Issue 2193: Default constructors for standard library containers are explicit

Title: Default constructors for standard library containers are explicit
Status: c++14
Section: [containers]
Submitter: Richard Smith

Created on 2012-10-04.00:00:00 last changed 123 months ago

Messages

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.

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

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

Change [string.cons] before p1 as indicated:

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

Change class template deque synopsis, [deque.overview] p2 as indicated:

deque() : deque(Allocator()) { }
explicit deque(const Allocator& = Allocator());

Change [deque.cons] before p1 as indicated:

explicit deque(const Allocator& = Allocator());

Change class template forward_list synopsis, [forwardlist.overview] p3 as indicated:

forward_list() : forward_list(Allocator()) { }
explicit forward_list(const Allocator& = Allocator());

Change [forwardlist.cons] before p1 as indicated:

explicit forward_list(const Allocator& = Allocator());

Change class template list synopsis, [list.overview] p2 as indicated:

list() : list(Allocator()) { }
explicit list(const Allocator& = Allocator());

Change [list.cons] before p1 as indicated:

explicit list(const Allocator& = Allocator());

Change class template vector synopsis, [vector.overview] p2 as indicated:

vector() : vector(Allocator()) { }
explicit vector(const Allocator& = Allocator());

Change [vector.cons] before p1 as indicated:

explicit vector(const Allocator& = Allocator());

Change class template specialization vector<bool> synopsis, [vector.bool] p1 as indicated:

vector() : vector(Allocator()) { }
explicit vector(const Allocator& = Allocator());

Change class template map synopsis, [map.overview] p2 as indicated:

map() : map(Compare()) { }
explicit map(const Compare& comp = Compare(),
             const Allocator& = Allocator());

Change [map.cons] before p1 as indicated:

explicit map(const Compare& comp = Compare(),
             const Allocator& = Allocator());

Change class template multimap synopsis, [multimap.overview] p2 as indicated:

multimap() : multimap(Compare()) { }
explicit multimap(const Compare& comp = Compare(),
                  const Allocator& = Allocator());

Change [multimap.cons] before p1 as indicated:

explicit multimap(const Compare& comp = Compare(),
                  const Allocator& = Allocator());

Change class template set synopsis, [set.overview] p2 as indicated:

set() : set(Compare()) { }
explicit set(const Compare& comp = Compare(),
             const Allocator& = Allocator());

Change [set.cons] before p1 as indicated:

explicit set(const Compare& comp = Compare(),
             const Allocator& = Allocator());

Change class template multiset synopsis, [multiset.overview] p2 as indicated:

multiset() : multiset(Compare()) { }
explicit multiset(const Compare& comp = Compare(),
                  const Allocator& = Allocator());

Change [multiset.cons] before p1 as indicated:

explicit multiset(const Compare& comp = Compare(),
                  const Allocator& = Allocator());

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());

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.

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());

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.

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());

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.

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());

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.

msg6861 (view)

Date: 2014-02-14.19:48:07

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

msg6676 (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.
Change class template basic_string synopsis, [basic.string] p5 as indicated:
basic_string() : basic_string(Allocator()) {}
explicit basic_string(const Allocator& a = Allocator());
Change [string.cons] before p1 as indicated:
explicit basic_string(const Allocator& a = Allocator());
Change class template deque synopsis, [deque.overview] p2 as indicated:
deque() : deque(Allocator()) {}
explicit deque(const Allocator& = Allocator());
Change [deque.cons] before p1 as indicated:
explicit deque(const Allocator& = Allocator());
Change class template forward_list synopsis, [forwardlist.overview] p3 as indicated:
forward_list() : forward_list(Allocator()) {}
explicit forward_list(const Allocator& = Allocator());
Change [forwardlist.cons] before p1 as indicated:
explicit forward_list(const Allocator& = Allocator());
Change class template list synopsis, [list.overview] p2 as indicated:
list() : list(Allocator()) {}
explicit list(const Allocator& = Allocator());
Change [list.cons] before p1 as indicated:
explicit list(const Allocator& = Allocator());
Change class template vector synopsis, [vector.overview] p2 as indicated:
vector() : vector(Allocator()) {}
explicit vector(const Allocator& = Allocator());
Change [vector.cons] before p1 as indicated:
explicit vector(const Allocator& = Allocator());
Change class template specialization vector<bool> synopsis, [vector.bool] p1 as indicated:
vector() : vector(Allocator()) {}
explicit vector(const Allocator& = Allocator());
Change class template map synopsis, [map.overview] p2 as indicated:
map() : map(Compare()) {}
explicit map(const Compare& comp = Compare(),
             const Allocator& = Allocator());
Change [map.cons] before p1 as indicated:
explicit map(const Compare& comp = Compare(),
             const Allocator& = Allocator());
Change class template multimap synopsis, [multimap.overview] p2 as indicated:
multimap() : multimap(Compare()) {}
explicit multimap(const Compare& comp = Compare(),
                  const Allocator& = Allocator());
Change [multimap.cons] before p1 as indicated:
explicit multimap(const Compare& comp = Compare(),
                  const Allocator& = Allocator());
Change class template set synopsis, [set.overview] p2 as indicated:
set() : set(Compare()) {}
explicit set(const Compare& comp = Compare(),
             const Allocator& = Allocator());
Change [set.cons] before p1 as indicated:
explicit set(const Compare& comp = Compare(),
             const Allocator& = Allocator());
Change class template multiset synopsis, [multiset.overview] p2 as indicated:
multiset() : multiset(Compare()) {}
explicit multiset(const Compare& comp = Compare(),
                  const Allocator& = Allocator());
Change [multiset.cons] before p1 as indicated:
explicit multiset(const Compare& comp = Compare(),
                  const Allocator& = Allocator());
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());
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());
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());
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());
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());
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());
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());
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());

msg6573 (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.

msg6198 (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());

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.

msg6152 (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.

msg6151 (view)

Date: 2012-10-06.00:00:00

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

msg6150 (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 17:03:20	admin	set	status: wp -> c++14
2014-02-20 13:52:38	admin	set	status: immediate -> wp
2014-02-14 19:48:07	admin	set	messages: + msg6869
2014-02-14 19:48:07	admin	set	status: ewg -> immediate
2014-02-14 09:40:24	admin	set	messages: + msg6861
2013-10-06 21:16:04	admin	set	messages: + msg6676
2013-08-27 19:42:08	admin	set	messages: + msg6573
2012-10-21 18:35:06	admin	set	status: open -> ewg
2012-10-20 00:32:14	admin	set	messages: + msg6198
2012-10-20 00:32:14	admin	set	status: new -> open
2012-10-06 11:55:45	admin	set	messages: + msg6152
2012-10-06 11:55:45	admin	set	messages: + msg6151
2012-10-04 00:00:00	admin	create