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

Adding to @sehe's answer, the following shows how to modify the code in case your moveable type is not default constructible:

Edited: changed code to properly deal with destruction of *extracted.
Edited: added alternative with std::unique_ptr.
Edited: added a second altrnative by sehe.

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#include 
#include 
#include 
#include 

struct moveonly {
    int x;
    moveonly(int x) noexcept : x(x) {}
    moveonly(moveonly&& o) noexcept : x(o.x) { o = {-1}; }
    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) {
    std::aligned_storage::type buffer;
    moveonly* extracted = reinterpret_cast(&buffer);

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

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

    try {
        moveonly ret = std::move(*extracted);
        extracted->~moveonly();
        return ret;
    } catch(...) {
        extracted->~moveonly();
        throw;
    }
}

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);
}

Same thing using std::unique_ptr for cleanup:

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#include 
#include 
#include 
#include 
#include 

struct moveonly {
    int x;
    moveonly(int x) noexcept : x(x) {}
    moveonly(moveonly&& o) noexcept : x(o.x) { o = {-1}; }
    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) {
    std::aligned_storage::type buffer;
    moveonly* extracted = reinterpret_cast(&buffer);

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

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

    std::unique_ptr ptr = {
        extracted,
        [](moveonly* p){ p->~moveonly(); }
    };

    return std::move(*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);
}

Sehe provides yet another alternative based on boost::optional which is the most elegant of all:

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#include 
#include 
#include 
#include 
#include 
#include 

struct moveonly {
    int x;
    moveonly(int x) noexcept : x(x) {}
    moveonly(moveonly&& o) noexcept : x(o.x) { o = {-1}; }
    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) {
    boost::optional 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 std::move(*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);
}