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EGD Eric

Deleting objects in std::list

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I''ve been trying to delete objects in std::list, but it never seems to work right. In the following example, I try to delete all the "dead" objects in a list by looping through them and using remove on the "dead" ones. After deleting the dead object, the next object in the loop causes a crash, as though I were trying to access a NULL pointer.
#include <iostream>

using std::cout;
using std::endl;

#include <list>
#include <algorithm>
#include <afxwin.h>
#include <fstream>
using std::ostream;
using std::istream;
using std::ofstream;

using std::list;
using std::fstream;
using std::ios;

#define SIZE 5

class Example
	Example() {}
	Example(int Value, bool bDead = false) {value = Value; dead =  bDead;}
	int getValue() {return value;}
	void setValue(int newValue) {value = newValue;}
	bool dead;
	int value;


//typedef for easier readability

typedef list<Example *> ExampleList;
typedef	list<Example *>::iterator exItor;

void printList(std::list< Example * > &listRef);
void saveList(std::list< Example * > &listRef, char* filename);
void loadList(std::list< Example * > &listRef, char* filename);

void main()
	ExampleList theList; //list of example classes

	ExampleList secondList;
	Example* ptr;

	//Populate the list

//	loadList(theList, "Example.txt");

	theList.push_back(new Example(5));
	theList.push_back(new Example(4));
	theList.push_back(ptr = new Example(3, true));
	theList.push_back(new Example(2));
	theList.push_back(new Example(1));

	secondList = theList;
	list<Example *>::iterator it;
	cout << "Deleting dead items:" << endl;
	for(it = theList.begin(); it != theList.end(); it++)
		if((*it)->dead == true)

	cout << "Here''s the second:\n";

void printList(std::list< Example * > &listRef)

		cout << "List is empty.\n";

	//	std::ostream_iterator< T > output(cout, " "); //declare output iterator

	//	std::copy(listRef.begin(), listRef.end(), output); //copy to output stream

		std::list< Example * >::iterator it;

		for(it = listRef.begin(); it != listRef.end(); it++)
			cout << (*it)->getValue() << endl;
		cout << endl;

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The loop you're using to remove dead items is error-prone. The problem looks like you're trying increment an invalidated iterator. Instead consider using the std::list::remove_if() member function.

edit: unless you're using MSVC 6 which doesn't properly support the remove_if() member function for std::list objects. In that case you'd need to do something like:

for(it = theList.begin(); it != theList.end(); ) {
if((*it)->dead == true) {
it = theList.erase(it);
} else {

[edited by - SiCrane on March 21, 2004 4:22:22 PM]

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A less elegant but more simple (and erase_if-free) solution might look a bit like...

list<Example*>::iterator thisOne = ExampleList.begin(),
max = ExampleList.end(),
lastOne = thisOne;


while (thisOne != max)
if (*thisOne->dead)
thisOne = lastOne;
lastOne = thisOne;

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