Htykuut

I have a non-copyable class, simplified as follows:

struct NonCopyable

{

    NonCopyable() = default;

    NonCopyable(const NonCopyable&) = delete;

    NonCopyable& operator = (const NonCopyable&) = delete;

    NonCopyable(NonCopyable&&) = default;

    NonCopyable& operator = (NonCopyable&&) = default;

};

I store the objects of this class in various std::lists. At one point I have an empty std::vector of these lists, which I would like to resize to contain a fixed number of empty lists. However, this complains about the lack of a NonCopyable copy constructor, even though (as far as I can see) it shouldn't be trying to construct any NonCopyables!

std::vector<std::list<NonCopyable>> v; // OK

v.resize(4);                           // ERROR

v.emplace_back();                      // ERROR

std::list<NonCopyable> l;              // OK

v.push_back(l);                        // ERROR (unsurprisingly)

v.push_back(std::move(l));             // ERROR

Why is this? Is there any way to do this without making the class copyable?

I'm using VS2017 with /std:c++17 in case that makes any difference.

Here's the full error output from just trying to resize (first two lines above).

1>------ Build started: Project: testcore, Configuration: Debug x64 ------

1>testcard.cpp

1>c:program files (x86)microsoft visual studio2017communityvctoolsmsvc14.15.26726includexmemory0(881): error C2280: 'NonCopyable::NonCopyable(const NonCopyable &)': attempting to reference a deleted function

1>[...]testcard.cpp(30): note: see declaration of 'NonCopyable::NonCopyable'

1>[...]testcard.cpp(30): note: 'NonCopyable::NonCopyable(const NonCopyable &)': function was explicitly deleted

1>c:program files (x86)microsoft visual studio2017communityvctoolsmsvc14.15.26726includelist(711): note: see reference to function template instantiation 'void std::_Default_allocator_traits<_Alloc>::construct<_Ty,const NonCopyable&>(_Alloc &,_Objty *const ,const NonCopyable &)' being compiled

1>        with

1>        [

1>            _Alloc=std::allocator<std::_List_node<NonCopyable,std::_Default_allocator_traits<std::allocator<NonCopyable>>::void_pointer>>,

1>            _Ty=NonCopyable,

1>            _Objty=NonCopyable

1>        ]

1>c:program files (x86)microsoft visual studio2017communityvctoolsmsvc14.15.26726includelist(716): note: see reference to function template instantiation 'void std::_Default_allocator_traits<_Alloc>::construct<_Ty,const NonCopyable&>(_Alloc &,_Objty *const ,const NonCopyable &)' being compiled

1>        with

1>        [

1>            _Alloc=std::allocator<std::_List_node<NonCopyable,std::_Default_allocator_traits<std::allocator<NonCopyable>>::void_pointer>>,

1>            _Ty=NonCopyable,

1>            _Objty=NonCopyable

1>        ]

1>c:program files (x86)microsoft visual studio2017communityvctoolsmsvc14.15.26726includelist(947): note: see reference to function template instantiation 'std::_List_node<_Ty,std::_Default_allocator_traits<_Alloc>::void_pointer> *std::_List_buy<_Ty,_Alloc>::_Buynode<const NonCopyable&>(std::_List_node<_Ty,std::_Default_allocator_traits<_Alloc>::void_pointer> *,std::_List_node<_Ty,std::_Default_allocator_traits<_Alloc>::void_pointer> *,const NonCopyable &)' being compiled

1>        with

1>        [

1>            _Ty=NonCopyable,

1>            _Alloc=std::allocator<NonCopyable>

1>        ]

1>c:program files (x86)microsoft visual studio2017communityvctoolsmsvc14.15.26726includelist(950): note: see reference to function template instantiation 'std::_List_node<_Ty,std::_Default_allocator_traits<_Alloc>::void_pointer> *std::_List_buy<_Ty,_Alloc>::_Buynode<const NonCopyable&>(std::_List_node<_Ty,std::_Default_allocator_traits<_Alloc>::void_pointer> *,std::_List_node<_Ty,std::_Default_allocator_traits<_Alloc>::void_pointer> *,const NonCopyable &)' being compiled

1>        with

1>        [

1>            _Ty=NonCopyable,

1>            _Alloc=std::allocator<NonCopyable>

1>        ]

