使用lambda表达式代替std::bind和std::function

原来

bool Myclass::connect() {
    std::function<void()>f = std::bind(&Myclass::loopCheckStatus, this);
    new std::thread(f);
    return true;
}

void Myclass::loopCheckStatus() {
    while (true) {
        //check something
    }
}

改造后

bool Myclass::connect() {
    // Using a lambda expression to call loopCheckStatus
    auto f = [*this]() { loopCheckStatus(); };
    std::thread t(f);
    return true;
}

注意这里的*this，这么写的原因是因为相当于通过lambda表达式通过拷贝来捕获this指针，防止lambda对无效数据进行操作。参考：1.https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#f53-avoid-capturing-by-reference-in-lambdas-that-will-be-used-non-locally-including-returned-stored-on-the-heap-or-passed-to-another-thread
2.https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#f54-when-writing-a-lambda-that-captures-this-or-any-class-data-member-dont-use--default-capture

更推荐的写法

std::thread thread(&mysql::do_flush_torrents, this);

因为传给thread构造函数的可调用对象的参数，默认是按值拷贝传递的，这样this指针就是拷贝了一份