This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of C++11 status.

774. Member swap undefined for most containers

Section: 23 [containers] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-01-14 Last modified: 2016-01-28

Priority: Not Prioritized

View other active issues in [containers].

View all other issues in [containers].

View all issues with C++11 status.

Discussion:

It appears most containers declare but do not define a member-swap function.

This is unfortunate, as all overload the swap algorithm to call the member-swap function! (required for swappable guarantees [Table 37] and Container Requirements [Table 87])

Note in particular that Table 87 gives semantics of a.swap(b) as swap(a,b), yet for all containers we define swap(a,b) to call a.swap(b) - a circular definition.

A quick survey of clause 23 shows that the following containers provide a definition for member-swap: 

array
queue
stack
vector

Whereas the following declare it, but do not define the semantics: 

deque
list
map
multimap
multiset
priority_queue
set
unordered_map
unordered_multi_map
unordered_multi_set
unordered_set

Suggested resolution:

Provide a definition for each of the affected containers...

[ Bellevue: ]

Move to Open and ask Alisdair to provide wording.

[ 2009-07 Frankfurt: ]

Daniel to provide wording. N2590 is no longer applicable.

[ 2009-07-28 Daniel provided wording. ]

  1. It assumes that the proposed resolution for 883(i) is applied, which breaks the circularity of definition between member swap and free swap.
  2. It uses the notation of the pre-concept allocator trait allocator_propagation_map, which might be renamed after the next refactoring phase of generalized allocators.
  3. It requires that compare objects, key equal functions and hash functions in containers are swapped via unqualified free swap according to 594(i).

[ 2009-09-30 Daniel adds: ]

The outcome of this issue should be considered with the outcome of 1198(i) both in style and in content (e.g. bullet 9 suggests to define the semantic of void priority_queue::swap(priority_queue&) in terms of the member swap of the container).

[ 2009-10 Santa Cruz: ]

Looked at, but took no action on as it overlaps too much with N2982. Waiting for a new draft WP.

[ 2009-10 Santa Cruz: ]

Leave as open. Pablo to provide wording.

[ 2009-10-26 Pablo updated wording. Here is the wording he replaced: ]

  1. Add a new Throws clause just after 99 [allocator.propagation.map]/5: 

    static void swap(Alloc& a, Alloc& b);

    Effects: [..]

    [ This exception requirement is added, such that it's combination with the general container requirements of N2723 [container.requirements.general]/9 make it unambiguously clear that the following descriptions of "swaps the allocators" have the following meaning: (a) This swap is done by calling allocator_propagation_map<allocator_type>::swap and (b) This allocator swap does never propagate an exception ]

  2. Change 23.2.7.2 [associative.reqmts.except]/3 as indicated:

    For associative containers, no swap function throws an exception unless that exception is thrown by the copy constructor or copy assignment operator of the container's Pred object (if any).

  3. Change 23.2.8.2 [unord.req.except]/3 as indicated:

    For unordered associative containers, no swap function throws an exception unless that exception is thrown by the copy constructor or copy assignment operator of the container's Hash or Pred object (if any).

  4. Insert a new paragraph just after 23.3 [sequences]/1:

    [ There is a new issue in process that will suggest a minimum header for swap and move. If this one is provided, this text can be removed and the header dependency should be added to <queue> ]

  5. Add one further clause at the end of 23.3.3.4 [array.special]:

    [This part is added, because otherwise array::swap would otherwise contradict the general contract of 23.2.2 [container.requirements.general] p. 10 b. 5]

    1. In 23.3.5 [deque], class template deque synopsis change as indicated: 

      void swap(deque<T,Alloc>&);

    2. At the end of 23.3.5.4 [deque.modifiers] add as indicated:

    1. In [forwardlist], class template forward_list synopsis change as indicated: 

      void swap(forward_list<T,Allocator>&);

    2. At the end of [forwardlist.modifiers] add as indicated:

    1. In 23.3.11 [list], class template list synopsis change as indicated: 

      void swap(list<T,Allocator>&);

    2. At the end of 23.3.11.4 [list.modifiers] add as indicated:

  9. At the end of 23.6.4.4 [priqueue.members] add a new prototype description:

    [ This requirement is added to ensure that even a user defined swap which is found by ADL for Compare satisfies the Swappable requirements ]

    [ This part is added, because otherwise priority_queue::swap would otherwise contradict the general contract of 23.2.2 [container.requirements.general] p. 10 b. 5 ]

    1. In 23.3.13 [vector], class template vector synopsis change as indicated: 

      void swap(vector<T,Allocator>&);

    2. Change 23.3.13.3 [vector.capacity] p. 8 as indicated: 

      void swap(vector<T,Allocator>& x);

      Effects: Exchanges the contents and capacity() of *this with that of x.

