I have a one-to-many map class - MyMap1N<WeakPtr_Parent,WeakPtr_Children>.
By design, it is supposed to store weak pointers of game-related instance.

Roughly speaking, it is called like :-

MyMap1N<WeakPtr<Room>,WeakPtr<RigidBody>> map;
WeakPtr<Room> room=create<Room>();
WeakPtr<RigidBody> body=create<RigidBody>();
map.add(room,body);
MyArray<WeakPtr<RigidBody>> bodys=map.getAllChildren(room);

By profiling, I found that std::unordered_map is too slow.
Thus, I had to find another way to implement it.

I decided to create an array (instead of unordered_map) in Room.
To increase speed of query, I also inject the indexInArray to store near every instance of RigidBody (see the below image).

With this indexInArray, it is possible to make operation add(room,body) and remove(room,body) get O(1), and guarantee that every slot of Room::bodys is occupied.

Question

A problem arises when some instances of child (RigidBody) are deleted.
MyMap1N cannot even know it.

How to clean the MyMap1N when some instances of RigidBody is deleted?

Note : (available tools / restriction)

In my case, fortunately, cost of checking "whether WeakPtr<> is nullptr" is very cheap.
Every instance has its own unique int ID.
The ID runs separating for each type and the ID's value is low (because I recycle it).
I use multi-threading.

(Edit:clarify) There are a lot of MyMap1N<Something,Something> that scatters around in many System-like class.
Thus, it is very unmaintainable to hardcode like this :-

rigidBody->destroy() ===> {     
        SystemA::mapRoomBody::removeParent(rigidBody) ;
        SystemA::mapCatBody::removeParent(rigidBody) ;
        SystemB::mapBodyDog::removeAllChildren(rigidBody) ;
}  //: Cat and Dog denotes some arbitrary GameObject-type class

My poor solution

Solution 1

I register every instances of MyMap1N to a central location automatically.

If a RigidBody is deleted, the central system will callback to every related MyMap1N.

(To determine whether a MyMap1N is related,
I used some template magic like MyMap1N::Type_Parent and MyMap1N::Type_Children.)

rigidBody->destroy()   
    ===> central->kill(RigidBody*) 
        ===> MyMap1N<WeakPtr<Room>,WeakPtr<RigidBody>>::removeParent(RigidBody*) 
              ... and many other related instances of MyMap1N

It works, but very slow.
I believe cache miss is the cause (not sure).

Solution 2 (my old version)

Whenever a user wants to delete a RigidBody, just marks it.
At the end of timestep, do same as workaround 1.
It is faster. Perhaps, it is because computer love batching. (e.g. less vtable cost)
However, it still uses CPU about 10-20% of the whole game.

Solution 3 (currently using)

If a RigidBody is deleted, don't do anything.
However, when I query add(room,body)/remove(room,body)/getAllChildren(room)/getParent(body), I have to check whether WeakPtr<>==nullptr.

It is fast. There is zero cost at the deleting and every query is also fast.

The disadvantage is that the array Room::bodys grows forever
because Room::Bodys gradually filled with X (Occupied but the object was deleted).
My program throws an assert-memory-fail at the 200th time-step.

Solution 4

I am considering using Solution 3,
but also creating a new function MyMap1N::periodicCleanUp to remove all the X i.e. repack it.

The function should be called periodically, perhaps once every 10 timesteps.
(like a big cleaning day)

I feel it is a hack and highly based on custom tuning (i.e. subjective adjustment).

From what has been gathered from the question and the comments, there appears to be a few viable solutions.

Solution 1

The first possible solution that others have been pointing out in the comments would be using a free index slot before appending to the array. This would involve each Room or object holding an array RigidBody to have a list of free indexes, std::forward_list or std::vector would be good for this. Then, you can add a RigidBody by first checking if there an available slot from the list. If there is, you pop off that index from the list, otherwise you append to the array. Removing a RigidBody simply involves pushing that freed up index to the list of available slots. Now, this solution would require that each RigidBody contains a list of parent and index pairs. That way, when the RigidBody is destroyed you simply notify each parent to free up the index the object was using.

Advantages

Could be a bit odd to implement.
Adding and removing is O(1).
Iteration speed is generally good.

Disadvantages

Uses a decent amount of memory.
The array will be growing.
Have to use a unique key per parent.

