List iteration problem, it resets values

This is a discussion on List iteration problem, it resets values within the C++ Programming forums, part of the General Programming Boards category; Hello! Sorry for asking a probably stupid question, but I can't solve the problem. I'm writting some application and got ...

  1. #1
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    List iteration problem, it resets values

    Hello!

    Sorry for asking a probably stupid question, but I can't solve the problem. I'm writting some application and got in stuck with a part of it. Here I post a simplified part of it.

    Code:
    #include <list>
    #include <iostream>
    #include <conio.h>
    using namespace std;
    class Object {
    	private: 
    		int value;
    	public:
    		Object(void);
    		int getValue(void);
    		void setValue(int value);
    };
    int main(void) {
    	Object one;
    	Object two;
    	list<Object> objects;
    	list<Object>::iterator i;
    	objects.push_back(one);
    	objects.push_back(two);
    	Object object;
    	for (i = objects.begin(); i != objects.end(); i++) {
    		object = *i;
    		object.setValue(9);
    		cout << object.getValue() << endl;
     	}
     	cout << "====" << endl;
     	for (i = objects.begin(); i != objects.end(); i++) {
    		object = *i;
    		cout << object.getValue() << endl;
     	}
    	getch();
    	return 0;
    }
    Object::Object(void) {
    	value = 0;
    }
    int Object::getValue(void) {
    	return value;
    }
    void Object::setValue(int value) {
    	this->value = value;
    }
    The output is:

    Code:
    9
    9
    ====
    0
    0
    In the first loop I change the values of 'value' property and output it immediately to check, then in the second loop I just output the values but they are zero again.

  2. #2
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    object = *i;

    makes a copy of the object in the list. you modify and output the copy in the first loop and make and output a new copy in the second loop

    this one should do it:

    Code:
    for (i = objects.begin(); i != objects.end(); i++) {
                    i->setValue(9);
                    cout << i->getValue() << endl;
    }
    cout << "====" << endl;
    for (i = objects.begin(); i != objects.end(); i++) {
          cout << i->getValue() << endl;
    }
    also it would be better to minimize the scope of your vars, for example like that:
    Code:
    for ( list<Object>::iterator i = objects.begin(); i != objects.end(); i++) {
    instead of declaring i outside the for-loop. that could lead to unwanted sideeffects later
    Last edited by pheres; 11-30-2006 at 07:34 AM.

  3. #3
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    Quote Originally Posted by pheres
    object = *i;

    makes a copy of the object in the list. you modify and output the copy in the first loop and make and output a new copy in the second loop
    Thanks a lot!

    Although, by the moment I've realized this myself. But still thanks.


    Quote Originally Posted by pheres
    also it would be better to minimize the scope of your vars, for example like that:
    Code:
    for ( list<Object>::iterator i = objects.begin(); i != objects.end(); i++) {
    instead of declaring i outside the for-loop. that could lead to unwanted sideeffects later
    Thanks for your advice, but I'm usually careful with variables.

  4. #4
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    Quote Originally Posted by Mike Borozdin
    I'm usually careful with variables.
    And that is exactly the reason why to limit their scope

  5. #5
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    Ok, just like

    Code:
    int i
    in the 'for' loop.

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