readerwriterqueue.h

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// ©2013-2015 Cameron Desrochers.
// Distributed under the simplified BSD license (see the license file that
// should have come with this header).
 
#pragma once
 
#include "atomicops.h"
#include <type_traits>
#include <utility>
#include <cassert>
#include <stdexcept>
#include <cstdint>
#include <cstdlib>              // For malloc/free & size_t
 
 
// A lock-free queue for a single-consumer, single-producer architecture.
// The queue is also wait-free in the common path (except if more memory
// needs to be allocated, in which case malloc is called).
// Allocates memory sparingly (O(lg(n) times, amortized), and only once if
// the original maximum size estimate is never exceeded.
// Tested on x86/x64 processors, but semantics should be correct for all
// architectures (given the right implementations in atomicops.h), provided
// that aligned integer and pointer accesses are naturally atomic.
// Note that there should only be one consumer thread and producer thread;
// Switching roles of the threads, or using multiple consecutive threads for
// one role, is not safe unless properly synchronized.
// Using the queue exclusively from one thread is fine, though a bit silly.
 
#define CACHE_LINE_SIZE 64
 
#ifdef AE_VCPP
#pragma warning(push)
#pragma warning(disable: 4324)  // structure was padded due to __declspec(align())
#pragma warning(disable: 4820)  // padding was added
#pragma warning(disable: 4127)  // conditional expression is constant
#endif
 
namespace moodycamel {
 
template<typename T, size_t MAX_BLOCK_SIZE = 512>
class ReaderWriterQueue
{
        // Design: Based on a queue-of-queues. The low-level queues are just
        // circular buffers with front and tail indices indicating where the
        // next element to dequeue is and where the next element can be enqueued,
        // respectively. Each low-level queue is called a "block". Each block
        // wastes exactly one element's worth of space to keep the design simple
        // (if front == tail then the queue is empty, and can't be full).
        // The high-level queue is a circular linked list of blocks; again there
        // is a front and tail, but this time they are pointers to the blocks.
        // The front block is where the next element to be dequeued is, provided
        // the block is not empty. The back block is where elements are to be
        // enqueued, provided the block is not full.
        // The producer thread owns all the tail indices/pointers. The consumer
        // thread owns all the front indices/pointers. Both threads read each
        // other's variables, but only the owning thread updates them. E.g. After
        // the consumer reads the producer's tail, the tail may change before the
        // consumer is done dequeuing an object, but the consumer knows the tail
        // will never go backwards, only forwards.
        // If there is no room to enqueue an object, an additional block (of
        // equal size to the last block) is added. Blocks are never removed.
 
public:
        // Constructs a queue that can hold maxSize elements without further
        // allocations. If more than MAX_BLOCK_SIZE elements are requested,
        // then several blocks of MAX_BLOCK_SIZE each are reserved (including
        // at least one extra buffer block).
        explicit ReaderWriterQueue(size_t maxSize = 15)
#ifndef NDEBUG
                : enqueuing(false)
                ,dequeuing(false)
#endif
        {
                assert(maxSize > 0);
                assert(MAX_BLOCK_SIZE == ceilToPow2(MAX_BLOCK_SIZE) && "MAX_BLOCK_SIZE must be a power of 2");
                assert(MAX_BLOCK_SIZE >= 2 && "MAX_BLOCK_SIZE must be at least 2");
 
