Issue 2067: packaged_task should have deleted copy c'tor with const parameter (original) (raw)

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2067. packaged_task should have deleted copy c'tor with const parameter

Section: 32.10.10 [futures.task] Status: C++14 Submitter: Daniel Krügler Opened: 2011-06-16 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Class template packaged_task is a move-only type with the following form of the deleted copy operations:

packaged_task(packaged_task&) = delete; packaged_task& operator=(packaged_task&) = delete;

Note that the argument types are non-const. This does not look like a typo to me, this form seems to exist from the very first proposing paper on N2276. Using either of form of the copy-constructor did not make much difference before the introduction of defaulted special member functions, but it makes now an observable difference. This was brought to my attention by a question on a German C++ newsgroup where the question was raised why the following code does not compile on a recent gcc:

#include #include #include #include

int main(){ std::packaged_task<void()> someTask([]{ std::cout << std::this_thread::get_id() << std::endl; }); std::thread someThread(std::move(someTask)); // Error here // Remainder omitted }

It turned out that the error was produced by the instantiation of some return type of std::bind which used a defaulted copy-constructor, which leads to a const declaration conflict with [class.copy] p8.

Some aspects of this problem are possibly core-language related, but I consider it more than a service to programmers, if the library would declare the usual form of the copy operations (i.e. those with const first parameter type) as deleted forpackaged_task to prevent such problems.

A similar problem exists for class template basic_ostream in 31.7.6.2 [ostream]:

namespace std { template <class charT, class traits = char_traits > class basic_ostream : virtual public basic_ios<charT,traits> { […]

// 27.7.3.3 Assign/swap basic_ostream& operator=(basic_ostream& rhs) = delete; basic_ostream& operator=(const basic_ostream&& rhs); void swap(basic_ostream& rhs); };

albeit this could be considered as an editorial swap of copy and move assignment operator, I suggest to fix this as part of this issue as well.

[ 2011 Bloomington. ]

Move to Ready.

Proposed resolution:

This wording is relative to the FDIS.

1. Modify the class template basic_ostream synopsis in 31.7.6.2 [ostream]as indicated (Note: The prototype signature of the move assignment operator in 31.7.6.2.3 [ostream.assign]is fine):

   namespace std {
   template <class charT, class traits = char_traits >
   class basic_ostream : virtual public basic_ios<charT,traits> {
   […]

   // 27.7.3.3 Assign/swap
   basic_ostream& operator=(const basic_ostream& rhs) = delete;
   basic_ostream& operator=(const basic_ostream&& rhs);
   void swap(basic_ostream& rhs);
   };

2. Modify the class template packaged_task synopsis in 32.10.10 [futures.task] p2 as indicated:

   namespace std {
   template class packaged_task; // undefined

   template<class R, class... ArgTypes>
   class packaged_task<R(ArgTypes...)> {
   public:
   […]

   // no copy
   packaged_task(const packaged_task&) = delete;
   packaged_task& operator=(const packaged_task&) = delete;

   […]
   };
   […]
   }