std::lock_guard

std::lock_guard是RAII模板类的简单实现，功能简单。

1. std::lock_guard 在构造函数中进行加锁，析构函数中进行解锁。
2. 锁在多线程编程中，使用较多，因此c++11提供了lock_guard模板类；在实际编程中，我们也可以根据自己的场景编写resource_guard RAII类，避免忘掉释放资源。

std::unique_lock

1. 类 unique_lock 是通用互斥包装器，允许延迟锁定、锁定的有时限尝试、递归锁定、所有权转移和与条件变量一同使用。
2. unique_lock比lock_guard使用更加灵活，功能更加强大。
3. 使用unique_lock需要付出更多的时间、性能成本。

std::condition_variable

C++11中引入了条件变量，其相关内容均在<condition_variable>中。这里主要介绍std::condition_variable类。

条件变量std::condition_variable用于多线程之间的通信，它可以阻塞一个或同时阻塞多个线程。std::condition_variable需要与std::unique_lock配合使用。

当std::condition_variable对象的某个wait函数被调用的时候，它使用std::unique_lock(通过std::mutex)来锁住当前线程。当前线程会一直被阻塞，直到另外一个线程在相同的std::condition_variable对象上调用了notification函数来唤醒当前线程。

std::condition_variable对象通常使用std::unique_lock<std::mutex>来等待，如果需要使用另外的lockable类型，可以使用std::condition_variable_any类。

需要维护两个计数器，分别是生产者已生产产品的数目和消费者已取走产品的数目。另外也需要保护产品库在多个生产者和多个消费者互斥地访问。

#include <cstdlib>
#include <condition_variable>
#include <iostream>
#include <mutex>
#include <thread>

static const int kItemRepositorySize  = 4; // Item buffer size.
static const int kItemsToProduce  = 10;   // How many items we plan to produce.

struct ItemRepository {
    int item_buffer[kItemRepositorySize];
    size_t read_position;
    size_t write_position;
    size_t produced_item_counter;
    size_t consumed_item_counter;
    std::mutex mtx;
    std::mutex produced_item_counter_mtx;
    std::mutex consumed_item_counter_mtx;
    std::condition_variable repo_not_full;
    std::condition_variable repo_not_empty;
} gItemRepository;

typedef struct ItemRepository ItemRepository;


void ProduceItem(ItemRepository *ir, int item)
{
    std::unique_lock<std::mutex> lock(ir->mtx);
    while(((ir->write_position + 1) % kItemRepositorySize)
        == ir->read_position) { // item buffer is full, just wait here.
        std::cout << "Producer is waiting for an empty slot...\n";
        (ir->repo_not_full).wait(lock);
    }

    (ir->item_buffer)[ir->write_position] = item;
    (ir->write_position)++;

    if (ir->write_position == kItemRepositorySize)
        ir->write_position = 0;

    (ir->repo_not_empty).notify_all();
    lock.unlock();
}

int ConsumeItem(ItemRepository *ir)
{
    int data;
    std::unique_lock<std::mutex> lock(ir->mtx);
    // item buffer is empty, just wait here.
    while(ir->write_position == ir->read_position) {
        std::cout << "Consumer is waiting for items...\n";
        (ir->repo_not_empty).wait(lock);
    }

    data = (ir->item_buffer)[ir->read_position];
    (ir->read_position)++;

    if (ir->read_position >= kItemRepositorySize)
        ir->read_position = 0;

    (ir->repo_not_full).notify_all();
    lock.unlock();

    return data;
}

void ProducerTask()
{
    bool ready_to_exit = false;
    while(1) {
        std::this_thread::sleep_for(std::chrono::seconds(1));
        std::unique_lock<std::mutex> lock(gItemRepository.produced_item_counter_mtx);
        if (gItemRepository.produced_item_counter < kItemsToProduce) {
            ++(gItemRepository.produced_item_counter);
            ProduceItem(&gItemRepository, gItemRepository.produced_item_counter);
            std::cout << "Producer thread " << std::this_thread::get_id()
                << " is producing the " << gItemRepository.produced_item_counter
                << "^th item" << std::endl;
        } else ready_to_exit = true;
        lock.unlock();
        if (ready_to_exit == true) break;
    }
    std::cout << "Producer thread " << std::this_thread::get_id()
                << " is exiting..." << std::endl;
}

void ConsumerTask()
{
    bool ready_to_exit = false;
    while(1) {
        std::this_thread::sleep_for(std::chrono::seconds(1));
        std::unique_lock<std::mutex> lock(gItemRepository.consumed_item_counter_mtx);
        if (gItemRepository.consumed_item_counter < kItemsToProduce) {
            int item = ConsumeItem(&gItemRepository);
            ++(gItemRepository.consumed_item_counter);
            std::cout << "Consumer thread " << std::this_thread::get_id()
                << " is consuming the " << item << "^th item" << std::endl;
        } else ready_to_exit = true;
        lock.unlock();
        if (ready_to_exit == true) break;
    }
    std::cout << "Consumer thread " << std::this_thread::get_id()
                << " is exiting..." << std::endl;
}

void InitItemRepository(ItemRepository *ir)
{
    ir->write_position = 0;
    ir->read_position = 0;
    ir->produced_item_counter = 0;
    ir->consumed_item_counter = 0;
}

int main()
{
    InitItemRepository(&gItemRepository);
    std::thread producer1(ProducerTask);
    std::thread producer2(ProducerTask);
    std::thread producer3(ProducerTask);
    std::thread producer4(ProducerTask);

    std::thread consumer1(ConsumerTask);
    std::thread consumer2(ConsumerTask);
    std::thread consumer3(ConsumerTask);
    std::thread consumer4(ConsumerTask);

    producer1.join();
    producer2.join();
    producer3.join();
    producer4.join();

    consumer1.join();
    consumer2.join();
    consumer3.join();
    consumer4.join();
}