C++ :: Finding A Specific Instance Of A Class
May 22, 2013
So I have a class that is like this:
class card {
public:
int id;
int val;
};
card card1;
card1.id = 1;
card1.val = 2;
card card2;
card2.id = 2;
card2.val = 45;
etc...
So my question is firstly, is there a better way to implement this? (a vector of classes or something maybe?) and how can I call up a specific instance of the class. For example, if I want the val of a specific instance of the class, how best can I do that?
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Mar 1, 2013
I am facing a real-life problem, it can be simplified as below:
#include <iostream>
using namespace std;
class B;
class A {
public:
void f1(A a) {}
void f2(B b) {}
[Code]...
There is no problem at all with the f1(), it compiles and executes without any problem. But f2() gives compilation error. How to solve this?
The error message is: error: 'b' has incomplete type This is just to define the function f2() in a class, that uses an instance of its child class as one of its arguments.
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Jul 3, 2014
There are two ways to access the members of class A inside class B:
1) Making an instance of class A in class B
2) Deriving class B from class A
So what is the basic difference in both ways as we can do same kind of work with both ways?
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Oct 4, 2014
I'm currently learning the Qt framework and doing my first tutorial. Straight away I saw something that baffled me:
notepad.h
Code: namespace Ui {
class Notepad;
}
class Notepad : public QMainWindow
[Code] ....
Note the ui pointer and the heap allocation in the class constructor; I can't wrap my head around why one would do this. What's going on here?
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Nov 6, 2014
I am putting a instance o the Vehicle Class inside the constructor of the Calculate Class then calling it later. I get a warning saying the variable is not used and a error when I try to used the functions from the vehicle class saying use of undeclared identifier.
Code:
#include <iostream>
#include "Calculate.h"
#include "Vehicle.h"
#include <fstream>
Calculate::Calculate(){
[Code] ....
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Feb 12, 2014
Okay so I have a class Student, which takes a number and a vector as a parameter for the constructor. Everything works well, until I output the values of the vector for every instance. The problem is that the same vector is being shared with EVERY instance I create, but I want it to be unique for every single one!
//Student.h
#ifndef __Grade_calculator__Student__
#define __Grade_calculator__Student__
#include <iostream>
[Code].....
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Nov 24, 2012
I was wondering if (in C++) you can instantiate a class (class foo) then have said class return the already instantiated object. (foo::instance())
In other words, can I have a class return it's-self via it's own methods? I want to be able to create a class (i.e. class foo) early in my program so it is already setup and ready to go. Then, farther down the line, I want to be able to call functions from that class without having to pass that object as an argument to my calling function. Can I do something like so:
MyClass::ReturnSelf()->foo();
or
MyClass::ReturnSelf().foo();
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Mar 2, 2014
I have a background in c# and am very frustrated with c++. If I created a class in c# like so:
public class Memory{
int x = 0;
int y = 0;
int height = 0;
int width = 0;
string firstname = "Bob";
string lastame = "Chester";
}
and then created a new instance of this class from a separate class by doing:
public class Main{
Memory mem = new Memory();
}
I have raked the internet for a way to create a new instance of a class in c++ while keeping its default values and have come up empty handed.
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Jan 17, 2014
I know that it is possible to pass a class instance to a function, but in my experience, if said function changes any variables of the class, they don't actually get changed. For example, we have class object, that has a member int number = 5. Lets say we have two functions, func1() and func2, which are not members of class object. If we pass object to func1() which, lets say, increases number by 5 (so now number = 10), at the end of that function number still = 5.
Is there a way to bypass this and have functions alter class variables permanently?
I know that I can pass variables by reference, but, in my experience, such a thing does not work with vectors (which I am also dealing with), so simple passing the desired variables by reference won't work.
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Sep 4, 2013
I'm trying to learn as much C++ as I can. I was writing a program that mixes linked lists and classes. There is the class "Obj" which only holds an integer called 'data' and the classic "struct node" structure for linked list, but this time the "node" structure will hold an instance of "Obj" Class and the next* pointer.
#include <iostream>
using namespace std;
class Obj {
private:
int data;
public:
[code]....
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May 6, 2014
I'm trying to change the values of some instance variables in my Controller Class so that when the user inserts values into main class it changes for Controller.
class Controller {
public:
Controller();
~Controller();
double PCFreq;
__int64 CounterStart;
[Code] ....
The user should be able to choose which foo the want to use. So I create an object of controller in main like this
Controller* con = new Controller()
Now my issues is, when I take user input (an integer) and try to do this
con->choice1 = choice1;
only the object of con's choice1 is = to user input.