                Block* firstBlock = nullptr;
 
                largestBlockSize = ceilToPow2(maxSize + 1);             // We need a spare slot to fit maxSize elements in the block
                if (largestBlockSize > MAX_BLOCK_SIZE * 2) {
                        // We need a spare block in case the producer is writing to a different block the consumer is reading from, and
                        // wants to enqueue the maximum number of elements. We also need a spare element in each block to avoid the ambiguity
                        // between front == tail meaning "empty" and "full".
                        // So the effective number of slots that are guaranteed to be usable at any time is the block size - 1 times the
                        // number of blocks - 1. Solving for maxSize and applying a ceiling to the division gives us (after simplifying):
                        size_t initialBlockCount = (maxSize + MAX_BLOCK_SIZE * 2 - 3) / (MAX_BLOCK_SIZE - 1);
                        largestBlockSize = MAX_BLOCK_SIZE;
                        Block* lastBlock = nullptr;
                        for (size_t i = 0; i != initialBlockCount; ++i) {
                                auto block = make_block(largestBlockSize);
                                if (block == nullptr) {
                                        throw std::bad_alloc();
                                }
                                if (firstBlock == nullptr) {
                                        firstBlock = block;
                                }
                                else {
                                        lastBlock->next = block;
                                }
                                lastBlock = block;
                                block->next = firstBlock;
                        }
                }
                else {
                        firstBlock = make_block(largestBlockSize);
                        if (firstBlock == nullptr) {
                                throw std::bad_alloc();
                        }
                        firstBlock->next = firstBlock;
                }
                frontBlock = firstBlock;
                tailBlock = firstBlock;
 
                // Make sure the reader/writer threads will have the initialized memory setup above:
                fence(memory_order_sync);
        }
 
        // Note: The queue should not be accessed concurrently while it's
        // being deleted. It's up to the user to synchronize this.
        ~ReaderWriterQueue()
        {
                // Make sure we get the latest version of all variables from other CPUs:
                fence(memory_order_sync);
 
                // Destroy any remaining objects in queue and free memory
                Block* frontBlock_ = frontBlock;
                Block* block = frontBlock_;
                do {
                        Block* nextBlock = block->next;
                        size_t blockFront = block->front;
                        size_t blockTail = block->tail;
 
                        for (size_t i = blockFront; i != blockTail; i = (i + 1) & block->sizeMask) {
                                auto element = reinterpret_cast<T*>(block->data + i * sizeof(T));
                                element->~T();
                                (void)element;
                        }
 
                        auto rawBlock = block->rawThis;
                        block->~Block();
                        std::free(rawBlock);
                        block = nextBlock;
                } while (block != frontBlock_);
        }
 
 
        // Enqueues a copy of element if there is room in the queue.
        // Returns true if the element was enqueued, false otherwise.
        // Does not allocate memory.
        AE_FORCEINLINE bool try_enqueue(T const& element)
        {
                return inner_enqueue<CannotAlloc>(element);
        }
 
        // Enqueues a moved copy of element if there is room in the queue.
        // Returns true if the element was enqueued, false otherwise.
        // Does not allocate memory.
        AE_FORCEINLINE bool try_enqueue(T&& element)
        {
                return inner_enqueue<CannotAlloc>(std::forward<T>(element));
        }
 
 
        // Enqueues a copy of element on the queue.
        // Allocates an additional block of memory if needed.
        // Only fails (returns false) if memory allocation fails.
        AE_FORCEINLINE bool enqueue(T const& element)
        {
                return inner_enqueue<CanAlloc>(element);
        }
 
        // Enqueues a moved copy of element on the queue.
        // Allocates an additional block of memory if needed.
        // Only fails (returns false) if memory allocation fails.
        AE_FORCEINLINE bool enqueue(T&& element)
        {
                return inner_enqueue<CanAlloc>(std::forward<T>(element));
        }
 
 
        // Attempts to dequeue an element; if the queue is empty,
        // returns false instead. If the queue has at least one element,
        // moves front to result using operator=, then returns true.
        template<typename U>
        bool try_dequeue(U& result)
        {
#ifndef NDEBUG
                ReentrantGuard guard(this->dequeuing);
#endif
 
                // High-level pseudocode:
                // Remember where the tail block is
                // If the front block has an element in it, dequeue it
                // Else
                //     If front block was the tail block when we entered the function, return false
                //     Else advance to next block and dequeue the item there
 
                // Note that we have to use the value of the tail block from before we check if the front
                // block is full or not, in case the front block is empty and then, before we check if the
                // tail block is at the front block or not, the producer fills up the front block *and
                // moves on*, which would make us skip a filled block. Seems unlikely, but was consistently
                // reproducible in practice.
                // In order to avoid overhead in the common case, though, we do a double-checked pattern
                // where we have the fast path if the front block is not empty, then read the tail block,
                // then re-read the front block and check if it's not empty again, then check if the tail
                // block has advanced.
 
