Issue2193

Title 2193. Default constructors for standard library containers are explicit
Status open Section [containers]
Submitter Richard Smith

Created on 2012-10-04.00:00:00 by admin, last changed by admin.

Messages
msg6861 (view) Date: 2014-02-14.19:48:07

Proposed resolution:

This wording is relative to N3376.

The more general criterion for performing the suggested transformation was: Any type with an initializer-list constructor that also has an explicit default constructor.

  1. Change class template basic_string synopsis, [basic.string] p5 as indicated:

    basic_string() : basic_string(Allocator()) { }
    explicit basic_string(const Allocator& a = Allocator());
    
  2. Change [string.cons] before p1 as indicated:

    explicit basic_string(const Allocator& a = Allocator());
    
  3. Change class template deque synopsis, [deque.overview] p2 as indicated:

    deque() : deque(Allocator()) { }
    explicit deque(const Allocator& = Allocator());
    
  4. Change [deque.cons] before p1 as indicated:

    explicit deque(const Allocator& = Allocator());
    
  5. Change class template forward_list synopsis, [forwardlist.overview] p3 as indicated:

    forward_list() : forward_list(Allocator()) { }
    explicit forward_list(const Allocator& = Allocator());
    
  6. Change [forwardlist.cons] before p1 as indicated:

    explicit forward_list(const Allocator& = Allocator());
    
  7. Change class template list synopsis, [list.overview] p2 as indicated:

    list() : list(Allocator()) { }
    explicit list(const Allocator& = Allocator());
    
  8. Change [list.cons] before p1 as indicated:

    explicit list(const Allocator& = Allocator());
    
  9. Change class template vector synopsis, [vector.overview] p2 as indicated:

    vector() : vector(Allocator()) { }
    explicit vector(const Allocator& = Allocator());
    
  10. Change [vector.cons] before p1 as indicated:

    explicit vector(const Allocator& = Allocator());
    
  11. Change class template specialization vector<bool> synopsis, [vector.bool] p1 as indicated:

    vector() : vector(Allocator()) { }
    explicit vector(const Allocator& = Allocator());
    
  12. Change class template map synopsis, [map.overview] p2 as indicated:

    map() : map(Compare()) { }
    explicit map(const Compare& comp = Compare(),
                 const Allocator& = Allocator());
    
  13. Change [map.cons] before p1 as indicated:

    explicit map(const Compare& comp = Compare(),
                 const Allocator& = Allocator());
    
  14. Change class template multimap synopsis, [multimap.overview] p2 as indicated:

    multimap() : multimap(Compare()) { }
    explicit multimap(const Compare& comp = Compare(),
                      const Allocator& = Allocator());
    
  15. Change [multimap.cons] before p1 as indicated:

    explicit multimap(const Compare& comp = Compare(),
                      const Allocator& = Allocator());
    
  16. Change class template set synopsis, [set.overview] p2 as indicated:

    set() : set(Compare()) { }
    explicit set(const Compare& comp = Compare(),
                 const Allocator& = Allocator());
    
  17. Change [set.cons] before p1 as indicated:

    explicit set(const Compare& comp = Compare(),
                 const Allocator& = Allocator());
    
  18. Change class template multiset synopsis, [multiset.overview] p2 as indicated:

    multiset() : multiset(Compare()) { }
    explicit multiset(const Compare& comp = Compare(),
                      const Allocator& = Allocator());
    
  19. Change [multiset.cons] before p1 as indicated:

    explicit multiset(const Compare& comp = Compare(),
                      const Allocator& = Allocator());
    
  20. Change class template unordered_map synopsis, [unord.map.overview] p3 as indicated:

    unordered_map();
    explicit unordered_map(size_type n = see below,
                           const hasher& hf = hasher(),
                           const key_equal& eql = key_equal(),
                           const allocator_type& a = allocator_type());
    
  21. Change [unord.map.cnstr] before p1 as indicated:

    unordered_map() : unordered_map(size_type(see below)) { }
    explicit unordered_map(size_type n = see below,
                           const hasher& hf = hasher(),
                           const key_equal& eql = key_equal(),
                           const allocator_type& a = allocator_type());
    
