读一读冰羚代码（10）publisher数据发布之神奇的Sample类

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已于 2025-04-21 14:49:49 修改 · 896 阅读

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#人工智能

于 2025-04-21 14:39:54 首次发布

在读一读冰羚代码（9）publisher数据发布之寻找共享内存上的可用空间-优快云博客一文结尾处，从共享内存获取得到ChunkHeader对象，并最终被转换成Sample类对象：

template <typename T, typename H, typename BasePublisherType>
inline expected<Sample<T, H>, AllocationError> PublisherImpl<T, H, BasePublisherType>::loanSample() noexcept
{
    static constexpr uint32_t USER_HEADER_SIZE{std::is_same<H, mepoo::NoUserHeader>::value ? 0U : sizeof(H)};

    auto result = port().tryAllocateChunk(sizeof(T), alignof(T), USER_HEADER_SIZE, alignof(H));
    if (result.has_error())
    {
        return err(result.error());
    }
    else
    {
        return ok(convertChunkHeaderToSample(result.value()));
    }
}

先看下publisher的demo（iceoryx_examples/icehello/iox_publisher_helloworld.cpp）：

while (!iox::hasTerminationRequested())
//! [wait for term]
{
    ++ct;

    // Retrieve a sample from shared memory
    //! [loan]
    auto loanResult = publisher.loan();
    //! [loan]
    //! [publish]
    if (loanResult.has_value())
    {
        auto& sample = loanResult.value();
        // Sample can be held until ready to publish
        sample->x = ct;      (1)
        sample->y = ct;
        sample->z = ct;
        sample.publish();    (2)
    }
}

（1）（2）两行有点出乎意料，因为，a. 要发布的数据结构其实有自己的定义（如下）。通常的做法，先初始化一个指向该数据结构的指针，然后利用该指针对数据成员赋值，代码中却利用Sample对象对要发布的数据结构RadarObject对象的数据成员赋值：

//! [radar object]
struct RadarObject
{
    double x = 0.0;
    double y = 0.0;
    double z = 0.0;
};
//! [radar object]

b. 既然Publisher对象已经构造完成，Published继承PublisherImpl，PublisherImpl定义有publish接口：

template <typename T, typename H = mepoo::NoUserHeader, typename BasePublisherType = BasePublisher<>>
class PublisherImpl : public BasePublisherType, private PublisherInterface<T, H>
{
  public:
    ///
    /// @brief publish Publishes the given sample and then releases its loan.
    /// @param sample The sample to publish.
    ///
    void publish(Sample<T, H>&& sample) noexcept override;
}；

不能直接调用Publiser对象的publish接口来发布数据吗？

再来看下subscriber的demo（iceoryx_examples/icehello/iox_subscriber_helloworld.cpp）：

int main()
{
    //! [initialize runtime]
    constexpr char APP_NAME[] = "iox-cpp-subscriber-helloworld";
    iox::runtime::PoshRuntime::initRuntime(APP_NAME);
    //! [initialize runtime]

    //! [initialize subscriber]
    iox::popo::Subscriber<RadarObject> subscriber({"Radar", "FrontLeft", "Object"});
    //! [initialize subscriber]

    // run until interrupted by Ctrl-C
    while (!iox::hasTerminationRequested())
    {
        //! [receive]
        auto takeResult = subscriber.take();    (1)
        if (takeResult.has_value())
        {
            std::cout << APP_NAME << " got value: " << takeResult.value()->x << std::endl;                                      (2)
        }
        //! [receive]
        else
        {
            //! [error]
            if (takeResult.error() == iox::popo::ChunkReceiveResult::NO_CHUNK_AVAILABLE)
            {
                std::cout << "No chunk available." << std::endl;
            }
            else
            {
                std::cout << "Error receiving chunk." << std::endl;
            }
            //! [error]
        }

        //! [wait]
        std::this_thread::sleep_for(std::chrono::milliseconds(100));
        //! [wait]
    }

    return (EXIT_SUCCESS);
}

（1）Subscriber对象从共享内存拿到的发布数据对象也被转换成了Sample对象

（2）利用Sample对象取出发布数据对象的数据成员

综合以上两个demo例程，Sample类至少要具备以下个功能：