c++ - 来自 http://www.cplusplus.com 的 std::packaged_task::reset 示例不起作用

Question

我尝试编译并运行来自 http://www.cplusplus.com/reference/future/packaged_task/reset/ 的 reset() 函数示例:

$ cat task.cpp
// packaged_task::get_future
#include <iostream>     // std::cout
#include <utility>      // std::move
#include <future>       // std::packaged_task, std::future
#include <thread>       // std::thread

// a simple task:
int triple (int x) { return x*3; }

int main ()
{
  std::packaged_task<int(int)> tsk (triple); // package task

  std::future<int> fut = tsk.get_future();
  std::thread (std::move(tsk),33).detach();
  std::cout << "The triple of 33 is " << fut.get() << ".\n";

  // re-use same task object:
  tsk.reset();
  fut = tsk.get_future();
  std::thread(std::move(tsk),99).detach();
  std::cout << "Thre triple of 99 is " << fut.get() << ".\n";

  return 0;
}

但我遇到了编译器错误或运行时错误，具体取决于使用的编译器:

• 海湾合作委员会:

$ g++ -o task -std=c++11 -I/opt/local/include task.cpp && ./task
The triple of 33 is 99.
terminate called after throwing an instance of 'std::future_error'
what(): No associated state
[1] 14850 abort ./task

• clang :

$ clang++ -o task -std=c++11 -stdlib=libc++ -I/opt/local/include task.cpp && ./task
In file included from task.cpp:2:
In file included from /opt/local/libexec/llvm-3.4/bin/../include/c++/v1/iostream:38:
In file included from /opt/local/libexec/llvm-3.4/bin/../include/c++/v1/ios:216:
In file included from /opt/local/libexec/llvm-3.4/bin/../include/c++/v1/__locale:15:
In file included from /opt/local/libexec/llvm-3.4/bin/../include/c++/v1/string:439:
In file included from /opt/local/libexec/llvm-3.4/bin/../include/c++/v1/algorithm:627:
/opt/local/libexec/llvm-3.4/bin/../include/c++/v1/memory:1627:45: error: multiple overloads of 'address' instantiate to the same
signature 'const_pointer (const_reference) const noexcept'
_LIBCPP_INLINE_VISIBILITY const_pointer address(const_reference __x) const _NOEXCEPT
^
/opt/local/libexec/llvm-3.4/bin/../include/c++/v1/type_traits:922:38: note: in instantiation of template class
'std::__1::allocator<int (int)>' requested here
: public integral_constant<bool, __is_empty(_Tp)> {};
^
/opt/local/libexec/llvm-3.4/bin/../include/c++/v1/memory:1908:40: note: in instantiation of template class
'std::__1::is_empty<std::__1::allocator<int (int)> >' requested here
bool = is_empty<_T2>::value
^
/opt/local/libexec/llvm-3.4/bin/../include/c++/v1/memory:1930:44: note: in instantiation of default argument for
'__libcpp_compressed_pair_switch<int (int), std::__1::allocator<int (int)>, false, false>' required here
template <class _T1, class _T2, unsigned = __libcpp_compressed_pair_switch<_T1, _T2>::value>
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/local/libexec/llvm-3.4/bin/../include/c++/v1/memory:2298:15: note: in instantiation of default argument for
'__libcpp_compressed_pair_imp<int (int), std::__1::allocator<int (int)> >' required here
: private __libcpp_compressed_pair_imp<_T1, _T2>
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/local/libexec/llvm-3.4/bin/../include/c++/v1/future:1770:36: note: in instantiation of template class
'std::__1::__compressed_pair<int (int), std::__1::allocator<int (int)> >' requested here
__compressed_pair<_Fp, _Alloc> __f_;
^
/opt/local/libexec/llvm-3.4/bin/../include/c++/v1/future:1877:9: note: in instantiation of template class
'std::__1::__packaged_task_func<int (int), std::__1::allocator<int (int)>, int (int)>' requested here
if (sizeof(_FF) <= sizeof(__buf_))
^
/opt/local/libexec/llvm-3.4/bin/../include/c++/v1/future:2026:45: note: in instantiation of function template specialization
'std::__1::__packaged_task_function<int (int)>::__packaged_task_function<int (&)(int)>' requested here
explicit packaged_task(_Fp&& __f) : __f_(_VSTD::forward<_Fp>(__f)) {}
^
task.cpp:12:32: note: in instantiation of function template specialization 'std::__1::packaged_task<int
(int)>::packaged_task<int (&)(int), void>' requested here
std::packaged_task<int(int)> tsk (triple); // package task
^
/opt/local/libexec/llvm-3.4/bin/../include/c++/v1/memory:1625:39: note: previous declaration is here
_LIBCPP_INLINE_VISIBILITY pointer address(reference __x) const _NOEXCEPT
^
/opt/local/libexec/llvm-3.4/bin/../include/c++/v1/memory:2121:9: error: data member instantiated with function type 'int (int)'
_T1 __first_;
^
/opt/local/libexec/llvm-3.4/bin/../include/c++/v1/memory:2298:15: note: in instantiation of template class
'std::__1::__libcpp_compressed_pair_imp<int (int), std::__1::allocator<int (int)>, 2>' requested here
: private __libcpp_compressed_pair_imp<_T1, _T2>
^
/opt/local/libexec/llvm-3.4/bin/../include/c++/v1/future:1770:36: note: in instantiation of template class
'std::__1::__compressed_pair<int (int), std::__1::allocator<int (int)> >' requested here
__compressed_pair<_Fp, _Alloc> __f_;
^
/opt/local/libexec/llvm-3.4/bin/../include/c++/v1/future:1877:9: note: in instantiation of template class
'std::__1::__packaged_task_func<int (int), std::__1::allocator<int (int)>, int (int)>' requested here
if (sizeof(_FF) <= sizeof(__buf_))
^
/opt/local/libexec/llvm-3.4/bin/../include/c++/v1/future:2026:45: note: in instantiation of function template specialization
'std::__1::__packaged_task_function<int (int)>::__packaged_task_function<int (&)(int)>' requested here
explicit packaged_task(_Fp&& __f) : __f_(_VSTD::forward<_Fp>(__f)) {}
^
task.cpp:12:32: note: in instantiation of function template specialization 'std::__1::packaged_task<int
(int)>::packaged_task<int (&)(int), void>' requested here
std::packaged_task<int(int)> tsk (triple); // package task
^
2 errors generated.

我正在使用 OSX Maverics 和编译器:

$ g++ --version
g++ (MacPorts gcc48 4.8.2_0) 4.8.2

$ clang++ --version
clang version 3.4 (tags/RELEASE_34/final)
Target: x86_64-apple-darwin13.1.0
Thread model: posix

有人可以帮帮我吗？

Answer 1

海湾合作委员会是正确的:

std::thread (std::move(tsk),33).detach(); // 1
//...
tsk.reset();  // 2

在 (1) 之后，tsk 没有更多状态(根据 packaged_task 的 move constructor)

在 (2) 处，reset() 为 required to throw no_state std::future_error

正如评论中所指出的，clang 有一个错误。