    -1- Effects: Constructs an empty unordered_map using the specified hash function, key equality func-
    tion, and allocator, and using at least n buckets. If n is not provided, For the default constructor
    the number of buckets is implementation-defined. max_load_factor() returns 1.0.
  22. Change class template unordered_multimap synopsis, [unord.multimap.overview] p3 as indicated:

    unordered_multimap();
    explicit unordered_multimap(size_type n = see below,
                                const hasher& hf = hasher(),
                                const key_equal& eql = key_equal(),
                                const allocator_type& a = allocator_type());
    
  23. Change [unord.multimap.cnstr] before p1 as indicated:

    unordered_multimap() : unordered_multimap(size_type(see below)) { }
    explicit unordered_multimap(size_type n = see below,
                                const hasher& hf = hasher(),
                                const key_equal& eql = key_equal(),
                                const allocator_type& a = allocator_type());
    
    -1- Effects: Constructs an empty unordered_multimap using the specified hash function, key equality
    function, and allocator, and using at least n buckets. If n is not provided, For the default constructor
    the number of buckets is implementation-defined. max_load_factor() returns 1.0.
  24. Change class template unordered_set synopsis, [unord.set.overview] p3 as indicated:

    unordered_set();
    explicit unordered_set(size_type n = see below,
                           const hasher& hf = hasher(),
                           const key_equal& eql = key_equal(),
                           const allocator_type& a = allocator_type());
    
  25. Change [unord.set.cnstr] before p1 as indicated:

    unordered_set() : unordered_set(size_type(see below)) { }
    explicit unordered_set(size_type n = see below,
                           const hasher& hf = hasher(),
                           const key_equal& eql = key_equal(),
                           const allocator_type& a = allocator_type());
    
    -1- Effects: Constructs an empty unordered_set using the specified hash function, key equality func-
    tion, and allocator, and using at least n buckets. If n is not provided, For the default constructor
    the number of buckets is implementation-defined. max_load_factor() returns 1.0.
  26. Change class template unordered_multiset synopsis, [unord.multiset.overview] p3 as indicated:

    unordered_multiset();
    explicit unordered_multiset(size_type n = see below,
                                const hasher& hf = hasher(),
                                const key_equal& eql = key_equal(),
                                const allocator_type& a = allocator_type());
    
  27. Change [unord.multiset.cnstr] before p1 as indicated:

    unordered_multiset() : unordered_multiset(size_type(see below)) { }
    explicit unordered_multiset(size_type n = see below,
                                const hasher& hf = hasher(),
                                const key_equal& eql = key_equal(),
                                const allocator_type& a = allocator_type());
    
    -1- Effects: Constructs an empty unordered_multiset using the specified hash function, key equality
    function, and allocator, and using at least n buckets. If n is not provided, For the default constructor
    the number of buckets is implementation-defined. max_load_factor() returns 1.0.
msg6855 (view) Date: 2017-07-05.22:14:34

[ Issaquah 2014-02-11: Move to Immediate after final review ]

msg6671 (view) Date: 2014-02-15.00:00:00

[ 2014-02-13, Issaquah, Jonathan revises wording ]

Previous resolution from Daniel [SUPERSEDED]:

This wording is relative to N3376.

The more general criterion for performing the suggested transformation was: Any type with an initializer-list constructor that also has an explicit default constructor.

  1. Change class template basic_string synopsis, [basic.string] p5 as indicated:

    basic_string() : basic_string(Allocator()) {}
    explicit basic_string(const Allocator& a = Allocator());
    
  2. Change [string.cons] before p1 as indicated:

    explicit basic_string(const Allocator& a = Allocator());
    
  3. Change class template deque synopsis, [deque.overview] p2 as indicated:

    deque() : deque(Allocator()) {}
    explicit deque(const Allocator& = Allocator());
    
  4. Change [deque.cons] before p1 as indicated:

    explicit deque(const Allocator& = Allocator());
    