                Block* frontBlock_ = frontBlock.load();
                size_t blockTail = frontBlock_->localTail;
                size_t blockFront = frontBlock_->front.load();
 
                if (blockFront != blockTail || blockFront != (frontBlock_->localTail = frontBlock_->tail.load())) {
                        fence(memory_order_acquire);
 
                non_empty_front_block:
                        // Front block not empty, dequeue from here
                        auto element = reinterpret_cast<T*>(frontBlock_->data + blockFront * sizeof(T));
                        result = std::move(*element);
                        element->~T();
 
                        blockFront = (blockFront + 1) & frontBlock_->sizeMask;
 
                        fence(memory_order_release);
                        frontBlock_->front = blockFront;
                }
                else if (frontBlock_ != tailBlock.load()) {
                        fence(memory_order_acquire);
 
                        frontBlock_ = frontBlock.load();
                        blockTail = frontBlock_->localTail = frontBlock_->tail.load();
                        blockFront = frontBlock_->front.load();
                        fence(memory_order_acquire);
 
                        if (blockFront != blockTail) {
                                // Oh look, the front block isn't empty after all
                                goto non_empty_front_block;
                        }
 
                        // Front block is empty but there's another block ahead, advance to it
                        Block* nextBlock = frontBlock_->next;
                        // Don't need an acquire fence here since next can only ever be set on the tailBlock,
                        // and we're not the tailBlock, and we did an acquire earlier after reading tailBlock which
                        // ensures next is up-to-date on this CPU in case we recently were at tailBlock.
 
                        size_t nextBlockFront = nextBlock->front.load();
                        size_t nextBlockTail = nextBlock->localTail = nextBlock->tail.load();
                        fence(memory_order_acquire);
 
                        // Since the tailBlock is only ever advanced after being written to,
                        // we know there's for sure an element to dequeue on it
                        assert(nextBlockFront != nextBlockTail);
                        AE_UNUSED(nextBlockTail);
 
                        // We're done with this block, let the producer use it if it needs
                        fence(memory_order_release);            // Expose possibly pending changes to frontBlock->front from last dequeue
                        frontBlock = frontBlock_ = nextBlock;
 
                        compiler_fence(memory_order_release);   // Not strictly needed
 
                        auto element = reinterpret_cast<T*>(frontBlock_->data + nextBlockFront * sizeof(T));
 
                        result = std::move(*element);
                        element->~T();
 
                        nextBlockFront = (nextBlockFront + 1) & frontBlock_->sizeMask;
 
                        fence(memory_order_release);
                        frontBlock_->front = nextBlockFront;
                }
                else {
                        // No elements in current block and no other block to advance to
                        return false;
                }
 
                return true;
        }
 
 
        // Returns a pointer to the front element in the queue (the one that
        // would be removed next by a call to `try_dequeue` or `pop`). If the
        // queue appears empty at the time the method is called, nullptr is
        // returned instead.
        // Must be called only from the consumer thread.
        T* peek()
        {
#ifndef NDEBUG
                ReentrantGuard guard(this->dequeuing);
#endif
                // See try_dequeue() for reasoning
 