Solution 2

There is also another similar type of solution that was discussed in the comments. However, instead of a RigidBody having multiple indexes for each parents, it has one unique ID that acts as an index. This unique ID should have a known range of minimum and maximum values. Then, each parent would allocate enough space to house the maximum amount of IDs and RigidBodies. The destruction and removal of a RigidBody is simple since you have to simply pass at ID/index to each registered parent. In addition, you could use a list to keep track of free ID's.

Advantages

Array will not be growing during runtime.
Adding and removing is O(1).
Less keys and indexes.
Same key/index for all parents.
Great if the array is going to be mostly filled.

Disadvantages

Uses a lot of memory.
Iteration will be inefficient if the array is mostly empty.

Solution 3

The periodic cleanup idea you suggested could work. However, it is likely that cleaning up all of the arrays in one-go could cost a lot of time. Hence, a possible adjustment would be to partially clear the array at the end of every timestep. That adjustment would require you having to store an index of where you last left off. To which, you would use that index to continue clearing sections of the array. Once the array has been fully cleared you can reset that index to 0 and start over. This solution and adjustment would only work if the rate you are removing bodies is usually greater than the rate of adding bodies.

Advantages

Easy to implement.
Easy to tune and adjust.

Disadvantages

Could fail depending on the rate of items being added and removed.
Could use more memory than necessary.

Solution 4

Another solution would involve using the address or ID of the rigid body to 'hash' or it into an array of vectors. This array of vectors could be accomplished by using a prime number to act as the size of the array. Then, we can use the RigidBodies ID or address and modulo that with the size of the array to place it into a vector. This makes erasing faster than a normal vector. In addition, it uses less memory than a massive static array of slots. Iterating over this structure would involve iterating over each bucket/vector. Or you can create a custom iterator that does this for you.

