Move element from boost multi_index array

Question

Let\'s say I have movable and not copyable object and I have boost multi-index array with random_access index. I need to move my object out of array front, but I cannot find

Answer 1

Non-const element operations are not supported because they could leave elements in a state which would break invariants placed on them by the various indexes.

The closest thing you can do is using modify:

moveonly pop_front(Table& table) {
    moveonly extracted;

    auto it = table.begin();
    if (it == table.end())
        throw std::logic_error("pop_front");

    if (table.modify(it, [&](moveonly& v) { extracted = std::move(v); })) {
        table.erase(it);
    }

    return extracted;
}

Note that modify does incur the cost of checking all indexes, and may fail. Fortunately, if it does fail, the effect is that iterator is erased:

• Effects: Calls mod(e) where e is the element pointed to by position and rearranges *position into all the indices of the multi_index_container. Rearrangement on sequenced indices does not change the position of the element with respect to the index; rearrangement on other indices may or might not succeed. If the rearrangement fails, the element is erased.
• Postconditions: Validity of position is preserved if the operation succeeds.

And here's a live demo:

Live On Coliru

#include 
#include 
#include 

struct moveonly {
    int x;
    moveonly(int x = -1) noexcept : x(x) {}
    moveonly(moveonly&& o) noexcept : x(o.x) { o = {}; }
    moveonly& operator=(moveonly o) noexcept { using std::swap; swap(x, o.x); return *this; }
};

static_assert(not std::is_copy_constructible{}, "moveonly");

namespace bmi = boost::multi_index;
using Table   = bmi::multi_index_container >
    > >;

template 
void dump(std::ostream& os, Container const& c) { 
    for (auto& r: c) os << r.x << " ";
    os << "\n";
}

moveonly pop_front(Table& table) {
    moveonly extracted;

    auto it = table.begin();
    if (it == table.end())
        throw std::logic_error("pop_front");

    if (table.modify(it, [&](moveonly& v) { extracted = std::move(v); })) {
        table.erase(it);
    }

    return extracted;
}

int main() {
    Table table;

    table.push_back({1});
    table.push_back({2});
    table.push_back({3});

    dump(std::cout << "table before: ", table);

    std::cout << "Extracted: " << pop_front(table).x << "\n";

    dump(std::cout << "table after: ", table);
}

Which prints:

table before: 1 2 3 
Extracted: 1
table after: 2 3