                Block* frontBlock_ = frontBlock.load();
                size_t blockTail = frontBlock_->localTail;
                size_t blockFront = frontBlock_->front.load();
 
                if (blockFront != blockTail || blockFront != (frontBlock_->localTail = frontBlock_->tail.load())) {
                        fence(memory_order_acquire);
                non_empty_front_block:
                        return reinterpret_cast<T*>(frontBlock_->data + blockFront * sizeof(T));
                }
                else if (frontBlock_ != tailBlock.load()) {
                        fence(memory_order_acquire);
                        frontBlock_ = frontBlock.load();
                        blockTail = frontBlock_->localTail = frontBlock_->tail.load();
                        blockFront = frontBlock_->front.load();
                        fence(memory_order_acquire);
 
                        if (blockFront != blockTail) {
                                goto non_empty_front_block;
                        }
 
                        Block* nextBlock = frontBlock_->next;
 
                        size_t nextBlockFront = nextBlock->front.load();
                        fence(memory_order_acquire);
 
                        assert(nextBlockFront != nextBlock->tail.load());
                        return reinterpret_cast<T*>(nextBlock->data + nextBlockFront * sizeof(T));
                }
 
                return nullptr;
        }
 
        // Removes the front element from the queue, if any, without returning it.
        // Returns true on success, or false if the queue appeared empty at the time
        // `pop` was called.
        bool pop()
        {
#ifndef NDEBUG
                ReentrantGuard guard(this->dequeuing);
#endif
                // See try_dequeue() for reasoning
 
                Block* frontBlock_ = frontBlock.load();
                size_t blockTail = frontBlock_->localTail;
                size_t blockFront = frontBlock_->front.load();
 
                if (blockFront != blockTail || blockFront != (frontBlock_->localTail = frontBlock_->tail.load())) {
                        fence(memory_order_acquire);
 
                non_empty_front_block:
                        auto element = reinterpret_cast<T*>(frontBlock_->data + blockFront * sizeof(T));
                        element->~T();
 
                        blockFront = (blockFront + 1) & frontBlock_->sizeMask;
 
                        fence(memory_order_release);
                        frontBlock_->front = blockFront;
                }
                else if (frontBlock_ != tailBlock.load()) {
                        fence(memory_order_acquire);
                        frontBlock_ = frontBlock.load();
                        blockTail = frontBlock_->localTail = frontBlock_->tail.load();
                        blockFront = frontBlock_->front.load();
                        fence(memory_order_acquire);
 
                        if (blockFront != blockTail) {
                                goto non_empty_front_block;
                        }
 
                        // Front block is empty but there's another block ahead, advance to it
                        Block* nextBlock = frontBlock_->next;
 
                        size_t nextBlockFront = nextBlock->front.load();
                        size_t nextBlockTail = nextBlock->localTail = nextBlock->tail.load();
                        fence(memory_order_acquire);
 
                        assert(nextBlockFront != nextBlockTail);
                        AE_UNUSED(nextBlockTail);
 
                        fence(memory_order_release);
                        frontBlock = frontBlock_ = nextBlock;
 
                        compiler_fence(memory_order_release);
 
                        auto element = reinterpret_cast<T*>(frontBlock_->data + nextBlockFront * sizeof(T));
                        element->~T();
 
                        nextBlockFront = (nextBlockFront + 1) & frontBlock_->sizeMask;
 
                        fence(memory_order_release);
                        frontBlock_->front = nextBlockFront;
                }
                else {
                        // No elements in current block and no other block to advance to
                        return false;
                }
 
                return true;
        }
 
        // Returns the approximate number of items currently in the queue.
        // Safe to call from both the producer and consumer threads.
        inline size_t size_approx() const
        {
                size_t result = 0;
                Block* frontBlock_ = frontBlock.load();
                Block* block = frontBlock_;
                do {
                        fence(memory_order_acquire);
                        size_t blockFront = block->front.load();
                        size_t blockTail = block->tail.load();
                        result += (blockTail - blockFront) & block->sizeMask;
                        block = block->next.load();
                } while (block != frontBlock_);
                return result;
        }
 
 
private:
        enum AllocationMode { CanAlloc, CannotAlloc };
 
        template<AllocationMode canAlloc, typename U>
        bool inner_enqueue(U&& element)
        {
#ifndef NDEBUG
                ReentrantGuard guard(this->enqueuing);
#endif
 