Basic Implementation of Structure

namespace {
    template<typename Int>
    constexpr bool isPrime(Int num, Int test = 2) {
        return (test * test > num ? true : (num % test == 0 ? false : isPrime(num, test + 1)));
    }
    //Buckets must be a size
    template<typename data_t, std::size_t PRIME_SIZE, typename = typename std::enable_if<isPrime(PRIME_SIZE)>::type>
    class BucketVector
    {
    public:
        constexpr static auto SIZE = PRIME_SIZE;
        template<bool is_const>
        using BucketIteratorBase = typename  std::iterator<std::bidirectional_iterator_tag, typename std::conditional<is_const, const data_t, data_t>::type>;
        using uint_t = std::uintptr_t;
        using BucketType = std::vector<data_t>;
        template<bool is_const>
        class BucketIterator : public BucketIteratorBase<is_const> {
        public:
            using Base = BucketIteratorBase<is_const>;
            using BucketOwner = BucketVector<data_t, PRIME_SIZE>;
            using typename Base::pointer;
            using typename Base::reference;
            using typename Base::value_type;
            friend class BucketIterator<!is_const>;
            std::size_t m_bucket;
            pointer m_value;
            BucketOwner* m_owner;
        public:
            BucketIterator(std::size_t bucket, pointer value, BucketOwner* owner)
                : m_bucket(bucket),
                m_value(value),
                m_owner(owner) {
                //validateIterator();
            }
            ~BucketIterator() {
            }
            template<bool value, typename = typename std::enable_if<!value || (value == is_const)>::type>
            BucketIterator(const BucketIterator<value>& iterator)
                : m_bucket(iterator.m_bucket),
                m_value(iterator.m_value),
                m_owner(iterator.m_owner) {
            }
            template<bool value, typename = typename std::enable_if<!value || (value == is_const)>::type>
            BucketIterator(BucketIterator<value>&& iterator)
                : m_bucket(std::move(iterator.m_bucket)),
                m_value(std::move(iterator.m_value)),
                m_owner(std::move(iterator.m_owner)) {
            }
            template<bool value, typename = typename std::enable_if<!value || (value == is_const)>::type>
            BucketIterator& operator=(BucketIterator<value>&& iterator) {
                m_bucket = std::move(iterator.m_bucket);
                m_value = std::move(iterator.m_value);
                m_owner = std::move(iterator.m_owner);
                return *this;
            }
            template<bool value, typename = typename std::enable_if<!value || (value == is_const)>::type>
            BucketIterator& operator=(const BucketIterator<value>& iterator) {
                m_bucket = iterator.m_bucket;
                m_value = iterator.m_value;
                m_owner = iterator.m_owner;
                return *this;
            }
            BucketIterator& operator++() {
                ++m_value;
                forwardValidate();
                return *this;
            }
            BucketIterator operator++(int) {
                BucketIterator copy(*this);
                ++(*this);
                return copy;
            }
            BucketIterator& operator--() {
                backwardValidate();
                --m_value;
                return *this;
            }
            BucketIterator operator--(int) {
                BucketIterator copy(*this);
                --(*this);
                return copy;
            }
            reference operator*() const {
                return *m_value;
            }
            pointer operator->() const {
                return m_value;
            }
            template<bool value>
            bool operator==(const BucketIterator<value>& iterator) const {
                return m_bucket == iterator.m_bucket && m_owner == iterator.m_owner && m_value == iterator.m_value;
            }
            template<bool value>
            bool operator!=(const BucketIterator<value>& iterator) const {
                return !(this->operator==(iterator));
            }
            BucketOwner* getSystem() const {
                return m_owner;
            }
            inline void backwardValidate() {
                while (m_value == m_owner->m_buckets[m_bucket].data() && m_bucket != 0) {
                    --m_bucket;
                    m_value = m_owner->m_buckets[m_bucket].data() + m_owner->m_buckets[m_bucket].size();
                }
            }
            inline void forwardValidate() {
                while (m_value == (m_owner->m_buckets[m_bucket].data() + m_owner->m_buckets[m_bucket].size()) && m_bucket != SIZE - 1) {
                    m_value = m_owner->m_buckets[++m_bucket].data();
                }
            }
        };
        using iterator = BucketIterator<false>;
        using const_iterator = BucketIterator<true>;
        friend class BucketIterator<false>;
        friend class BucketIterator<true>;
    private:
        std::array<BucketType, SIZE> m_buckets;
        std::size_t m_size;
    public:
        BucketVector()
            : m_size(0) {
        }
        ~BucketVector() {
        }
        BucketVector(const BucketVector&) = default;
        BucketVector(BucketVector&&) = default;
        BucketVector& operator=(const BucketVector&) = default;
        BucketVector& operator=(BucketVector&&) = default;
        data_t& operator[](std::size_t index) {
            const auto bucketIndex = findBucketIndex(index);
            return m_buckets[bucketIndex.first][bucketIndex.second];
        }
        const data_t& operator[](std::size_t index) const {
            return static_cast<BucketVector*>(this)->operator[](index);
        }
        data_t& at(std::size_t index) {
            if (index >= m_size) {
                throw std::out_of_range("BucketVector::at index out of range");
            }
            return this->operator[](index);
        }
        const data_t& at(std::size_t index) const {
            return static_cast<BucketVector*>(this)->at(index);
        }
        void erase(const_iterator iter) {
            auto& bucket = m_buckets[iter.m_bucket];
            std::size_t index = iter.m_value - bucket.data();
            bucket[index] = bucket.back();
            bucket.pop_back();
            --m_size;
        }
        void push_back(uint_t id, const data_t& data) {
            const auto slot = get_slot(id);
            m_buckets[slot].push_back(data);
            ++m_size;
        }
        void push_back(uint_t id, data_t&& data) {
            const auto slot = get_slot(id);
            m_buckets[slot].push_back(std::move(data));
            ++m_size;
        }
        template<typename... args>
        void emplace_back(uint_t id, args&&... parameters) {
            const auto slot = get_slot(id);
            m_buckets[slot].emplace_back(std::forward<args>(parameters)...);
            ++m_size;
        }

