将元素从向量复制到另一个

Question

I have two vectors of pointers: arr that contains some elements already and temp , the new vector I want to copy specific elements from arr to. For example I'd like to copy second element of arr to be copied into temp and deleted from arr . How can it be done?

I tried this, but it's not good:

void deleteobject(vector < figure3d *> &arr,int index,vector < figure3d *> &temp)
{
     vector < figure3d * > :: iterator i=arr.begin();
     temp.insert(temp.begin(),*i);
     delete *i;
     arr.erase(i);
     temp[0]->print();
}

Answer 1

您不应该delete要复制的对象，因为您希望将其保持在temp -只需将其从arr erase 。

Answer 2

arr.begin() gives you an iterator pointing to the first element, so if you want the second element, you should advance i one:

++i;

If you want the index th element, you should advance i by index :

i += index;

Storing raw pointers in a vector, and having to remember to delete them when removing them from the vector is a memory leak waiting to happen. And ass molbdnilo mentioned in another answer, you actually have a memory handling error since you're deleting the object that temp has a pointer to.

Answer 3

From what I understand you want to copy an element at specified index (argument index ) and remove it from vector temp . This function could look simple like this:

void deleteobject(std::vector<figure3d*> &arr, int index, std::vector<figure3d*> &temp)
{
    temp.insert(temp.begin(), arr[index]);
    arr.erase(arr.begin() + index);
}

In this case, you don't copy the object itself, but only a reference to it - thus you don't need to free the memory where this object is stored.

Also note that in your function you call delete *i; which frees the memory where the object pointed by i is stored - and the pointer that stored in arr becomes invalid ( dangling pointer ), because it points to memory that was freed already.

And I also suggest you to use vector of objects rather than vector of pointers to objects - although elements get copied, it's usually fast enough and there is no troublesome memory management connected with it. And in case there is a good reason why you use vector of pointers ( When to use pointers in C++ ), I suggest you to use vector of smart pointers instead: if you have TR1 or C++11 support, use std::shared_ptr , otherwise use boost::shared_ptr ( Where is shared_ptr? )

Answer 4

This looks like a job for !
something like this is what I would do:

Class removePredicate{
Public: 
    bool operator() (figure3d** item) {
       //remove logic goes here
    }
};

remove_copy_if(arr.begin(), arr.end(), back_inserter(tmp), not1(removePredicate));

This will copy all the elements that you want to remove into tmp. The to actually remove them from arr it's just a simple application of the remove-erase idiom:

erase(remove_if(arr.begin(), arr.end(), not1(removePredicate)), arr.end());

There's probably a way to combine these two steps into one line but you run the risk of losing readability at point.