1>c:program files (x86)microsoft visual studio2017communityvctoolsmsvc14.15.26726includelist(1308): note: see reference to function template instantiation 'void std::list<NonCopyable,std::allocator<_Ty>>::_Insert<const NonCopyable&>(std::_List_unchecked_const_iterator<std::_List_val<std::_List_simple_types<_Ty>>,std::_Iterator_base0>,const NonCopyable &)' being compiled

1>        with

1>        [

1>            _Ty=NonCopyable

1>        ]

1>c:program files (x86)microsoft visual studio2017communityvctoolsmsvc14.15.26726includelist(1308): note: see reference to function template instantiation 'void std::list<NonCopyable,std::allocator<_Ty>>::_Insert<const NonCopyable&>(std::_List_unchecked_const_iterator<std::_List_val<std::_List_simple_types<_Ty>>,std::_Iterator_base0>,const NonCopyable &)' being compiled

1>        with

1>        [

1>            _Ty=NonCopyable

1>        ]

1>c:program files (x86)microsoft visual studio2017communityvctoolsmsvc14.15.26726includelist(1265): note: see reference to function template instantiation 'void std::list<NonCopyable,std::allocator<_Ty>>::_Insert_range<_Iter>(std::_List_unchecked_const_iterator<std::_List_val<std::_List_simple_types<_Ty>>,std::_Iterator_base0>,_Iter,_Iter,std::forward_iterator_tag)' being compiled

1>        with

1>        [

1>            _Ty=NonCopyable,

1>            _Iter=std::_List_const_iterator<std::_List_val<std::_List_simple_types<NonCopyable>>>

1>        ]

1>c:program files (x86)microsoft visual studio2017communityvctoolsmsvc14.15.26726includelist(1264): note: see reference to function template instantiation 'void std::list<NonCopyable,std::allocator<_Ty>>::_Insert_range<_Iter>(std::_List_unchecked_const_iterator<std::_List_val<std::_List_simple_types<_Ty>>,std::_Iterator_base0>,_Iter,_Iter,std::forward_iterator_tag)' being compiled

1>        with

1>        [

1>            _Ty=NonCopyable,

1>            _Iter=std::_List_const_iterator<std::_List_val<std::_List_simple_types<NonCopyable>>>

1>        ]

1>c:program files (x86)microsoft visual studio2017communityvctoolsmsvc14.15.26726includelist(800): note: see reference to function template instantiation 'std::_List_iterator<std::_List_val<std::_List_simple_types<_Ty>>> std::list<_Ty,std::allocator<_Ty>>::insert<std::_List_const_iterator<std::_List_val<std::_List_simple_types<_Ty>>>,void>(std::_List_const_iterator<std::_List_val<std::_List_simple_types<_Ty>>>,_Iter,_Iter)' being compiled

1>        with

1>        [

1>            _Ty=NonCopyable,

1>            _Iter=std::_List_const_iterator<std::_List_val<std::_List_simple_types<NonCopyable>>>

1>        ]

1>c:program files (x86)microsoft visual studio2017communityvctoolsmsvc14.15.26726includelist(800): note: see reference to function template instantiation 'std::_List_iterator<std::_List_val<std::_List_simple_types<_Ty>>> std::list<_Ty,std::allocator<_Ty>>::insert<std::_List_const_iterator<std::_List_val<std::_List_simple_types<_Ty>>>,void>(std::_List_const_iterator<std::_List_val<std::_List_simple_types<_Ty>>>,_Iter,_Iter)' being compiled

1>        with

1>        [

1>            _Ty=NonCopyable,

1>            _Iter=std::_List_const_iterator<std::_List_val<std::_List_simple_types<NonCopyable>>>

1>        ]

1>c:program files (x86)microsoft visual studio2017communityvctoolsmsvc14.15.26726includelist(796): note: while compiling class template member function 'std::list<NonCopyable,std::allocator<_Ty>>::list(const std::list<_Ty,std::allocator<_Ty>> &)'

1>        with

1>        [

1>            _Ty=NonCopyable

1>        ]

1>c:program files (x86)microsoft visual studio2017communityvctoolsmsvc14.15.26726includexmemory0(881): note: see reference to function template instantiation 'std::list<NonCopyable,std::allocator<_Ty>>::list(const std::list<_Ty,std::allocator<_Ty>> &)' being compiled