        void pop_back(uint_t index) {
            const auto slot = get_slot(index);
            m_buckets[slot].pop_back();
            --m_size;
        }
        void pop_front(uint_t index) {
            const auto slot = get_slot(index);
            m_buckets[slot].pop_front();
            --m_size;
        }
        void reserve(std::size_t size) {
            const std::size_t slotSize = size / SIZE + 1;
            for (auto& bucket : m_buckets) {
                bucket.reserve(slotSize);
            }
        }
        void clear() {
            for (auto& bucket : m_buckets) {
                bucket.clear();
            }
        }
        bool empty() const {
            return m_size != 0;
        }
        std::size_t size() const {
            return m_size;
        }
        iterator find(uint_t index, const data_t& value) {
            const std::size_t slot = get_slot(index);
            auto& bucket = m_buckets[slot];
            for (auto it = bucket.begin(), end = bucket.end(); it != end; ++it) {
                if (*it == value) {
                    return { slot, &(*it), this };
                }
            }
            return end();
        }
        template<typename fn_t>
        iterator find(uint_t index, const fn_t& fn) {
            const std::size_t slot = get_slot(index);
            auto& bucket = m_buckets[slot];
            for (auto it = bucket.begin(), end = bucket.end(); it != end; ++it) {
                if (fn(*it)) {
                    return { slot, &(*it), this };
                }
            }
            return end();
        }
        const_iterator find(uint_t index, const data_t& value) const {
            return cfind(index, value);
        }
        const_iterator cfind(uint_t index, const data_t& value) const {
            return static_cast<BucketVector*>(this)->find(index, value);
        }
        iterator begin(uint_t index = 0) {
            auto bucketIndex = findBucketIndex(index);
            iterator it{ bucketIndex.first, m_buckets[bucketIndex.first].data() + bucketIndex.second, this };
            it.forwardValidate();
            return it;
        }
        iterator end(uint_t index = 0) {
            iterator it{ SIZE - 1, m_buckets.back().data() + m_buckets.back().size(), this };
            return it;
        }
        const_iterator begin(uint_t index = 0) const {
            auto bucketIndex = findBucketIndex(index);
            const_iterator it{ bucketIndex.first, m_buckets[bucketIndex.first].data() + bucketIndex.second, this };
            it.forwardValidate();
            return it;
        }
        const_iterator end(uint_t index = 0) const {
            const_iterator it{ SIZE - 1, m_buckets.back().data() + m_buckets.back().size(), this };
            return it;
        }
        std::size_t get_slot(uint_t id) {
            return id % SIZE;
        }
    private:
        inline std::pair<std::size_t, std::size_t> findBucketIndex(std::size_t index) {
            std::size_t bucket = 0;
            std::size_t count = 0;
            while (index >= m_buckets[bucket].size() + count) {
                count += m_buckets[bucket].size();
                ++bucket;
            }
            return { bucket, index - count };
        }
    };
}

Advantages

Appending is O(1).
Use's less memory than Solution 1 and 2.
Can be used to quickly find out if a RigidBody belongs to a parent.
Erasing is fast for the size of vector you are going to use.
Iteration is faster than Solution's 1 and 2 if the array is more than 50% empty.

Disadvantages

Erasing is fast but not as fast as Solution's 1 and 2.
Vectors will grow.
Iteration is slower than Solution's 1 and 2 if the array is more than 50% full.

Basic Benchmark Program

You can use this program to test various inputs such as size and amount of values to remove to see the performance.

#include <chrono>
#include <iostream>
#include <vector>
#include <algorithm>
#include <random>
#include <set>
#include <iomanip>
#include <unordered_set>
#include <array>
#include <vector>
#include <iterator>
#include <type_traits>


template<typename mclock_t = typename std::conditional<std::chrono::high_resolution_clock::is_steady, std::chrono::high_resolution_clock, std::chrono::steady_clock>::type>
class Benchmarker {
public:
    using ClockType = mclock_t;
    using TimePoint = std::chrono::time_point<ClockType>;
private:
    TimePoint m_start;
    TimePoint m_end;
    bool m_running;
public:
    Benchmarker(bool run = false) {
        m_running = run;

        if (m_running) {
            start();
        }
    }

    Benchmarker& start() {
        m_start = ClockType::now();
        m_running = true;

        return *this;
    }

    Benchmarker& stop() {
        m_end = ClockType::now();
        m_running = false;

        return *this;
    }

    template<typename T = std::chrono::microseconds>
    Benchmarker& printDuration(std::ostream& out) {
        out << std::chrono::duration_cast<T>(m_end - m_start).count();

        return *this;
    }

    template<typename T = std::chrono::microseconds>
    long long getDurationCount() {
        return std::chrono::duration_cast<T>(m_end - m_start).count();
    }

    friend std::ostream& operator<<(std::ostream& out, Benchmarker& benchmarker) {
        out << std::chrono::duration_cast<std::chrono::microseconds>(benchmarker.m_end - benchmarker.m_start).count();

        return out;
    }

    TimePoint getDuration() {
        return m_end - m_start;
    }

    TimePoint getStartTime() {
        return m_start;
    }

    TimePoint getEndTime() {
        return m_end;
    }

    bool isRunning() {
        return m_running;
    }
};