                // High-level pseudocode (assuming we're allowed to alloc a new block):
                // If room in tail block, add to tail
                // Else check next block
                //     If next block is not the head block, enqueue on next block
                //     Else create a new block and enqueue there
                //     Advance tail to the block we just enqueued to
 
                Block* tailBlock_ = tailBlock.load();
                size_t blockFront = tailBlock_->localFront;
                size_t blockTail = tailBlock_->tail.load();
 
                size_t nextBlockTail = (blockTail + 1) & tailBlock_->sizeMask;
                if (nextBlockTail != blockFront || nextBlockTail != (tailBlock_->localFront = tailBlock_->front.load())) {
                        fence(memory_order_acquire);
                        // This block has room for at least one more element
                        char* location = tailBlock_->data + blockTail * sizeof(T);
                        new (location) T(std::forward<U>(element));
 
                        fence(memory_order_release);
                        tailBlock_->tail = nextBlockTail;
                }
                else {
                        fence(memory_order_acquire);
                        if (tailBlock_->next.load() != frontBlock) {
                                // Note that the reason we can't advance to the frontBlock and start adding new entries there
                                // is because if we did, then dequeue would stay in that block, eventually reading the new values,
                                // instead of advancing to the next full block (whose values were enqueued first and so should be
                                // consumed first).
 
                                fence(memory_order_acquire);            // Ensure we get latest writes if we got the latest frontBlock
 
                                // tailBlock is full, but there's a free block ahead, use it
                                Block* tailBlockNext = tailBlock_->next.load();
                                size_t nextBlockFront = tailBlockNext->localFront = tailBlockNext->front.load();
                                nextBlockTail = tailBlockNext->tail.load();
                                fence(memory_order_acquire);
 
                                // This block must be empty since it's not the head block and we
                                // go through the blocks in a circle
                                assert(nextBlockFront == nextBlockTail);
                                tailBlockNext->localFront = nextBlockFront;
 
                                char* location = tailBlockNext->data + nextBlockTail * sizeof(T);
                                new (location) T(std::forward<U>(element));
 
                                tailBlockNext->tail = (nextBlockTail + 1) & tailBlockNext->sizeMask;
 
                                fence(memory_order_release);
                                tailBlock = tailBlockNext;
                        }
                        else if (canAlloc == CanAlloc) {
                                // tailBlock is full and there's no free block ahead; create a new block
                                auto newBlockSize = largestBlockSize >= MAX_BLOCK_SIZE ? largestBlockSize : largestBlockSize * 2;
                                auto newBlock = make_block(newBlockSize);
                                if (newBlock == nullptr) {
                                        // Could not allocate a block!
                                        return false;
                                }
                                largestBlockSize = newBlockSize;
 
                                new (newBlock->data) T(std::forward<U>(element));
 
                                assert(newBlock->front == 0);
                                newBlock->tail = newBlock->localTail = 1;
 
                                newBlock->next = tailBlock_->next.load();
                                tailBlock_->next = newBlock;
 
                                // Might be possible for the dequeue thread to see the new tailBlock->next
                                // *without* seeing the new tailBlock value, but this is OK since it can't
                                // advance to the next block until tailBlock is set anyway (because the only
                                // case where it could try to read the next is if it's already at the tailBlock,
                                // and it won't advance past tailBlock in any circumstance).
 
