提升ASIO異常傳播

Question

我注意到，在許多Boost ASIO示例中，正在對可能拋出錯誤的函數進行調用，但未使用try/catch。例如，封鎖UDP客戶端的例子here具有以下功能：

void check_deadline() 
    { 
    // Check whether the deadline has passed. We compare the deadline against 
    // the current time since a new asynchronous operation may have moved the 
    // deadline before this actor had a chance to run. 
    if (deadline_.expires_at() <= deadline_timer::traits_type::now()) 
    { 
     // The deadline has passed. The outstanding asynchronous operation needs 
     // to be cancelled so that the blocked receive() function will return. 
     // 
     // Please note that cancel() has portability issues on some versions of 
     // Microsoft Windows, and it may be necessary to use close() instead. 
     // Consult the documentation for cancel() for further information. 
     socket_.cancel(); 

     // There is no longer an active deadline. The expiry is set to positive 
     // infinity so that the actor takes no action until a new deadline is set. 
     deadline_.expires_at(boost::posix_time::pos_infin); 
    } 

    // Put the actor back to sleep. 
    deadline_.async_wait(boost::bind(&client::check_deadline, this)); 
    } 

爲deadline_.expires_at（here）的機制的文檔指出，這個函數拋出一個boost ::系統:: SYSTEM_ERROR例外。

這是不是在這個例子中被捕獲，因爲它只是一個例子，或者像這樣的調用拋出的異常通過調用run，run-one等來傳播？換句話說，用try catch來調用io_service.run（）是否足以處理這些類型的異常？

此外，我還注意到deadline_.async_wait文檔here指出處理程序需要一個引用boost :: system :: system_error :: error_code的簽名。我在check_deadline（）函數中看不到引用或佔位符。

Answer 1

basic_deadline_timer::async_wait文檔狀態：

Regardless of whether the asynchronous operation completes immediately or not, the handler will not be invoked from within this function. Invocation of the handler will be performed in a manner equivalent to using boost::asio::io_service::post().

這意味着該處理器將會從內部io_service::run()調用（調用它的線程），所以異常會被自動傳播到用戶代碼，並沒有特殊的處理是需要在Asio內部。通常，爲了簡化，樣本不包含錯誤處理。

對不起，我不知道爲什麼樣本中沒有指定error_code佔位符。需要看看Asio的消息來源。