived
Press any key to continue . . .
I would like to know if there's a way to make a method from a derived class a friend of its base class. Something like:
class Derived;
class Base {
int i, j;
friend void Derived::f();
protected:
Base();
[Code] ......
I have an example where I have a variable belonging to a base class, but I would like to tell the compiler that it actually belongs to a derived class. How can I do this?
// base class: R0
// derived class: R1
// see function SetR1 for the problem
class R0 {
public:
int a;
[Code] .....
I just wondering if a base class can call the overridden function from a Derived class?
Here's an example:
//Base Class H
class BaseClass {
public:
BaseClass();
virtual ~BaseClass();
virtual void functionA();
[Code] ....
So basically, when I am creating a new object of Derived class, it will initialize BaseClass and the BaseClass will call functionA but I want it to call the function overridden by Derived class.
I know that if I call newObj->functionA it will call the overridden function. Right now I want the base class to call the overridden function "this->functionA(); in BaseClass" during its initialization. Is it possible to do that?
Basically, I have a base class called MainShop and it has 3 derived classes which are SwordShop, SpellBookShop and BowShop. I want the base class to be able to call a function from one of the derived classes but no matter what i do, it doesn't seem to work!
Here is my code:
#include "MainShop.h"
//BaseClass cpp
void MainShop::EnterShop(Hero& hero)
[Code]....
I have two other derived classes, but its basically the same concept. I have a function in one of the derived classes and i would like to call it from the base class. This is one my derived classes:
//SwordShop derived cpp
#include "SwordShop.h"
void SwordShop::soldierShop(Hero& hero)
{
/* some code here*/
}
Base class has an array, whose size is controlled by the derived class.
I can't use the STL and use a 2003 compiler, so things like std::vector and std::array are out. I also can't use dynamic memory allocation.
So I thought of a few options:
1.
template <int N> class myBaseClass { ... int array[N]; ... }
then class MyClass: public myBaseClass<8> ... etc ...
2.
have a int **array in the base and assign in the derived class.
3.
give the base some virtual methods such as int *getArray or even int &getInt for more safety.
I understand it is done like this
// Calling the base class constructor
explicit CCandyBox(double lv, double wv, double hv, const char* str="Candy"): CBox(lv, wv, hv)
{
...
}
But how does the compiler know that you are initializing the base "part" of the object?
Or is this the entire reason initialization lists exist, to separate this confusion?
I'm having some difficulties in understanding the topic which I stated above.
View 5 Replies View RelatedIf Yes, then why this syntax does not works :
class Derived : public Base {
public:
Derived& operator=(const Derived &rhs) {
operator =(static_cast<const Base&>(rhs));
[Code] ....
I have an abstract base class - let's call it MyInterface - and a class that most classes in my program inherit from, let's call it MyBaseclass.
Let's assume that all my objects inherit MyBaseclass, some of which also inherit MyInterface. Now I want to collect objects in a container class, MyContainerclass. The container class is only interested in objects that implement MyInterface.
Now I know that all objects that inherit MyInterface also inherit MyBaseclass, but the compiler doesn't know that. MyContainerclass wants to call methods in MyBaseclass, but it collects pointers to MyInterface classes. I can't make MyInterface inherit MyBaseclass, because I will be using classes that I don't want to change (they are part of a framework) that already inherit MyContainerclass. IOW, I can't use virtual inheritance to get a nice inheritance diamond.
To sum up, I want to create a container class that:
1. Collects objects that implement MyInterface.
2. Calls MyBaseclass methods on the collected objects.
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'"
#ifndef ENTITY_H
#define ENTITY_H
#include <SFMLGraphics.hpp>
class Entity {
public:
Entity();
Entity(sf::Vector2f position, sf::Vector2f size, sf::Color fillColor, sf::Color outlineColor);
[Code] ....
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'm trying to implement a class hierarchy and a wrapper class with a pointer to the base class. The base class has operator< overloaded and the implementation makes use of virtual functions as some of the logic for sorting is in the derived classes. Unfortunately, when trying to use the base class operator< from the wrapper, I get a "pure virtual method called".
Below code is meant to illustrate my problem. Unfortunately it crashes on me (upon destruction of vec) and I cannot quite see, why. So two questions:
1. spot the error I made in the code below (having lived in Java-land for the last 5 years, I'm sure I just did some stupid error)?
2. How can I implement Wrapper::operator< to use Base::operator<? I know I could write a function and pass it to sort but I'm interessted if there is a way to actually use Base::operator<.
Code:
#include <vector>
#include <algorithm>
#include <cmath>
[Code].....
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;
}
What Could be causing this error?
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.
Code:
class Base {
public:
class BaseIterator {
};
virtual const BaseIterator& begin();
virtual const BaseIterator& end();
[Code] .....
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.
All the involved classes are templates,not sure if that makes the situation different.