                                fence(memory_order_release);
                                tailBlock = newBlock;
                        }
                        else if (canAlloc == CannotAlloc) {
                                // Would have had to allocate a new block to enqueue, but not allowed
                                return false;
                        }
                        else {
                                assert(false && "Should be unreachable code");
                                return false;
                        }
                }
 
                return true;
        }
 
 
        // Disable copying
        ReaderWriterQueue(ReaderWriterQueue const&) {  }
 
        // Disable assignment
        ReaderWriterQueue& operator=(ReaderWriterQueue const&) {  }
 
 
 
        AE_FORCEINLINE static size_t ceilToPow2(size_t x)
        {
                // From http://graphics.stanford.edu/~seander/bithacks.html#RoundUpPowerOf2
                --x;
                x |= x >> 1;
                x |= x >> 2;
                x |= x >> 4;
                for (size_t i = 1; i < sizeof(size_t); i <<= 1) {
                        x |= x >> (i << 3);
                }
                ++x;
                return x;
        }
 
        template<typename U>
        static AE_FORCEINLINE char* align_for(char* ptr)
        {
                const std::size_t alignment = std::alignment_of<U>::value;
                return ptr + (alignment - (reinterpret_cast<std::uintptr_t>(ptr) % alignment)) % alignment;
        }
private:
#ifndef NDEBUG
        struct ReentrantGuard
        {
                ReentrantGuard(bool& _inSection)
                        : inSection(_inSection)
                {
                        assert(!inSection);
                        if (inSection) {
                                throw std::runtime_error("ReaderWriterQueue does not support enqueuing or dequeuing elements from other elements' ctors and dtors");
                        }
 
                        inSection = true;
                }
 
                ~ReentrantGuard() { inSection = false; }
 
        private:
                ReentrantGuard& operator=(ReentrantGuard const&);
 
        private:
                bool& inSection;
        };
#endif
 
        struct Block
        {
                // Avoid false-sharing by putting highly contended variables on their own cache lines
                weak_atomic<size_t> front;      // (Atomic) Elements are read from here
                size_t localTail;                       // An uncontended shadow copy of tail, owned by the consumer
 
                char cachelineFiller0[CACHE_LINE_SIZE - sizeof(weak_atomic<size_t>) - sizeof(size_t)];
                weak_atomic<size_t> tail;       // (Atomic) Elements are enqueued here
                size_t localFront;
 
                char cachelineFiller1[CACHE_LINE_SIZE - sizeof(weak_atomic<size_t>) - sizeof(size_t)];  // next isn't very contended, but we don't want it on the same cache line as tail (which is)
                weak_atomic<Block*> next;       // (Atomic)
 
                char* data;             // Contents (on heap) are aligned to T's alignment
 
                const size_t sizeMask;
 
 
                // size must be a power of two (and greater than 0)
                Block(size_t const& _size, char* _rawThis, char* _data)
                        : front(0), localTail(0), tail(0), localFront(0), next(nullptr), data(_data), sizeMask(_size - 1), rawThis(_rawThis)
                {
                }
 
        private:
                // C4512 - Assignment operator could not be generated
                Block& operator=(Block const&);
 
        public:
                char* rawThis;
        };
 
 
        static Block* make_block(size_t capacity)
        {
                // Allocate enough memory for the block itself, as well as all the elements it will contain
                auto size = sizeof(Block) + std::alignment_of<Block>::value - 1;
                size += sizeof(T) * capacity + std::alignment_of<T>::value - 1;
                auto newBlockRaw = static_cast<char*>(std::malloc(size));
                if (newBlockRaw == nullptr) {
                        return nullptr;
                }
 
                auto newBlockAligned = align_for<Block>(newBlockRaw);
                auto newBlockData = align_for<T>(newBlockAligned + sizeof(Block));
                return new (newBlockAligned) Block(capacity, newBlockRaw, newBlockData);
        }
 
private:
        weak_atomic<Block*> frontBlock;         // (Atomic) Elements are enqueued to this block
 
        char cachelineFiller[CACHE_LINE_SIZE - sizeof(weak_atomic<Block*>)];
        weak_atomic<Block*> tailBlock;          // (Atomic) Elements are dequeued from this block
 
        size_t largestBlockSize;
 
#ifndef NDEBUG
        bool enqueuing;
        bool dequeuing;
#endif
};
 
