使用asio::io_context::strand同步任务无输出，求正确实现方式

我尝试用strand同步完成处理程序，但未得到预期结果：不包装strand直接调用asio::post时，输出正常但任务不同步；用strand包装后完全没有输出。

最小可复现示例 #

#include <asio.hpp>
#include <thread>
#include <iostream>
#include <vector>
#include <random>

struct Task
{
    Task(int id, int wait_time) 
        : id_{id}
        , wait_time_{wait_time}
    {}

    void operator()()
    {
        std::cout << "Tast-" << id_ << " started. [" << std::this_thread::get_id() << "]" << std::endl;
        std::this_thread::sleep_for(std::chrono::milliseconds(wait_time_));
        std::cout << "Task-" << id_ << " finished after (" << wait_time_
                  << ") milliseconds. [" << std::this_thread::get_id() << "]" << std::endl;
    }
    int id_;
    int wait_time_;
};

int main()
{
    std::random_device rd;
    std::mt19937 engine(rd());
    std::uniform_int_distribution d(500, 2'000);

    asio::io_context ctx;
    asio::io_context::strand strand(ctx);

    std::vector<std::jthread> threads;
    auto count = 4;
    for (int i = 0; i < count; ++i)
    {
        threads.emplace_back([&]{ ctx.run(); });
    }

    for (int i = 0; i < count * 2; ++i)
    {
        asio::post(Task(i + 1, d(engine)));
        // asio::post(strand.wrap(Task(i + 1, d(engine))));  /* THIS DOESN'T PRODUCE ANY OUTPUT */
    }
    ctx.run();
}

无Strand时的输出 #

Tast-1 started. [17652]
Tast-7 started. [26096]
Tast-3 started. [56484]
Tast-8 started. [32000]
Tast-5 started. [Tast-6 started. [79448]
61340]Tast-2Tast-4 started. [55696]
 started. [84880]

Task-6 finished after (784) milliseconds. [79448]
Task-2 finished after (835) milliseconds. [84880]
Task-1 finished after (923) milliseconds. [17652]
Task-4 finished after (1281) milliseconds. [55696]
Task-3 finished after (1668) milliseconds. [56484]
Task-7 finished after (1763) milliseconds. [26096]
Task-8 finished after (1888) milliseconds. [32000]
Task-5 finished after (1982) milliseconds. [61340]

问题原因与修复方案 #

核心问题 #

你错误地使用了strand.wrap()：将包装后的handler直接post到io_context，而非通过strand来投递任务。这种用法无法触发strand的串行调度逻辑，反而会导致任务无法被正确执行。

正确使用方式 #

有两种标准用法可以实现strand的同步效果：

1. 直接通过strand的post方法投递任务（推荐，写法更简洁）：

   asio::post(strand, Task(i + 1, d(engine)));
   

2. 用strand.wrap()包装后，仍投递到strand：

   asio::post(strand, strand.wrap(Task(i + 1, d(engine))));
   

修复后的完整代码 #

#include <asio.hpp>
#include <thread>
#include <iostream>
#include <vector>
#include <random>

struct Task
{
    Task(int id, int wait_time) 
        : id_{id}
        , wait_time_{wait_time}
    {}

    void operator()()
    {
        std::cout << "Task-" << id_ << " started. [" << std::this_thread::get_id() << "]" << std::endl;
        std::this_thread::sleep_for(std::chrono::milliseconds(wait_time_));
        std::cout << "Task-" << id_ << " finished after (" << wait_time_
                  << ") milliseconds. [" << std::this_thread::get_id() << "]" << std::endl;
    }
    int id_;
    int wait_time_;
};

int main()
{
    std::random_device rd;
    std::mt19937 engine(rd());
    std::uniform_int_distribution d(500, 2'000);

    asio::io_context ctx;
    asio::io_context::strand strand(ctx);

    std::vector<std::jthread> threads;
    auto count = 4;
    for (int i = 0; i < count; ++i)
    {
        threads.emplace_back([&]{ ctx.run(); });
    }

    for (int i = 0; i < count * 2; ++i)
    {
        asio::post(strand, Task(i + 1, d(engine)));
    }
    ctx.run();
}

修复后效果 #

任务会严格串行执行，同一时间只有一个任务在运行，输出类似：

Task-1 started. [12345]
Task-1 finished after (800) milliseconds. [12345]
Task-2 started. [12345]
Task-2 finished after (1200) milliseconds. [12345]
...

内容的提问来源于stack exchange，提问作者DailyLearner