However back at the class for Controller, choice1 hasn't received a value.
I can't initialize through Controllers constructor because I get the user input through a switch statement and the range of con would only be as far as the case.
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Dec 4, 2014
I am creating a program that allows a user to create multiple 'sequences' and multiple 'filters' and then apply a filter to a sequence.
Each sequence and filter is an array of values.
How do I go about allowing the user to create a 'new' sequence and then store the location so I can access it again later? Once they have created a new sequence they can go on to create another sequence and ten maybe a filter and then another sequence etc etc .. and then they can select sequence 1 and edit the values if they so wish.
They would be asked how many sample values for the sequence, and then I would create a sequence with that many values and an id (1,2,3,4...). They could then enter this id to view/edit the sequence.
The entering/editing values part I am fine with. I just don't know how to allow them to create multiple new instances of a class without using an array so something like..
sequenceClassName somearray[10];
int i;
*create a new array*
somearray[i].create_class(how_many_samples)
i++ //so next sequence they create is 2,3,4.. etc
- this then calls the member function that creates an array using 'sample_values = new float[how_many_samples]' and the user can input their data and edit it whenever by entering the id which will correspond to the somearray[i].
However that approach only allows them to enter a maximum of 11 sequences. It all depends on how big I make that initial array and it just seems like the wrong way to do it.
( how to interact with them, just how to create multiple classes and recall them later to access the data!)
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Apr 27, 2014
I am new to C++. I am trying to create 5 instance object of class Test. I wonder if these two codes are the same.
Test testArray[5];
//which one is the correct way or what is the correct way?
for(i=0;i<5;i++)
{
Test testArray;
}
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Aug 25, 2013
I'm new in object oriented programming. I need creating a global object/instance of a class, that can be used by any function.
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Jan 15, 2015
Let's say I reference a dll in my project that has an abstract class in it:
public abstract class Module {
public string type;
public abstract string doSomething();
}
And then I have an Assembly object, loaded from a dll that is not referenced in project, but rather loaded during run-time, containing:
public class Tester : Module
{
public static string type = "Test";
public override void doSomething() {
//Stuff being done
return "hello";
}
}
How can I get the value of "type" in the class that is loaded during runtime? How do I use the "doSomething" method of it and get the returned object?
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Jan 13, 2015
Here's the problem I want to intialise different classes based on user input. I've tried using an IF statement but the scope of an if statement would mean the rest main program wouldn't be able to see it.
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Oct 20, 2012
I have a class that is a template, I have to declare it in my main but i want the user to choose what type of data they will use in the class, I cant just declare myclass my, i have to use myclass<int> my, how can I change so user can select the proper datatype to use in the class.
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May 23, 2013
I need to create an object of a mfc derived CFormView class that's not in the doc/template (a second view class). But it was generated with a protected ctor. Here's the code explanation with comments.
I'm thinking all the normal classes of the Doc/View template are created starting with this code, but within the template code base.
Code:
CSingleDocTemplate* pDocTemplate;
pDocTemplate = new CSingleDocTemplate(IDR_MAINFRAME,
RUNTIME_CLASS(CViewSwitchDoc), //<-expands to-> ((CRuntimeClass*)(&CViewSwitchDoc::classCViewSwitchDoc)),
RUNTIME_CLASS(CMainFrame), // main SDI frame window
RUNTIME_CLASS(CViewSwitchView));
But I have generated "another view" using the "Add Class" Wizard, it's a derived class of mfc CFormView which I named ViewForm. However I'm having a problem creating an instance of it because of the generated protected ctor and pulls a compile error of not being able to access ctor. Below are the header and implementation files of this said ViewForm class. How to create an object of this view ? Did I go about it all the wrong way since it's not in the doc/template group ?
// ViewForm.h file
#pragma once
// ViewForm form view
class ViewForm : public CFormView {
DECLARE_DYNCREATE(ViewForm)
[Code] ....
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May 3, 2013
I'll just tell you in short how my Problem looks like: I should implement a New Class in a SourceCode i didnt write myself. The source code is extremely sized (i think approx >100.000 Lines of Code), so i dont want to change too much in it in order to get my Implementation done.
MY problem looks simplicified like that: Starting from 3 classes and my new class the pseudo-code looks like that:
Class1(){
float* m_CalibX, m_CalibY;
.. }
Class2(){
char* m_ImageData[];
[Code] .....