namespace {
    template<typename Int>
    constexpr bool isPrime(Int num, Int test = 2) {
        return (test * test > num ? true : (num % test == 0 ? false : isPrime(num, test + 1)));
    }
    //Buckets must be a size
    template<typename data_t, std::size_t PRIME_SIZE, typename = typename std::enable_if<isPrime(PRIME_SIZE)>::type>
    class BucketVector
    {
    public:
        constexpr static auto SIZE = PRIME_SIZE;
        template<bool is_const>
        using BucketIteratorBase = typename  std::iterator<std::bidirectional_iterator_tag, typename std::conditional<is_const, const data_t, data_t>::type>;
        using uint_t = std::uintptr_t;
        using BucketType = std::vector<data_t>;
        template<bool is_const>
        class BucketIterator : public BucketIteratorBase<is_const> {
        public:
            using Base = BucketIteratorBase<is_const>;
            using BucketOwner = BucketVector<data_t, PRIME_SIZE>;
            using typename Base::pointer;
            using typename Base::reference;
            using typename Base::value_type;
            friend class BucketIterator<!is_const>;
            std::size_t m_bucket;
            pointer m_value;
            BucketOwner* m_owner;
        public:
            BucketIterator(std::size_t bucket, pointer value, BucketOwner* owner)
                : m_bucket(bucket),
                m_value(value),
                m_owner(owner) {
                //validateIterator();
            }
            ~BucketIterator() {
            }
            template<bool value, typename = typename std::enable_if<!value || (value == is_const)>::type>
            BucketIterator(const BucketIterator<value>& iterator)
                : m_bucket(iterator.m_bucket),
                m_value(iterator.m_value),
                m_owner(iterator.m_owner) {
            }
            template<bool value, typename = typename std::enable_if<!value || (value == is_const)>::type>
            BucketIterator(BucketIterator<value>&& iterator)
                : m_bucket(std::move(iterator.m_bucket)),
                m_value(std::move(iterator.m_value)),
                m_owner(std::move(iterator.m_owner)) {
            }
            template<bool value, typename = typename std::enable_if<!value || (value == is_const)>::type>
            BucketIterator& operator=(BucketIterator<value>&& iterator) {
                m_bucket = std::move(iterator.m_bucket);
                m_value = std::move(iterator.m_value);
                m_owner = std::move(iterator.m_owner);
                return *this;
            }
            template<bool value, typename = typename std::enable_if<!value || (value == is_const)>::type>
            BucketIterator& operator=(const BucketIterator<value>& iterator) {
                m_bucket = iterator.m_bucket;
                m_value = iterator.m_value;
                m_owner = iterator.m_owner;
                return *this;
            }
            BucketIterator& operator++() {
                ++m_value;
                forwardValidate();
                return *this;
            }
            BucketIterator operator++(int) {
                BucketIterator copy(*this);
                ++(*this);
                return copy;
            }
            BucketIterator& operator--() {
                backwardValidate();
                --m_value;
                return *this;
            }
            BucketIterator operator--(int) {
                BucketIterator copy(*this);
                --(*this);
                return copy;
            }
            reference operator*() const {
                return *m_value;
            }
            pointer operator->() const {
                return m_value;
            }
            template<bool value>
            bool operator==(const BucketIterator<value>& iterator) const {
                return m_bucket == iterator.m_bucket && m_owner == iterator.m_owner && m_value == iterator.m_value;
            }
            template<bool value>
            bool operator!=(const BucketIterator<value>& iterator) const {
                return !(this->operator==(iterator));
            }
            BucketOwner* getSystem() const {
                return m_owner;
            }
            inline void backwardValidate() {
                while (m_value == m_owner->m_buckets[m_bucket].data() && m_bucket != 0) {
                    --m_bucket;
                    m_value = m_owner->m_buckets[m_bucket].data() + m_owner->m_buckets[m_bucket].size();
                }
            }
            inline void forwardValidate() {
                while (m_value == (m_owner->m_buckets[m_bucket].data() + m_owner->m_buckets[m_bucket].size()) && m_bucket != SIZE - 1) {
                    m_value = m_owner->m_buckets[++m_bucket].data();
                }
            }
        };
        using iterator = BucketIterator<false>;
        using const_iterator = BucketIterator<true>;
        friend class BucketIterator<false>;
        friend class BucketIterator<true>;
    private:
        std::array<BucketType, SIZE> m_buckets;
        std::size_t m_size;
    public:
        BucketVector()
            : m_size(0) {
        }
        ~BucketVector() {
        }
        BucketVector(const BucketVector&) = default;
        BucketVector(BucketVector&&) = default;
        BucketVector& operator=(const BucketVector&) = default;
        BucketVector& operator=(BucketVector&&) = default;
        data_t& operator[](std::size_t index) {
            const auto bucketIndex = findBucketIndex(index);
            return m_buckets[bucketIndex.first][bucketIndex.second];
        }
        const data_t& operator[](std::size_t index) const {
            return static_cast<BucketVector*>(this)->operator[](index);
        }
        data_t& at(std::size_t index) {
            if (index >= m_size) {
                throw std::out_of_range("BucketVector::at index out of range");
            }
            return this->operator[](index);
        }
        const data_t& at(std::size_t index) const {
            return static_cast<BucketVector*>(this)->at(index);
        }
        void erase(const_iterator iter) {
            auto& bucket = m_buckets[iter.m_bucket];
            std::size_t index = iter.m_value - bucket.data();
            bucket[index] = bucket.back();
            bucket.pop_back();
            --m_size;
        }
        void push_back(uint_t id, const data_t& data) {
            const auto slot = get_slot(id);
            m_buckets[slot].push_back(data);
            ++m_size;
        }
        void push_back(uint_t id, data_t&& data) {
            const auto slot = get_slot(id);
            m_buckets[slot].push_back(std::move(data));
            ++m_size;
        }
        template<typename... args>
        void emplace_back(uint_t id, args&&... parameters) {
            const auto slot = get_slot(id);
            m_buckets[slot].emplace_back(std::forward<args>(parameters)...);
            ++m_size;
        }

