Taskflow关键概念与使用方法详解

A Task in Taskflow is a callable object for which the operation std::invoke is applicable. It can be either a functor, a lambda expression, a bind expression, or a class object with operator() overloaded. All tasks are created from tf::Taskflow, the class that manages a task dependency graph.

tf::Taskflow

可以理解成计算图，用于管理计算节点，添加节点的方法：

• placeholder 返回一个task对象的handle
• emplace 参数可以是多个callable对象，返回对应的task handle

示例：

tf::Taskflow taskflow;
tf::Task A = taskflow.placeholder();
tf::Task B = taskflow.emplace([] () { std::cout << "task B\n"; });
 
auto [D, E, F] = taskflow.emplace(
  [](){ std::cout << "Task A\n"; },
  [](){ std::cout << "Task B\n"; },
  [](){ std::cout << "Task C\n"; }
);

tf::Task

可以理解成计算图的计算节点，管理计算任务。常用方法：

• name(const std::string &name) 赋予名字给任务
• name() 获取任务名字
• work(CallableObj&& callable) 赋予新“工作”给节点
• template <typename... Ts> Task& precede(Ts&&... tasks) 指定前继节点
• template <typename... Ts> Task& Task::succeed(Ts&&... tasks) 指定后继节点
• template <typename T> Task& Task::composed_of(T& object) 添加子图

在每个task，都是使用通用的多态函数包装器— std::function 来存储和调用每个 _callable，_因此函数callable需要满足function的要求：

1. 可复制

   #include <iostream>
   #include <functional>
   #include <memory>
    
   int main() {
       // 创建一个 std::function 对象
       std::function<void()> task = [ptr = std::make_unique<int>(1)]() {
           std::cout << "captured unique pointer is not copyable";
       };
    
       // 调用 task
       task();
    
       // 尝试复制 task
       std::function<void()> task_copy = task;  // 编译失败
    
       return 0;
   }
    
   // compile error:
   /usr/include/c++/11/bits/std_function.h: In instantiation of ‘std::function<_Res(_ArgTypes ...)>::function(_Functor&&) [with _Functor = main()::<lambda()>; _Constraints = void; _Res = void; _ArgTypes = {}]’:
   main.cpp:17:5:   required from here
   /usr/include/c++/11/bits/std_function.h:439:69: error: static assertion failed: std::function target must be copy-constructible
     439 |           static_assert(is_copy_constructible<__decay_t<_Functor>>::value,
   // 过不了可复制这一静态断言

2. …

tf::Executor

特点：

• 可以前后用于多次执行相同/不同的taskflow
• 线程安全
• run_* 方法是非阻塞返回
• 使用 work-stealing 算法来调度任务执行

Important

Attention

A running taskflow must remain alive during its execution. It is your responsibility to ensure a taskflow not being destructed when it is running.

Similarly, you should avoid touching a taskflow while it is running.