Created on 2011-06-16.00:00:00 last changed 131 months ago
Proposed resolution:
This wording is relative to the FDIS.
Modify the class template basic_ostream synopsis in [ostream] as indicated (Note: The prototype signature of the move assignment operator in [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=(constbasic_ostream&& rhs); void swap(basic_ostream& rhs); };
Modify the class template packaged_task synopsis in [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; […] }; […] }
[ 2011 Bloomington. ]
Move to Ready.
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 [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.
History | |||
---|---|---|---|
Date | User | Action | Args |
2014-02-20 13:20:35 | admin | set | status: wp -> c++14 |
2012-02-12 18:36:43 | admin | set | status: voting -> wp |
2012-02-09 04:07:48 | admin | set | status: ready -> voting |
2011-08-16 23:35:18 | admin | set | messages: + msg5857 |
2011-08-16 23:35:18 | admin | set | status: new -> ready |
2011-06-16 21:35:33 | admin | set | messages: + msg5820 |
2011-06-16 00:00:00 | admin | create |