[cfe-dev] thread(F&&) fails when F is not MoveAssignable

[Howard Hinnant]

On Dec 16, 2012, at 5:22 PM, Gordon Henriksen <gordonhenriksen at me.com> wrote:

> [Sorry for the cross-post, but cfe-users doesn't seem to be active yet.]
> 
> The snippet at the end of this message fails to compile with clang-libc++.
> 
> Is LaunchByMove() invalid C++11, or is this a libc++ bug?
> 
> I have a body of code written using the failing LaunchByMove pattern. I guess
> I'll have to work around the issue, perhaps with the LaunchByCopy pattern.
> Enforcing that style may prove error-prone, though.
> 
> The relevant section in N3242 is this:
> 
>  30.3.1.2  thread constructors
>  …
> 
>  template <class F, class ...Args> explicit thread(F&& f, Args&&... args);
> 
>  Given a function as follows:
> 
>    template <class T> typename decay<T>::type decay_copy(T&& v)
>      { return std::forward<T>(v); }
> 
>  Requires: F and each Ti in Args shall satisfy the MoveConstructible
>  requirements. INVOKE(decay_copy(std::forward<F>(f)),
>  decay_copy(std::forward<Args>(args))...) (20.8.2) shall be a valid
>  expression.
> 
> There doesn't appear to be any Swappable or MoveAssignable requirement for
> template argument F, at least in the draft.
> 
> — Gordon
> 
> 
> cat <<'EOT' >test.cpp
> #include <thread>
> class CopyableNotAssignable {
>  int &n_;
> public:
>  CopyableNotAssignable(int &n) : n_(n) { }
> };
> void LaunchByMove() {
>  int n;
>  std::thread(CopyableNotAssignable(n)).join(); // expect-error
> }
> void LaunchByCopy() {
>  int n;
>  CopyableNotAssignable cna(n);
>  std::thread(cna).join(); // successful
> }
> EOT
> xcrun clang -std=c++0x -stdlib=libc++ -c test.cpp
> 
> 
> Output:
> In file included from test.cpp:1:
> /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/thread:330:5: error: 
>    attempt to use a deleted function
>  __invoke(_VSTD::move(_VSTD::get<0>(__t)), ...
>  ^
> /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/thread:340:5: note: 
>    in instantiation of function template specialization
>    'std::__1::__threaad_execute<CopyableNotAssignable, , >' requested here
>  __threaad_execute(*__p, _Index());
>  ^
> /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/thread:352:41: note: 
>    in instantiation of function template specialization
>    'std::__1::__thread_proxy<std::__1::tuple<CopyableNotAssignable> >'
>    requested here
>  int __ec = pthread_create(&__t_, 0, &__thread_proxy<_Gp>, __p.get());
>                                      ^
> test.cpp:9:3: note: in instantiation of function template specialization
>    'std::__1::thread::thread<CopyableNotAssignable, , void>' requested here
> std::thread(CopyableNotAssignable(n)).join();
> ^
> /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/type_traits:825:5: note: 
>    function has been explicitly marked deleted here
>  ~__nat() = delete;
>  ^
> 1 error generated.

CopyableNotAssignable does not appear to be a callable object with zero parameters.  LaunchByMove and LaunchByCopy should both fail.  Or technically, they exhibit undefined behavior, so anything could happen.  A compile-time error is the highest quality solution.

Howard