So, i need Parameters from 3 different classes to insert in my NewClass. The 3 Classes dont know anyting about each other. So, i need to implement a Class-Instance from Type NewClass which is known by the other 3 Classes. I did solve it in this way:
//ClassInstance.h
#include "NewClass.h"
static NewClass ClassInstance
I just wrote a headerfile with a class-instance which is getting included by the other 3 Classes. So they all know the same Instance and writing their Parameters into it. Is this a decent solution or could it happen to get bugs/ logical mistakes with it?
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Mar 19, 2013
Below code produces run-time segmentation fault (core dumped) when execution reached line 53: delete [] array
Code 1:
#include <cstdlib>
#include <iostream>
using namespace std;
#define QUANTITY 5
class Parent {
protected:
int ID;
[Code] ....
Output of code 1:
Constructor of A: Instance created with ID=1804289383
Constructor of A: Instance created with ID=846930886
Constructor of A: Instance created with ID=1681692777
Constructor of A: Instance created with ID=1714636915
Constructor of A: Instance created with ID=1957747793
A::showID() -- ID is 1804289383
A::showID() -- ID is 846930886
A::showID() -- ID is 1681692777
A::showID() -- ID is 1714636915
A::showID() -- ID is 1957747793
Try to delete [] array..
Segmentation fault (core dumped)
Question: Why does segmentation fault happen in code 1 above?
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Mar 24, 2015
I am making a simple arkanoid game. When the ball hits the block, i want the block to disappear. How would i go around this? i am working in visual studio. I got the collision and everything working.. Here is my code;
int main(int argc, char** argv){
//initialisation
SDL_Init(SDL_INIT_EVERYTHING);
SDL_WM_SetCaption("Graphics Window",NULL); //set name of project
//variable declaration
SDL_Surface*screen;
boolrunning;
intgameState;
float startTime;
[Code] ....
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Oct 31, 2013
I have a .cpp file which contains 5 smaller defined classes. In my missile class I have a default constructor and a constructor that I invoke
class Missile{
private:
bool isHuman;
[Code]...
My issue is when creating and adding the pointer object; it doesn't seem to create a new instance of the class-the Missile objects all share the same xPos value which is always the first xPos when the "fire" command is given. The "Missile *missile = new Missile(xPos, yPos, true);" line does not seem to create a new instance of the object with different variables but instead creates a new object with the same variables. Is it necessary for me to always make a separate .cpp and .h file for any class I want to create multiple instances of or can I keep the smaller classes in the same file and still create a new separate instance of the class?
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Jan 24, 2012
Hey I am trying to use the getline() function to read a line from a file. For some reason Visual Studio 2010 gives me the following error. "No instance of overloaded function "getline" matches the argument list". The piece of code that produces the error is in a class in a separate .h file and is executed as a method of the object. I'm almost certain it has something to do with either the compiler thinking I am calling another getline in a different namespace or my parameters for the function are incorrect. Here is the code:
Code:
#include <iostream>
#include <string>
#include <vector>
#include <fstream>
using namespace std;
class InsultGenerator
[Code] .....
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Jan 23, 2013
I am trying to delete a speific element in an array of class objects. i am overwriting the element i waant to delete with the element after it. My algorithm works but the output is not correct, after debugging it seems my objects just dont copy, is there a way to copy a class object, i have looked up copy constructors and attempted to write one but it does not seem to have any effect on the output.
below is my code
class user {
string firstname, lastname, currentteam, position, status ;
int age ;
public:
user() {};
[Code] .....
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May 4, 2014
I tried to implement a string class and i want to modify a specific char in my string . I did operator over load but this code crash in this line
name [2] = 'k' ;
Why ?!!!!!!!!!!!!!
this is my code
# include <iostream>
using namespace std;
#include <assert.h>
class String {
public :
char *content ;
int size ;
[Code] ....
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Dec 30, 2013
I have `MainShop`(base class) then `SwordShop` and `BowShop`(derived classes) and another derived class called `Inventory`. I have a vector under protected in my base class, and when I access it from my derived classes it's okay, but each one has its own value. How can I set them all to have the same value?
//MainShop.h
#pragma once
class MainShop {
private:
//some variables
protected:
vector <string> WeaponInventory;
[code]......
Here every time I run it, it goes straight to the `else` and it displays that I do not have any items. I want to just have one vector for both my bow and Sword shops because vectors take up a lot of memory and my teacher said keep them to a minimum. So I just want one vector that takes in items from my bow class as well as my sword class, but for some reason it's acting as if I have multiple vectors, each with its own set of items.
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