Taskflow：异常处理（Exception Handling）

从运行的Taskflow中捕捉异常

当Task抛出异常时，执行器将以tf::Future句柄引用的共享状态存储该异常。

#include <taskflow/taskflow.hpp>
void print_str(char const* str) {std::cout << str << std::endl;
}
int main() {tf::Executor executor;tf::Taskflow taskflow;// task抛出一个异常taskflow.emplace([](){ throw std::runtime_error("exception"); });// Task抛出的异常将会存入future中tf::Future<void> future = executor.run(taskflow); try {future.get(); // 并在get() 获取}catch(const std::runtime_error& e) {std::cerr << e.what() << std::endl;}// 上面逻辑的简化版// try {//     executor.run(taskflow).get();// }// catch(const std::runtime_error& e) {//     std::cerr << e.what() << std::endl;// }
}

由于tf::Future是从std::future派生的，它继承了C++标准定义的所有异常处理行为。

异常将会自动取消Taskflow的执行。所有依赖该异常Task的后续任务将无法运行。

#include <taskflow/taskflow.hpp>
void print_str(char const* str) {std::cout << str << std::endl;
}
int main() {tf::Executor executor;tf::Taskflow taskflow;tf::Task A = taskflow.emplace([](){ throw std::runtime_error("exception on A"); });tf::Task B = taskflow.emplace([](){ std::cout << "Task B\n"; });A.precede(B);try {executor.run(taskflow).get();}catch(const std::runtime_error& e) {std::cerr << e.what() << std::endl;}
}

当A抛出异常时，执行者将取消任务流的执行，停止在A之后运行的每个任务。在这种情况下，B不会运行。

当并发任务中，多个Task抛出异常，Taskflow只会存储一个异常，并忽略其他异常：

#include <taskflow/taskflow.hpp>
int main() {tf::Executor executor;tf::Taskflow taskflow;auto [A, B, C, D] = taskflow.emplace([]() { std::cout << "TaskA\n"; },[]() { std::cout << "TaskB\n";throw std::runtime_error("Exception on Task B");},[]() { std::cout << "TaskC\n"; throw std::runtime_error("Exception on Task C");},[]() { std::cout << "TaskD will not be printed due to exception\n"; });A.precede(B, C);  // A runs before B and CD.succeed(B, C);  // D runs after  B and Ctry {executor.run(taskflow).get();}catch(const std::runtime_error& e) {// catched either B's or C's exceptionstd::cout << e.what() << std::endl;}
}

同样，异步Task也依靠future传播异常：

tf::Executor executor;
auto fu = executor.async([](){ throw std::runtime_error("exception"); });
try {fu.get();
}
catch(const std::runtime_error& e) {std::cerr << e.what() << std::endl;
}

tf::Executor::silent_async 不会返回future，所以其异常会被传播到其父Task，或者忽略（没有父Task时）：

tf::Taskflow taskflow;
tf::Executor executor;// execption will be silently ignored
executor.silent_async([](){ throw std::runtime_error("exception"); });// exception will be propagated to the parent tf::Runtime task and then its Taskflow
taskflow.emplace([&](tf::Runtime& rt){rt.silent_async([](){ throw std::runtime_error("exception"); });
});
try {taskflow.get();
}
catch(const std::runtime_error& re) {std::cout << re.what() << std::endl;
}