  5. Change class template forward_list synopsis, [forwardlist.overview] p3 as indicated:

    forward_list() : forward_list(Allocator()) {}
    explicit forward_list(const Allocator& = Allocator());
    
  6. Change [forwardlist.cons] before p1 as indicated:

    explicit forward_list(const Allocator& = Allocator());
    
  7. Change class template list synopsis, [list.overview] p2 as indicated:

    list() : list(Allocator()) {}
    explicit list(const Allocator& = Allocator());
    
  8. Change [list.cons] before p1 as indicated:

    explicit list(const Allocator& = Allocator());
    
  9. Change class template vector synopsis, [vector.overview] p2 as indicated:

    vector() : vector(Allocator()) {}
    explicit vector(const Allocator& = Allocator());
    
  10. Change [vector.cons] before p1 as indicated:

    explicit vector(const Allocator& = Allocator());
    
  11. Change class template specialization vector<bool> synopsis, [vector.bool] p1 as indicated:

    vector() : vector(Allocator()) {}
    explicit vector(const Allocator& = Allocator());
    
  12. Change class template map synopsis, [map.overview] p2 as indicated:

    map() : map(Compare()) {}
    explicit map(const Compare& comp = Compare(),
                 const Allocator& = Allocator());
    
  13. Change [map.cons] before p1 as indicated:

    explicit map(const Compare& comp = Compare(),
                 const Allocator& = Allocator());
    
  14. Change class template multimap synopsis, [multimap.overview] p2 as indicated:

    multimap() : multimap(Compare()) {}
    explicit multimap(const Compare& comp = Compare(),
                      const Allocator& = Allocator());
    
  15. Change [multimap.cons] before p1 as indicated:

    explicit multimap(const Compare& comp = Compare(),
                      const Allocator& = Allocator());
    
  16. Change class template set synopsis, [set.overview] p2 as indicated:

    set() : set(Compare()) {}
    explicit set(const Compare& comp = Compare(),
                 const Allocator& = Allocator());
    
  17. Change [set.cons] before p1 as indicated:

    explicit set(const Compare& comp = Compare(),
                 const Allocator& = Allocator());
    
  18. Change class template multiset synopsis, [multiset.overview] p2 as indicated:

    multiset() : multiset(Compare()) {}
    explicit multiset(const Compare& comp = Compare(),
                      const Allocator& = Allocator());
    
  19. Change [multiset.cons] before p1 as indicated:

    explicit multiset(const Compare& comp = Compare(),
                      const Allocator& = Allocator());
    
  20. Change class template unordered_map synopsis, [unord.map.overview] p3 as indicated:

    unordered_map() : unordered_map(see below) {}
    explicit unordered_map(size_type n = see below,
                           const hasher& hf = hasher(),
                           const key_equal& eql = key_equal(),
                           const allocator_type& a = allocator_type());
    
  21. Change [unord.map.cnstr] before p1 as indicated:

    unordered_map() : unordered_map(see below) {}
    explicit unordered_map(size_type n = see below,
                           const hasher& hf = hasher(),
                           const key_equal& eql = key_equal(),
                           const allocator_type& a = allocator_type());
    
  22. Change class template unordered_multimap synopsis, [unord.multimap.overview] p3 as indicated:

    unordered_multimap() : unordered_multimap(see below) {}
    explicit unordered_multimap(size_type n = see below,
                                const hasher& hf = hasher(),
                                const key_equal& eql = key_equal(),
                                const allocator_type& a = allocator_type());
    
  23. Change [unord.multimap.cnstr] before p1 as indicated:

    unordered_multimap() : unordered_multimap(see below) {}
    explicit unordered_multimap(size_type n = see below,
                                const hasher& hf = hasher(),
                                const key_equal& eql = key_equal(),
                                const allocator_type& a = allocator_type());
    
  24. Change class template unordered_set synopsis, [unord.set.overview] p3 as indicated:

    unordered_set() : unordered_set(see below) {}
    explicit unordered_set(size_type n = see below,
                           const hasher& hf = hasher(),
                           const key_equal& eql = key_equal(),
                           const allocator_type& a = allocator_type());
    
