{"id":411,"date":"2007-11-30T16:21:26","date_gmt":"2007-11-30T22:21:26","guid":{"rendered":"http:\/\/blog.codedread.com\/archives\/2007\/11\/30\/c-stl-safely-removing-items-from-a-container\/"},"modified":"2007-11-30T16:21:26","modified_gmt":"2007-11-30T22:21:26","slug":"c-stl-safely-removing-items-from-a-container","status":"publish","type":"post","link":"https:\/\/www.codedread.com\/blog\/archives\/2007\/11\/30\/c-stl-safely-removing-items-from-a-container\/","title":{"rendered":"C++ STL: Safely Removing Items From A Container"},"content":{"rendered":"

A short article about removing items from a standard C++ container. <\/p>\n

When using the C++ Standard Template Library, one has to be careful not to invalidate iterators in a container that you're iterating over. If that makes any sense to you, then <\/p>\n

<\/object><\/div>\n

A colleague of mine had code like this:<\/p>\n

<\/p>\n

class TaskList {<\/p>\n

  vector<Task*> mTasks;<\/p>\n

  typedef vector<Task*>::iterator TaskIter;<\/p>\n

 <\/p>\n

  void removeTask(Task* t) {<\/p>\n

    TaskIter it = find(mTasks.begin(),mTasks.end(),t);<\/p>\n

    if(it != mTasks.end()) { mTasks.erase(it); }<\/p>\n

  }<\/p>\n

 <\/p>\n

 public:<\/p>\n

  void addTask(Task* t) { mTasks.push_back(t); }<\/p>\n

 <\/p>\n

  void processQueue() {<\/p>\n

    for(TaskIter loopit=mTasks.begin();loopit!=mTasks.end();++loopit) {<\/p>\n

      Task* task = *loopit;<\/p>\n

      if(task->isNotDone()) {<\/p>\n

        task->doSomething();<\/p>\n

      }<\/p>\n

      else {<\/p>\n

        removeTask(task);<\/p>\n

      }<\/p>\n

    }<\/p>\n

  }<\/p>\n

};<\/p>\n

<\/code><\/p>\n

You might be surprised to hear that the program crashed when the first task had to be removed. The reason is that the removeTask() function erases an item out the collection which invalidates the iterator loopit. When the for-loop tries to increment the iterator, we get unexpected results<\/i>.<\/p>\n

This is a frustrating problem, but it can be solved. In short you have to:<\/p>\n