为什么是std :: packed_task< void（）>无效？ [英] Why is std::packaged_task<void()> not valid?

问题描述

使用MSVC2012，

以下代码将按预期进行编译和运行

  std :: packaged_task< int（）> task（[]（） - > int {std :: cout<<hello world<< std :: endl; return 0;}）; 
 std :: thread t（std :: move（task））; 
 t.join（）;以下代码将无法编译和运行
 
 
 
 $ b $ 
 
 
 b 
  std :: packaged_task< void（）> task（[]（）{std :: cout<<hello world<< std :: endl;}）; 
 std :: thread t（std :: move（task））; 
 t.join（）; 
  
为什么会这样？
 
 
 
 ：
作为一种解决方法，可以使用std :: promise在返回void的函数上获取std :: future。
  std :: promise< void>诺言; 
 auto future = promise.get_future（）; 
 std :: thread thread（[]（std :: promise< void>& p）{std :: cout<<hello world<< std :: endl; p.set_value ）;}，std :: move（promise））; 
 future.wait（）; 
 
请注意，vs2012库中有一个带std :: thread的错误，承诺作为l值引用并移动promise，如果你通过值或r值引用传递promise，它将不会编译。这是因为实现使用的std :: bind（）的行为不符合预期。
解决方案
 MSVC2012。在MSVC2012附带的线程库实现中有相当多的错误。我发布了一个部分列表在我的博客文章比较它我的商业Just :: Thread图书馆： http://www.justsoftwaresolutions.co.uk/news/just-thread-v1.8.0-released.html  
 
Using MSVC2012,

The following code will compile and run as expected
std::packaged_task< int() > task( []()->int{ std::cout << "hello world" << std::endl; return 0; } );
std::thread t( std::move(task) );
t.join();
while the following code will fail to compile and run
std::packaged_task< void() > task( [](){ std::cout << "hello world" << std::endl; } );
std::thread t( std::move(task) );
t.join();
Why is this so?

Edit:
As a workaround, it is possible to use std::promise to get a std::future on a function that returns void
std::promise<void> promise;
auto future = promise.get_future();
std::thread thread( [](std::promise<void> &p){ std::cout << "hello world" << std::endl; p.set_value(); }, std::move(promise) );
future.wait();
Note that there is a bug in the vs2012 library with std::thread that forces you to pass the promise in as a l-value reference and move the promise in, it would not compile if you pass the promise by value or by r-value reference. This is supposedly because the implementation uses std::bind() which does not behave as expected.
解决方案
This is a bug in MSVC2012. There are quite a few bugs in the thread library implementation that ships with MSVC2012. I posted a partial list in my blog post comparing it to my commercial Just::Thread library: http://www.justsoftwaresolutions.co.uk/news/just-thread-v1.8.0-released.html

                        
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