        void pop_back(uint_t index) {
            const auto slot = get_slot(index);
            m_buckets[slot].pop_back();
            --m_size;
        }
        void pop_front(uint_t index) {
            const auto slot = get_slot(index);
            m_buckets[slot].pop_front();
            --m_size;
        }
        void reserve(std::size_t size) {
            const std::size_t slotSize = size / SIZE + 1;
            for (auto& bucket : m_buckets) {
                bucket.reserve(slotSize);
            }
        }
        void clear() {
            for (auto& bucket : m_buckets) {
                bucket.clear();
            }
        }
        bool empty() const {
            return m_size != 0;
        }
        std::size_t size() const {
            return m_size;
        }
        iterator find(uint_t index, const data_t& value) {
            const std::size_t slot = get_slot(index);
            auto& bucket = m_buckets[slot];
            for (auto it = bucket.begin(), end = bucket.end(); it != end; ++it) {
                if (*it == value) {
                    return { slot, &(*it), this };
                }
            }
            return end();
        }
        template<typename fn_t>
        iterator find(uint_t index, const fn_t& fn) {
            const std::size_t slot = get_slot(index);
            auto& bucket = m_buckets[slot];
            for (auto it = bucket.begin(), end = bucket.end(); it != end; ++it) {
                if (fn(*it)) {
                    return { slot, &(*it), this };
                }
            }
            return end();
        }
        const_iterator find(uint_t index, const data_t& value) const {
            return cfind(index, value);
        }
        const_iterator cfind(uint_t index, const data_t& value) const {
            return static_cast<BucketVector*>(this)->find(index, value);
        }
        iterator begin(uint_t index = 0) {
            auto bucketIndex = findBucketIndex(index);
            iterator it{ bucketIndex.first, m_buckets[bucketIndex.first].data() + bucketIndex.second, this };
            it.forwardValidate();
            return it;
        }
        iterator end(uint_t index = 0) {
            iterator it{ SIZE - 1, m_buckets.back().data() + m_buckets.back().size(), this };
            return it;
        }
        const_iterator begin(uint_t index = 0) const {
            auto bucketIndex = findBucketIndex(index);
            const_iterator it{ bucketIndex.first, m_buckets[bucketIndex.first].data() + bucketIndex.second, this };
            it.forwardValidate();
            return it;
        }
        const_iterator end(uint_t index = 0) const {
            const_iterator it{ SIZE - 1, m_buckets.back().data() + m_buckets.back().size(), this };
            return it;
        }
        std::size_t get_slot(uint_t id) {
            return id % SIZE;
        }
    private:
        inline std::pair<std::size_t, std::size_t> findBucketIndex(std::size_t index) {
            std::size_t bucket = 0;
            std::size_t count = 0;
            while (index >= m_buckets[bucket].size() + count) {
                count += m_buckets[bucket].size();
                ++bucket;
            }
            return { bucket, index - count };
        }
    };
}