  11. Insert a new paragraph just before 23.4 [associative]/1:

    1. In 23.4.3 [map], class template map synopsis change as indicated: 

      void swap(map<Key,T,Compare,Allocator>&);

    2. At the end of 23.4.3.4 [map.modifiers] add as indicated:

      [ This requirement is added to ensure that even a user defined swap which is found by ADL for Compare satisfies the Swappable requirements ]

    1. In 23.4.4 [multimap], class template multimap synopsis change as indicated: 

      void swap(multimap<Key,T,Compare,Allocator>&);

    2. At the end of 23.4.4.3 [multimap.modifiers] add as indicated:

    1. In 23.4.6 [set], class template set synopsis change as indicated: 

      void swap(set<Key,Compare,Allocator>&);

    2. After section 23.4.6.2 [set.cons] add a new section and add the following paragraphs:

    1. In 23.4.7 [multiset], class template multiset synosis, change as indicated: 

      void swap(multiset<Key,Compare,Allocator>&);

    2. After section 23.4.7.2 [multiset.cons] add a new section and add the following paragraphs:

  16. Insert a new paragraph just before 23.5 [unord] p. 1:

  17. After section 23.5.3.3 [unord.map.elem] add a new section and add the following paragraphs:

    [ This requirement is added to ensure that even a user defined swap which is found by ADL for Hash and Pred satisfies the Swappable requirements ]

  18. After section 23.5.4.2 [unord.multimap.cnstr] add a new section and add the following paragraphs:

  19. After section 23.5.6.2 [unord.set.cnstr] add a new section and add the following paragraphs:

  20. After section 23.5.7.2 [unord.multiset.cnstr] add a new section and add the following paragraphs:

[ 2009-10-30 Pablo and Daniel updated wording. ]

[ 2010 Pittsburgh: Ready for Pittsburgh. ]

Proposed resolution:

[ This resolution is based on the September 2009 WP, N2960, except that it assumes that N2982 and issues 883(i) and 1232(i) have already been applied. Note in particular that Table 91 in N2960 is refered to as Table 90 because N2982 removed the old Table 90. This resolution also addresses issue 431(i). ]

In 23.2.2 [container.requirements.general], replace the a.swap(b) row in table 90, "container requirements" (was table 91 before the application of N2982 to the WP):

a.swap(b) void     swap(a,b) (Note A)
   

Modify the notes immediately following Table 90 in 23.2.2 [container.requirements.general] as follows (The wording below is after the application of N2982 to N2960. The editor might also want to combine Notes A and B into one.):

Notes: the algorithms swap(), equal() and lexicographical_compare() are defined in Clause 25. Those entries marked "(Note A)" or "(Note B)" should have constant complexity .

In 23.2.2 [container.requirements.general], before paragraph 8, add:

[ Note to the editor: Paragraph 2 starts with a sentence fragment, clearly from an editing or source-control error. ]

Modify 23.2.7.2 [associative.reqmts.except] as follows:

23.2.4.1 Exception safety guarantees 23.2.7.2 [associative.reqmts.except]

For associative containers, no clear() function throws an exception. erase(k) does not throw an exception unless that exception is thrown by the container's Pred object (if any).

For associative containers, if an exception is thrown by any operation from within an insert() function inserting a single element, the insert() function has no effect.

For associative containers, no swap function throws an exception unless that exception is thrown by the copy constructor or copy assignment operator of the container's Pred object (if any).

Modify 23.2.8.2 [unord.req.except], paragraph 3 as follows:

For unordered associative containers, no swap function throws an exception unless that exception is thrown by the copy constructor or copy assignment operator of the container's Hash or Pred object (if any).

Modify section 23.3.3.4 [array.special]:

array specialized algorithms 23.3.3.4 [array.special]

template <class T, size_t N> void swap(array<T,N>& x,array<T,N>& y);

Effects: swap_ranges(x.begin(), x.end(), y.begin() );

Add a new section after [array.fill] (Note to the editor: array::fill make use of a concept requirement that must be removed or changed to text.):

Insert a new paragraph just after 23.6 [container.adaptors]/1: