C++ :: Cannot Change Virtual Functions From Object?
Oct 14, 2013
I try the functions pointers too, but without success. I understand the objects are the way for work with class's. until here fine. But why i can't change the virtual functions from an object? is there anyway for do it? Ican't do, outside of classfunctions, these code:
#include <iostream>
using namespace std;
class test {
public:
virtual void created(){};
[Code] ....
How i can validate these line:
void a::created()
???
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Jan 31, 2014
I was reading about the CRTP, and I can't for the life of me understand it's advantages over virtual functions.
Unless you're coding embedded systems, and can't afford the few extra bytes for the vptr, or coding something requiring high-performance, where every nanosecond counts, I just don't see why the CRTP is so attractive. It just adds more text and forces every user class that wants to use the CRTP'd hierarchy to become a template class.
I tried implementing my Functor hierarchy with the CRTP instead of virtual functions...All it did was clutter my files with angle brackets and made the whole thing look very ugly.
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May 2, 2014
if we do a virtual functions(polymorphism) why we need re-declare the functions(when we create a new class derived from other)?
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Oct 19, 2014
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?
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Feb 15, 2013
Is it possible to do something like this:
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)))
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Jan 26, 2013
when I should use pure virtual functions.On the one hand, "TOY" for example should be an abstract class since theres no such thing as "TOY" , there are "toy cars", "toy fighters" etc , but on the other hand I need to force it somehow to be abstract since theres no really a function that any toy should have and implement on his own way (except PRINT maybe).
when I should REALLY use pure virtual functions? And if I want to avoid people from creating TOY objects (for example), the only way is PURE virtual functions. right?
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Sep 11, 2014
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.
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Aug 7, 2013
I'm working with inheritance and pure virtual functions, and I want to overload an output stream operator. However, every time I run the program I get this: 0x7fff00ee98c0.
I'll include a base class and a derived class so you can see what I'm talking about.
Base:
#include <iostream>
using namespace std;
#ifndef _Insurance_h_
#define _Insurance_h_
[Code]....
The application is something like this (I'm assuming the user has already inputted the name, salesperson, make, model, etc):
#include "Auto.h"
#include <iostream>
using namespace std;
#include <vector>
vector<Insurance *> sales;
[Code] .....
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Jul 29, 2014
I have found that when I dump a C++ object from memory to a file - it seems that there is a misplacement of the last Virtual-Function-Table pointer - in that appears at the beginning. The result is that the gdump information based on this object dump (using green hills) is incorrect. I copied the contents of the gdump information below. The executable is compiled in linux.
Basically MEIO::CameraStatus contains an item that relates to its parent class (line 188). Then it has 18 items that are all Diagnostics::EventsCounter items. Now for each Diagnostics::EventsCounter item there is a Virtual-Function-Table Info Pointer as its last item. All is fine and good except that the last item of MEIO::CameraStatus which is _selfReset is missing its last item of 4 bytes (which is the Virtual-Function-Table Info Pointer ). On the other hand - right before the first Diagnostics::EventsCounter item ("_vidErrors") - there is an extra 4 bytes which happens to be the Virtual-Function-Table Info Pointer. As I said the gdump information file does not see this.
Why the object memory "moves" the last Virtual-Function-Table Info Pointer to the beginning (right before _vidErrors) and is there a way to "fix" this?
///////////////////////////////////////////////////////////////////////////
"MEIO::CameraStatus" val:0x000002f0 ind208,-1) Struct-Begin Info
188: "" offset 0, Parent-Class Private Info C++ Struct ref = 114
189: "_vidErrors" offset 160, Member Info C++ Struct ref = 128
190: "_vdiErrors" offset 480, Member Info C++ Struct ref = 128
[Code] .....
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Apr 13, 2014
I'm writing a program that calculate the carbon footprint for car, building, and bicycle. i have three classes building, car, bicycle. class called carbonfootprint have the pure virtual and should have the formula, but i didn't find it. having a little bit hard understanding some requests. like,
• Write an abstract class CarbonFootprint with only a pure virtual getCarbonFootprint method. Have each of your classes inherit from that abstract class and implement the getCarbonFootprint method to calculate an appropriate carbon footprint for that class.
• The main() function in the given program creates objects of each of the three classes, places pointers to those objects in a vector of CarbonFootprint pointers. You need to iterate through the vector, polymorphically invoking each object’s getCarbonFootprint method.
// Test program for CarbonFootprint and implementing classes.
#include <iostream>
#include <vector>
using namespace std;
int main() {
vector< CarbonFootprint* > list;
// add elements to list
[Code] ....
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Jan 17, 2014
the book I learn from gave a task to write a program which gets a matrix , and we have to write a function that switches 2 columns or rows the user inputs .as far as I know a function can not change variables in the main function without using pointers .so , theoretically, can a function described here can be written without using pointers ? as far as I tried - it can not.
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Jul 20, 2013
I know this is pretty basic script the problem I have is that I have to use functions for each calculation. The problem I face is that when the user types in the amount of quarters, dimes, nickels, and pennies when a function tries to add them together it displays a huge number instead of the right number.
Here is my code:
#include <stdio.h>
void insert_money(int p, int n, int d, int q, int total){
printf("How much of each coin do you want to insert:
");
printf("Quarters: ");
scanf("%d", &q);
[Code] ....
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Mar 9, 2014
And for some reason my program pauses (nothing displays, and my cursor changes to the little "loading" cursor thingy) when it hits line 139. That is where I added an additional piece of code that should change the shape of the triangle while the program is running.
#include <Windows.h>
#include <WindowsX.h>
#include <d3d9.h>
#define CUSTOMFVF (D3DFVF_XYZRHW | D3DFVF_DIFFUSE)
[Code].....
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Jan 21, 2014
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].....
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Mar 29, 2013
Code:
void lexer(ifstream& inputfile) {
string line;
getline(inputfile,line);
[Code] ......
I am trying to pass input file between two functions. The code compiles but immediately upon running the program, there is a "bad cast" run time error.
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Oct 21, 2013
My question is why when I attach this function to an object, it produces a different result? If I attach div() to goose, it produces 4 as the value of alpha, but if it's not attached, it produces 3, why is this?
main.cpp
#include <iostream>
#include <cstdlib>
#include "main.h"
using namespace std;
int main() {
Oreo fox;
[code].....
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Feb 25, 2014
I need to send same instance of object of a class in two function (Main function and thread function). The class is something like this:
//The class need to have constructor.
Class ABC {
public:
DWORD *IdG;
ABC(int number) {
IdG = new DWORD[number];
}
}obj(32);
The obj(32) is called in following two function. Function F1 is called using thread in main function.
void F1() {
obj.test;
}
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nShowCmd) {
obj.test1;
_beginthread(F1,0,(void*)number);
}
The code works well when the object of class ABC is created as shown above. My problem is the value that is passed in the object ('32') is to be read from the file.
If I read the file and create object separately in Main function and function 'F1' then , function 'F1' is not executed.
How to create same instance of object for Main function and function 'F1' with value passed in the object taken from the file.
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Feb 29, 2012
I am having an issue with passing an ifstream object to functions. Here is the code:
Code:
#include <fstream>
using namespace std;
void otherfunction (ifstream *ifs) {
...does stuff, like ifs->open(), then reads from the file...
}
int main () {
ifstream ifs();
otherfunction(&ifs);
}
Here is the error message:
Code: error: cannot convert ‘std::ifstream (*)()’ to ‘std::ifstream*’ for argument ‘1’ to ‘void otherfunction(std::ifstream*)’
Why can't I do that? What does "ifstream (*)()" even mean? And I don't want to change the structure of the program. I have reasons for declaring the ifstream object in the main function (because there are actually two functions that need access to the ifstream object -- neither of which is working).
Also, if I change the main function to be this instead:
Code:
int main () {
ifstream ifs();
ifstream *ifsptr = &ifs; //EDIT 2: forgot the ampersand
otherfunction(ifsptr);
}
I get the same error as above. However, if I change the main function to this:
Code:
int main () {
ifstream *ifsptr = new ifstream();
otherfunction(ifsptr);
}
I get all kinds of crazy errors about "undefined symbols for architecture _____". Here is the actual error message from my program (parseArgs is the real name of otherfunction)
Code:
Undefined symbols for architecture x86_64:
"std::ios_base::Init::Init()", referenced from:
__static_initialization_and_destruction_0(int, int)in cchumGBV.o
"std::ios_base::Init::~Init()", referenced from:
[Code] .....
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Dec 27, 2012
class Parent{
public:
virtual int width();
virtual int height();
int area(){return width()*height();};
[Code] ....