constexpr std::size_t SIZE = 1'000;
constexpr std::size_t INDEXES = 400;
constexpr std::size_t SPACING = 26;

void vectorFindErase(std::vector<int>& values, int value) {
    const auto end = values.end();
    for (auto it = values.begin(); it != end; ++it) {
        if (*it == value) {
            values.erase(it);
            break;
        }
    }
}
void vectorEraseSorted(std::vector<int>& values, int value) {
    auto it = std::lower_bound(values.begin(), values.end(), value);
    if (it != values.end() && !(value < *it)) {
        values.erase(it);
    }
}

void setErase(std::unordered_set<int>& values, int value) {
    values.erase(value);
}
int main() {
    std::mt19937 rng;
    rng.seed(std::random_device()());


    std::vector<int> values(SIZE);
    std::generate_n(values.begin(), SIZE, []() {
        static int index = 0;
        return index++;
    });
    auto sorted = values;
    auto preallocate = values;
    auto vnf = values;

    std::random_shuffle(vnf.begin(), vnf.end(), [&](auto i) {
        return rng() % i;
    });
    std::vector<int> indexes(INDEXES);
    std::generate(indexes.begin(), indexes.end(), [&]() {
        return rng() % SIZE;
    });

    //APPEND VALUES TO BUCKET VECTOR, USE VALUE AS IT'S OWN KEY
    BucketVector<int, 23> bucket;
    for (auto& value : values) {
        bucket.push_back(value, value);
    }



    Benchmarker<> bench(true);

    //NAIVE FIND AND ERASE
    for (auto& index : indexes) {
        vectorFindErase(vnf, index);
    }
    std::cout << std::left;
    std::cout << std::setw(SPACING) << "Naive Find and Erase: " << bench.stop() << '\n';

    //SORTED ERASE
    bench.start();
    for (auto& index : indexes) {
        vectorEraseSorted(sorted, index);
    }
    std::cout << std::setw(SPACING) << "Sorted erase: " << bench.stop() << '\n';

    //PRELLOCATED ERASE
    bench.start();
    for (auto& index : indexes) {
        preallocate[index] = std::numeric_limits<int>::min();
    }
    std::cout << std::setw(SPACING) << "Prellocated erase: " << bench.stop() << '\n';

    //BUCKETVECTOR ERASE
    bench.start();
    for (auto& index : indexes) {
        auto it = bucket.find(index, index);
        if (it == bucket.end()) {
            continue;
        }
        bucket.erase(it);
    }

    std::cout << std::setw(SPACING) << "BucketVector erase: " << bench.stop() << '\n';

    //BUCKET SUM/ITERATE
    bench.start();
    long long bucketSum = 0;
    for (std::size_t index = 0; index != 10'000; ++index) {
        for (auto& val : bucket) {
            bucketSum += val;
        }
    }
    std::cout << std::setw(SPACING) << "Bucket Sum/Iterate: " << bench.stop() << ' ' << bucketSum << '\n';


    //PREALLOCATE SUM/ITERATE
    bench.start();
    long long vfsum = 0;
    for (std::size_t index = 0; index != 10'000; ++index) {
        for (auto& val : preallocate) {
            if (val != std::numeric_limits<int>::min()) {
                vfsum += val;
            }
        }
    }

    std::cout << std::setw(SPACING) << "Preallocate sum/Iterate: " << bench.stop() << ' ' << vfsum << '\n';
    std::cin.get();

    return 0;
}

On my machine, I found that the BucketVector was slightly faster to iterate over than a preallocated array when the preallocated array was 50% or more empty with a size of 1000.

来源：https://stackoverflow.com/questions/55898783/cleaning-nullptr-in-one-to-many-relation-that-use-custom-weak-pointer

标签

c++

arrays

c++17

one-to-many

handle

cleaning nullptr in one-to-many relation that use custom weak pointer

Question

My poor solution

Solution 1

Solution 2 (my old version)

Solution 3 (currently using)

Solution 4

Solution 1

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Solution 2

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Solution 3

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Solution 4

Basic Implementation of Structure

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Basic Benchmark Program