// Like ReaderWriterQueue, but also providees blocking operations
template<typename T, size_t MAX_BLOCK_SIZE = 512>
class BlockingReaderWriterQueue
{
private:
        typedef ::moodycamel::ReaderWriterQueue<T, MAX_BLOCK_SIZE> ReaderWriterQueue;
 
public:
        explicit BlockingReaderWriterQueue(size_t maxSize = 15)
                : inner(maxSize)
        { }
 
 
        // Enqueues a copy of element if there is room in the queue.
        // Returns true if the element was enqueued, false otherwise.
        // Does not allocate memory.
        AE_FORCEINLINE bool try_enqueue(T const& element)
        {
                if (inner.try_enqueue(element)) {
                        sema.signal();
                        return true;
                }
                return false;
        }
 
        // Enqueues a moved copy of element if there is room in the queue.
        // Returns true if the element was enqueued, false otherwise.
        // Does not allocate memory.
        AE_FORCEINLINE bool try_enqueue(T&& element)
        {
                if (inner.try_enqueue(std::forward<T>(element))) {
                        sema.signal();
                        return true;
                }
                return false;
        }
 
 
        // Enqueues a copy of element on the queue.
        // Allocates an additional block of memory if needed.
        // Only fails (returns false) if memory allocation fails.
        AE_FORCEINLINE bool enqueue(T const& element)
        {
                if (inner.enqueue(element)) {
                        sema.signal();
                        return true;
                }
                return false;
        }
 
        // Enqueues a moved copy of element on the queue.
        // Allocates an additional block of memory if needed.
        // Only fails (returns false) if memory allocation fails.
        AE_FORCEINLINE bool enqueue(T&& element)
        {
                if (inner.enqueue(std::forward<T>(element))) {
                        sema.signal();
                        return true;
                }
                return false;
        }
 
 
        // Attempts to dequeue an element; if the queue is empty,
        // returns false instead. If the queue has at least one element,
        // moves front to result using operator=, then returns true.
        template<typename U>
        bool try_dequeue(U& result)
        {
                if (sema.tryWait()) {
                        bool success = inner.try_dequeue(result);
                        assert(success);
                        AE_UNUSED(success);
                        return true;
                }
                return false;
        }
 
 
        // Attempts to dequeue an element; if the queue is empty,
        // waits until an element is available, then dequeues it.
        template<typename U>
        void wait_dequeue(U& result)
        {
                sema.wait();
                bool success = inner.try_dequeue(result);
                AE_UNUSED(result);
                assert(success);
                AE_UNUSED(success);
        }
 
 
        // Returns a pointer to the front element in the queue (the one that
        // would be removed next by a call to `try_dequeue` or `pop`). If the
        // queue appears empty at the time the method is called, nullptr is
        // returned instead.
        // Must be called only from the consumer thread.
        AE_FORCEINLINE T* peek()
        {
                return inner.peek();
        }
 
        // Removes the front element from the queue, if any, without returning it.
        // Returns true on success, or false if the queue appeared empty at the time
        // `pop` was called.
        AE_FORCEINLINE bool pop()
        {
                if (sema.tryWait()) {
                        bool result = inner.pop();
                        assert(result);
                        AE_UNUSED(result);
                        return true;
                }
                return false;
        }
 
        // Returns the approximate number of items currently in the queue.
        // Safe to call from both the producer and consumer threads.
        AE_FORCEINLINE size_t size_approx() const
        {
                return sema.availableApprox();
        }
 
 
private:
        // Disable copying & assignment
        BlockingReaderWriterQueue(ReaderWriterQueue const&) {  }
        BlockingReaderWriterQueue& operator=(ReaderWriterQueue const&) {  }
 
private:
        ReaderWriterQueue inner;
        spsc_sema::LightweightSemaphore sema;
};
 
}    // end namespace moodycamel
 
#ifdef AE_VCPP
#pragma warning(pop)
#endif