How to erase while iterating boost::intrusive::list

Question

How can I erase elements from a boost::intrusive::list while iterating through it? The following code fails with an assertion failure https://wandbox.org/permlink/nzFshFSsaIrvBiTa

#include <iostream>
#include <vector>
#include <boost/intrusive/list.hpp>

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

class Integer : public boost::intrusive::list_base_hook<> {
public:
    explicit Integer(int a_in) : a{a_in} {}
    int a;
};

int main() {
    auto vec = std::vector<Integer>{};
    vec.push_back(Integer{1});
    vec.push_back(Integer{2});
    vec.push_back(Integer{3});
    auto list = boost::intrusive::list<Integer>{};
    for (auto ele : vec) {
        list.push_back(ele);
    }

    for (auto it = list.begin(); it != list.end();) {
        if (it->a == 2) {
            it = list.erase(it);
        } else {
            ++it;
        }
    }

    for (auto ele : list) {
        cout << ele.a << endl;
    }
}

Answer 1

Your problem is that you have pushed temporaries into the list:

for (auto ele : vec) {
    list.push_back(ele);
}

You probably meant to write:

for (auto& ele : vec) {
    list.push_back(ele);
}

This is classic confusion when starting to work with intrusive containers: nothing is by value like all the with all the standard library containers.

To avoid situations like this, consider using the auto-unlinking hook mode.

Demo

Even safer than having to think about reference-qualifying the loop variable, is having no loop at all:

boost::intrusive::list<X> list(vec.begin(), vec.end());

Live On Coliru

#include <iostream>
#include <iterator>
#include <vector>
#include <boost/intrusive/list.hpp>

struct X : public boost::intrusive::list_base_hook<> {
    X(int a_in) : a{a_in} {}
    int a;

    friend std::ostream& operator<<(std::ostream& os, X const& x) { return os << "{" << x.a << "}"; }
};

int main() {
    std::ostream_iterator<X> out(std::cout << std::unitbuf, " ");

    std::vector<X> vec { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
    boost::intrusive::list<X> list(vec.begin(), vec.end());

    std::cout << "before: "; std::copy(list.begin(), list.end(), out);

    list.remove_if([](X const& x) { return 0 == (x.a % 2); });

    std::cout << "\nafter: "; std::copy(list.begin(), list.end(), out);
}

Prints

before: {1} {2} {3} {4} {5} {6} {7} {8} {9} {10} 
after: {1} {3} {5} {7} {9} 

With auto_unlink:

struct X : public bi::list_base_hook<bi::link_mode<bi::auto_unlink> > {

Note, you need to disable constant-time size() support for the list<> (see reference)

With that in place, even adding

vec.erase(vec.begin()+4);

will correctly unlink the corresponding node from the intrusive list:

Live On Coliru

#include <iostream>
#include <iterator>
#include <vector>
#include <boost/intrusive/list.hpp>

namespace bi = boost::intrusive;

struct X : public bi::list_base_hook<bi::link_mode<bi::auto_unlink> > {
    X(int a_in) : a{a_in} {}
    int a;

    friend std::ostream& operator<<(std::ostream& os, X const& x) { return os << "{" << x.a << "}"; }
};

int main() {
    std::ostream_iterator<X> out(std::cout << std::unitbuf, " ");

    std::vector<X> vec { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
    bi::list<X, bi::constant_time_size<false> > list(vec.begin(), vec.end());

    std::cout << "before: "; std::copy(list.begin(), list.end(), out);

    list.remove_if([](X const& x) { return 0 == (x.a % 2); });

    std::cout << "\nafter: "; std::copy(list.begin(), list.end(), out);

    vec.erase(vec.begin()+4);
    std::cout << "\nauto-unlinked: "; std::copy(list.begin(), list.end(), out);
}

Prints

before: {1} {2} {3} {4} {5} {6} {7} {8} {9} {10} 
after: {1} {3} {5} {7} {9} 
auto-unlinked: {1} {3} {6} {8} {10}