Is it possible to remove elements from a vector of shared_ptr?

Question

Say I have

vector<shared_ptr<string>> enemy;

how do I remove elements from the enemy vector?

Thanks for your help in advance

**Edit (code in context)

void RemoveEnemy( vector<shared_ptr<Enemy>> & chart, string id )
{
  int i = 0;
  bool found = FALSE;
  for(auto it = chart.begin(); it != chart.end(); i++)
  {
    if(id == chart[i]->GetEnemyID() )
    {
        found = TRUE;
        chart.erase(it);
    }
}

the code above segfaults me

The same way you remove them from any other `vector`? What problem are you having? — UnholySheep, Apr 26 '19 at 22:28
https://stackoverflow.com/questions/347441/erasing-elements-from-a-vector — cplusplusrat, Apr 26 '19 at 22:29
Look here https://en.cppreference.com/w/cpp/container/vector/erase — Zan Lynx, Apr 26 '19 at 22:31
Yes, it's very possible. There a couple of ways to do this, depending on what you might want to remove. Do you want to remove `shared_ptrs` based on the _addresses_ or _values_ of the shared strings? — alter_igel, Apr 26 '19 at 22:32
Here's `remove_if` which you'd want to use with a lambda probably. See the example for the erase/remove idiom: https://en.cppreference.com/w/cpp/algorithm/remove — Zan Lynx, Apr 26 '19 at 22:34
There are dozens of ways to remove elements from a vector. You should check the manual and come back if you have trouble using one of the functions that removes elements from the vector. — Galik, Apr 27 '19 at 00:20

Remy Lebeau · Accepted Answer · 2019-04-27T01:13:03.487

You remove elements the same way you remove any elements from any std::vector - via the std::vector::erase() method, for instance. All you need for that is an iterator to the desired element to remove.

In your case, since you are storing std::shared_ptr<std::string> objects rather than storing actual std::string objects, you may need to use something like std::find_if() to find the vector element containing the desired string value, eg:

void removeEnemy(string name)
{
    auto iter = std::find_if(enemy.begin(), enemy.end(),
        [&](auto &s){ return (*s == name); }
    );
    if (iter != enemy.end())
        enemy.erase(iter);
}

UPDATE: in the new code you have added, you are incorrectly mixing indexes and iterators together. You are creating an infinite loop if the vector is not empty, as you never increment the it iterator that controls your loop, you are incrementing your index i variable instead (see what happens when you don't give your variables unique and meaningful names?). So you end up going out of bounds of the vector into surrounding memory. That is why you get the segfault error.

Even though you are (trying to) use an iterator to loop through the vector, you are using indexes to access the elements, instead of dereferencing the iterator to access the elements. You don't need to use indexes at all in this situation, the iterator alone will suffice.

Try this instead:

void RemoveEnemy( vector<shared_ptr<Enemy>> & chart, string id )
{
  for(auto it = chart.begin(); it != chart.end(); ++it)
  {
    if (id == it->GetEnemyID() )
    {
      chart.erase(it);
      return;
    }
}

Or, using the kind of code I suggested earlier:

void RemoveEnemy( vector<shared_ptr<Enemy>> & chart, string id )
{
    auto iter = std::find_if(chart.begin(), chart.end(),
        [&](auto &enemy){ return (enemy->GetEnemyID() == id); }
    );
    if (iter != chart.end())
        chart.erase(iter);
}

whenever I try this I get this error: error: no matching function for call to ‘std::vector >::erase(int&)’ — dcjboi, Apr 27 '19 at 00:40
@dcjboi The code I showed can't produce that error. `vector::erase()` takes a `vector::iterator` as input, calling `std::find_if()` on a `vector` returns a `vector::iterator`, and a `vector::iterator` is not implemented as an `int&`. Are you trying to pass an *index* to `erase()` instead of an `iterator`? Please edit your question to show the actual code you are using. — Remy Lebeau, Apr 27 '19 at 00:50
@dcjboi you are mixing indexes with iterators. There is no reason to do that. — Remy Lebeau, Apr 27 '19 at 01:03

score 1 · Answer 2 · answered Apr 27 '19 at 02:03

1

The problem with your code is that erase() invalidates the iterator. You must use it = chart.erase(it).

answered Apr 27 '19 at 02:03

Zan Lynx

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Zan Lynx · Answer 3 · 2019-07-08T16:46:26.847

I like mine which will remove aliens at high speed and without any care for the ordering of the other items. Removal with prejudice!

Note: remove_if is most often used with erase and it will preserve the order of the remaining elements. However, partition does not care about the ordering of elements and is much faster.

partition-test.cpp:
make partition-test && echo 1 alien 9 alien 2 8 alien 4 7 alien 5 3 | ./partition-test

#include <algorithm>
#include <iostream>
#include <iterator>
#include <memory>
#include <string>
#include <vector>

using namespace std;

template <typename T>
ostream &operator<<(ostream &os, const vector<T> &container) {
  bool comma = false;
  for (const auto &x : container) {
    if (comma)
      os << ", ";
    os << *x;
    comma = true;
  }
  return os;
}

int main() {
  vector<shared_ptr<string>> iv;
  auto x = make_shared<string>();
  while (cin >> *x) {
    iv.push_back(x);
    x = make_shared<string>();
  }
  cout << iv << '\n';

  iv.erase(partition(begin(iv), end(iv),
                     [](const auto &x) { return *x != "alien"s; }),
           end(iv));
  cout << iv << '\n';
  return 0;
}

Is it possible to remove elements from a vector of shared_ptr?

3 Answers3