1>        with

1>        [

1>            _Ty=NonCopyable

1>        ]

1>c:program files (x86)microsoft visual studio2017communityvctoolsmsvc14.15.26726includevector(1812): note: see reference to class template instantiation 'std::list<NonCopyable,std::allocator<_Ty>>' being compiled

1>        with

1>        [

1>            _Ty=NonCopyable

1>        ]

1>c:program files (x86)microsoft visual studio2017communityvctoolsmsvc14.15.26726includevector(1811): note: while compiling class template member function 'std::list<NonCopyable,std::allocator<_Ty>> *std::vector<std::list<_Ty,std::allocator<_Ty>>,std::allocator<std::list<_Ty,std::allocator<_Ty>>>>::_Udefault(std::list<_Ty,std::allocator<_Ty>> *,const unsigned __int64)'

1>        with

1>        [

1>            _Ty=NonCopyable

1>        ]

1>c:program files (x86)microsoft visual studio2017communityvctoolsmsvc14.15.26726includevector(1479): note: see reference to function template instantiation 'std::list<NonCopyable,std::allocator<_Ty>> *std::vector<std::list<_Ty,std::allocator<_Ty>>,std::allocator<std::list<_Ty,std::allocator<_Ty>>>>::_Udefault(std::list<_Ty,std::allocator<_Ty>> *,const unsigned __int64)' being compiled

1>        with

1>        [

1>            _Ty=NonCopyable

1>        ]

1>[...]testcard.cpp(37): note: see reference to class template instantiation 'std::vector<std::list<NonCopyable,std::allocator<_Ty>>,std::allocator<std::list<_Ty,std::allocator<_Ty>>>>' being compiled

1>        with

1>        [

1>            _Ty=NonCopyable

1>        ]

1>Done building project "testcore.vcxproj" -- FAILED.

========== Build: 0 succeeded, 1 failed, 2 up-to-date, 0 skipped ==========

std::list is not noexcept-movable, but is copyable. Therefore, std::vector prefers list's the copy-constructor over its move-constructor, regardless.

To work around the issue, you have to wrap your std::list in a class that is not copyable.

As @j6t pointed out, it seems it can't be done. A vector with a std::list with non-copyable elements can not be expanded according to the standard (...). If it was my private project I'd actually consider something like this:

#include <iostream>

#include <vector>

#include <list>



namespace xxx {

    // <flame_bait>

    template<class T, class Enable = void>

    struct list : std::list<T> {

        list() : std::list<T>() {}

        list(list&&) = default;

        list& operator=(list&&) = default;

        list(const list&) = delete;

        list& operator=(const list&) = delete;

    };



    template<class T>

    struct list<T, typename std::enable_if<std::is_copy_constructible<T>::value>::type> : std::list<T> {};

    // </flame_bait>

}



struct NonCopyable

{

    int m_id;

    NonCopyable(int id = 0) : m_id(id) {}

    NonCopyable(const NonCopyable&) = delete;

    NonCopyable& operator = (const NonCopyable&) = delete;

    NonCopyable(NonCopyable&&) noexcept = default;

    NonCopyable& operator = (NonCopyable&&) noexcept = default;

    ~NonCopyable() = default;

    friend std::ostream& operator<<(std::ostream&, const NonCopyable&);

};



std::ostream& operator<<(std::ostream& os, const NonCopyable& nc) {

    os << "I am not a free man, I am number " << nc.m_id;

    return os;

}



int main() {

    std::vector<xxx::list<NonCopyable>> v;

    v.resize(4);                    // now working 



    int id = 0;

    for (auto& l : v) {

        l.emplace_back(++id);       // emplace_back one NonCopyable per list  

    }



    xxx::list<NonCopyable> li;      // Create a separate list

    li.emplace_back(++id);          // create one...

    v.emplace_back(std::move(li));  // and move list to vector is now working



    for (auto& l : v) {

        for (auto& nc : l) {

            std::cout << nc << "n";

        }

    }

}

Expected output:

I am not a free man, I am number 1

I am not a free man, I am number 2

I am not a free man, I am number 3

I am not a free man, I am number 4

I am not a free man, I am number 5