C++核心准则CP.61:使用async启动并发任务

CP.61: Use async() to spawn concurrent tasks

CP.61:使用async启动并发任务

Reason（原因）

Similar to R.12, which tells you to avoid raw owning pointers, you should also avoid raw threads and raw promises where possible. Use a factory function such as std::async, which handles spawning or reusing a thread without exposing raw threads to your own code.

R.12告诉我们避免原始的所有权指针，本规则告诉我们：如果可能应该避免原始线程和promise。使用像std::async一样的工厂函数，它可以可以启动和重新使用线程而不必向外部代码保护原始线程。

Example（示例）

int read_value(const std::string& filename)
{
    std::ifstream in(filename);
    in.exceptions(std::ifstream::failbit);
    int value;
    in >> value;
    return value;
}

void async_example()
{
    try {
        std::future<int> f1 = std::async(read_value, "v1.txt");
        std::future<int> f2 = std::async(read_value, "v2.txt");
        std::cout << f1.get() + f2.get() << '\n';
    } catch (const std::ios_base::failure& fail) {
        // handle exception here
    }
}

Note（注意）

Unfortunately, std::async is not perfect. For example, it doesn't use a thread pool, which means that it may fail due to resource exhaustion, rather than queuing up your tasks to be executed later. However, even if you cannot use std::async, you should prefer to write your own future-returning factory function, rather than using raw promises.

不幸的是，std::async并不完美。例如，它没有使用线程池，这意味着它可能因为资源枯竭而失败，而不是将任务排队等候执行。然而，即使你不能使用std::async，你还是可以编写自己的返回future的工厂函数，而不是使用原始的promise。

Example (bad)（反面示例）

This example shows two different ways to succeed at using std::future, but to fail at avoiding raw std::thread management.

示例代码演示了两种不同的成功使用std::future的方式，但是没能避免使用原始的std::thread。

void async_example()
{
    std::promise<int> p1;
    std::future<int> f1 = p1.get_future();
    std::thread t1([p1 = std::move(p1)]() mutable {
        p1.set_value(read_value("v1.txt"));
    });
    t1.detach(); // evil

    std::packaged_task<int()> pt2(read_value, "v2.txt");
    std::future<int> f2 = pt2.get_future();
    std::thread(std::move(pt2)).detach();

    std::cout << f1.get() + f2.get() << '\n';
}

Example (good)（范例）

This example shows one way you could follow the general pattern set by std::async, in a context where std::async itself was unacceptable for use in production.

示例代码显示了你可以遵守的，由std::async设定的通常模式，这种模式可以用于开发过程中std::async无法直接使用的情况。

void async_example(WorkQueue& wq)
{
    std::future<int> f1 = wq.enqueue([]() {
        return read_value("v1.txt");
    });
    std::future<int> f2 = wq.enqueue([]() {
        return read_value("v2.txt");
    });
    std::cout << f1.get() + f2.get() << '\n';
}

Any threads spawned to execute the code of read_value are hidden behind the call to WorkQueue::enqueue. The user code deals only with future objects, never with raw thread, promise, or packaged_task objects.

任何启动并调用read_value的线程都被隐藏在WorkQueue::enqueue的调用后面。用户代码智能处理future对象，永远不会使用原始线程，promise或者打包的task对象。

Enforcement（实施建议）

???

原文链接https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#cp61-use-async-to-spawn-concurrent-tasks