How to transfer nodes between two boost::intrusive::slist objects

Question

Is it valid to transfer nodes between two boost::intrusive::slist<boost::intrusive::cache_last<true>> objects? Something like the following

auto one = boost::intrusive::slist<Node, boost::intrusive::cache_last<true>>{};
auto two = boost::intrusive::slist<Node, boost::intrusive::cache_last<true>>{};

auto node = std::make_unique<Node>();
one.push_back(*node);

auto& front = one.front();
one.pop_front();
two.push_back(front);

I'm getting a segmentation fault, and an assertion failure with boost version 1.70.0 https://wandbox.org/permlink/nWHakTYUiVBGKH6I. How can I fix this?

Note: I can't allocate a new node and copy the old one because I am using an intrusive list to have control over when and where the allocation happens.

Sorry I deleted my answer. You are dereferencing when you push_back. I think sth happens in the dtor. I got it working but can't delete the same problem unique ptr has: wandbox.org/permlink/q5pBdd0JBdJN5ewD — Oblivion
@oblivion Sorry, I forgot to mention, I need to transfer the nodes. In particular I want to avoid allocating a new node for the purposes of transfering a node from one to another. — Curious

spectras spectras · Accepted Answer · 2019-06-28T09:42:11

It seems to be the purpose of the splice method:

Effects: Transfers all the elements of list x to this list, before the the element pointed by it. No destructors or copy constructors are called.

two.splice(two.end(), one, one.begin());

Fully working demo:

#include <iostream>
#include <string>
#include <boost/intrusive/slist.hpp>

struct Node : public boost::intrusive::slist_base_hook<>
{
    int         i;
    std::string s;

    Node(int i, std::string s) : i(i), s(std::move(s)) {}
};

using NodeList = boost::intrusive::slist<Node, boost::intrusive::cache_last<true>>;

int main()
{
    NodeList one;
    NodeList two;

    auto show = [&](auto text){
        std::cout <<text <<"\n  one\n";
        for (auto & item : one) { std::cout <<"    " <<item.i <<' ' <<item.s <<'\n'; }
        std::cout <<"  two\n";
        for (auto & item : two) { std::cout <<"    " <<item.i <<' ' <<item.s <<'\n'; }
    };


    one.push_back(*new Node(42, "hello"));
    show("before splice");

    two.splice(two.end(), one, one.begin());
    show("after splice");

    one.clear_and_dispose([](Node * ptr){ delete ptr; });
    two.clear_and_dispose([](Node * ptr){ delete ptr; });
    return 0;
}

When run, will write:

before splice
  one
    42 hello
  two
after splice
  one
  two
    42 hello

How to transfer nodes between two boost::intrusive::slist objects

2 Answers