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Feb 20, 2013
My coin/money change code works when there can be an exact change each time, i.e. when the 1 cent option is available. However, when the change options are only $10, $5, $1, 25 cents and 10 cents, it does not give me what I want for instance, I wanted to get change for $237.80, I was expecting to get:
23 10's, one 5, two 1's and 8 dimes. However, the code below is giving me 23 10's, one 5, two 1's and 3 quarters (there is no option left for the 5 remaining cents).how to fix it?
Code:
#include<iostream>
#include<algorithm>
#include<vector>
using namespace std;
void change(double cents, int a[]);
int main() {
double Dollars;
double cents;
[code]...
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Mar 20, 2014
I'm a little confused by my programming assignment this week. I've been working at it Wednesday and I've made progress but I'm still confused as to how I'm supposed to do this. The class I made is called Stack, and it's derived from a template class called StackADT. We also utilize a class called unorderedLinkedList, which is derived from a class called linkedList.
We're supposed to implement all of the virtual functions from stackADT in the Stack class. The Stack data is stored in a an unorderedLinkedList, so what I'm confused by is how to implement a few of the Stack functions because there are no functions in unorderedLinkedList which we could call to manipulate the data.
As you can see from my attached code, I'm really confused by how I'm supposed to implement the pop() and top() functions, and I also think my initializeList() function is wrong. We don't have any similar functions in unorderedLinkedList to call, so I'm at a loss of how i'd access my unorderedLinkedList. My initial thought was to call the similar functions in the class that unorderedLinkedList was derived from, linkedList, but I'm unsure of this is what we're supposed to do, or if theres actually a way to access my unorderedLinkedList without having to use the functions from the base class.
NOTE: We're not allowed to modify stackADT, unorderedLinkedList, and linkedList.
Stack.h
#include "stackADT.h"
#include "unorderedLinkedList.h"
template<class Type>
class Stack: public stackADT<Type>{
template <class T>
struct nodeType
{
T info;
nodeType<T> *link;
[Code]...
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Mar 16, 2013
will copy constructor does object initialization using another already created object? I understand that it can be applied for object initialization and not for assignment.Is it correct?
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Jul 3, 2014
I have a method to take a Tile object and make an instances of it based on some data from the original object. Than it is suppose to manipulate the a specific instance and save the results. The first loop through it works but it changes all instance as well as the base.
public static int recurse(int count, Tile[,] b,Huristic h,int check) {
if (check==1) {
boardState.Add(B)/>;
return check;
} if (check == 0)
[Code] .....
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Dec 13, 2012
#include "B.h"
class A {
public :
A()
{
s_b = new B();
b = new B();
[Code] ....
In my project i have seen static object as above . But not able to know what is the exact use of it and how they are different from general object .
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May 3, 2013
At the moment im trying out with pointing to an array of functions. I got this working as following:
typedef void (* functionPtr) ();
functionPtr functions[2][2]={{do11,do12}, {do21,do22}};
void do11(){DEBUG_PRINTLN("11");}
void do12(){DEBUG_PRINTLN("12");}
void do21(){DEBUG_PRINTLN("21");}
void do22(){DEBUG_PRINTLN("22");}
void loop(){
A=0;
B=1;
functions[A][b]();
}
But now I'm trying to use this to point to a function inside a class so instead of do11, i want to be able to point to Basic.Do11. Somehow this doesnt work and I keep on getting this message:
error: argument of type 'void (Basic::)()' does not match 'void (*)()'
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Jan 6, 2014
I have questions about multiple inheritance and virtual methods. I have a class called solid. All objects of this class have hitboxes and can collide with others. I have the following methods:
void testCollision(something begin, something end);
/* This method takes a container's begin and end iterators to test if the object collides with any other object of the list of all the solids currently in the game area. Each time there is a collision, it calls collide(other) and other.collide(*this) */
virtual bool collide(solid& other);
/* This method always returns false and does nothing */
This class will be inherited by another class which will have overloads for a few specific collisions. For example:
class player : public solid{
public:
bool collide(projectile& other);
bool collide(enemy& other);
bool collide(wall& other);
};
My question is quite simple actually. If I have a loop which calls testCollision() with all elements in the list of all solids (a list of pointers to solids to be exact) and there is a collision between the player and a projectile, will testCollision call player::colide(projectile& other) or will it call solid::collide(solid& other). And in any case, did I understand how to use the virtual keyword? If I'm right, it should call the player::colide method if it's there for the specific type, else it will call the solid::colide which only returns 0, ignoring collision.
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