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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 <utility>
#include <future>
#include <iostream>
#include <thread>

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 for packaged_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<charT> >
  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<charT> >
     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> 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;
       
       […]
     };
     […]
   }