  25. Change [unord.set.cnstr] before p1 as indicated:

    unordered_set() : unordered_set(see below) {}
    explicit unordered_set(size_type n = see below,
                           const hasher& hf = hasher(),
                           const key_equal& eql = key_equal(),
                           const allocator_type& a = allocator_type());
    
  26. Change class template unordered_multiset synopsis, [unord.multiset.overview] p3 as indicated:

    unordered_multiset() : unordered_multiset(see below) {}
    explicit unordered_multiset(size_type n = see below,
                                const hasher& hf = hasher(),
                                const key_equal& eql = key_equal(),
                                const allocator_type& a = allocator_type());
    
  27. Change [unord.multiset.cnstr] before p1 as indicated:

    unordered_multiset() : unordered_multiset(see below) {}
    explicit unordered_multiset(size_type n = see below,
                                const hasher& hf = hasher(),
                                const key_equal& eql = key_equal(),
                                const allocator_type& a = allocator_type());
    
msg6568 (view) Date: 2013-10-15.00:00:00

[ 2013-10-06, Daniel comments: ]

Issue 2303 describes the more general problem related to explicit instantiation requests in the current library and may help to solve this problem here as well.

msg6057 (view) Date: 2013-08-15.00:00:00

[ 2013-08-27, Joaquín M López Muñoz comments: ]

For the record, I'd like to point out that the resolution proposed by the submitter, namely replacing

explicit basic_string(const Allocator& a = Allocator());

by

basic_string() : basic_string(Allocator()) {}
explicit basic_string(const Allocator& a);

(and similarly for other container and container-like classes) might introduce a potential backwards-compatibility problem related with explicit instantiation. Consider for instance

struct my_allocator
{
  my_allocator(...); // no default ctor
  ...
};

template class std::basic_string<char, std::char_traits<char>, my_allocator<char>>;

This (which I understand is currently a valid explicit instantiation of std::basic_string) will break if std::basic_string ctors are modified as proposed by this issue, since my_allocator doesn't have a default ctor.

msg6014 (view) Date: 2012-10-20.00:32:14

[ 2012, Portland: Move to Open ]

This may be an issue better solved by a core language tweak. Throw the issue over to EWG and see whether they believe the issue is better resolved in Core or Library.

AJM suggest we spawn a new status of 'EWG' to handle such issues - and will move this issue appropriately when the software can record such resolutions.

msg6013 (view) Date: 2012-10-06.00:00:00

[ 2012-10-06: Daniel adds concrete wording. ]

msg6012 (view) Date: 2012-10-04.00:00:00

Most (all?) of the standard library containers have explicit default constructors. Consequently:

std::set<int> s1 = { 1, 2 }; // ok
std::set<int> s2 = { 1 }; // ok
std::set<int> s3 = {}; // ill-formed, copy-list-initialization selected an explicit constructor

Note that Clang + libc++ rejects the declaration of s3 for this reason. This cannot possibly match the intent.

Suggested fix: apply this transformation throughout the standard library:

set() : set(Compare()) {}
explicit set(const Compare& comp = Compare(),
             const Allocator& = Allocator());
History
Date User Action Args
2014-02-27 17:03:20adminsetstatus: wp -> open
2014-02-20 13:52:38adminsetstatus: immediate -> wp
2014-02-14 19:48:07adminsetstatus: ewg -> immediate
2014-02-14 19:48:07adminsetmessages: + msg6861
2014-02-14 09:40:24adminsetmessages: + msg6855
2013-10-06 21:16:04adminsetmessages: + msg6671
2013-08-27 19:42:08adminsetmessages: + msg6568
2012-10-21 18:35:06adminsetstatus: open -> ewg
2012-10-20 00:32:14adminsetstatus: new -> open
2012-10-20 00:32:14adminsetmessages: + msg6057
2012-10-06 11:55:45adminsetmessages: + msg6014
2012-10-06 11:55:45adminsetmessages: + msg6013
2012-10-04 00:00:00admincreate