Issue 2193: Default constructors for standard library containers are explicit

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2193. Default constructors for standard library containers are explicit

Section: 24 [containers] Status: C++14 Submitter: Richard Smith Opened: 2012-10-04 Last modified: 2017-07-05

Priority: 1

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Discussion:

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());

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

[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.

[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.

[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.

[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Previous resolution from Daniel [SUPERSEDED]: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, 23.4.3 [basic.string] p5 as indicated:
basic_string() : basic_string(Allocator()) {}
explicit basic_string(const Allocator& a = Allocator());
Change 23.4.3.3 [string.cons] before p1 as indicated:
explicit basic_string(const Allocator& a = Allocator());
Change class template deque synopsis, 24.3.8.1 [deque.overview] p2 as indicated:
deque() : deque(Allocator()) {}
explicit deque(const Allocator& = Allocator());
Change 24.3.8.2 [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, 24.3.10.1 [list.overview] p2 as indicated:
list() : list(Allocator()) {}
explicit list(const Allocator& = Allocator());
Change 24.3.10.2 [list.cons] before p1 as indicated:
explicit list(const Allocator& = Allocator());
Change class template vector synopsis, 24.3.11.1 [vector.overview] p2 as indicated:
vector() : vector(Allocator()) {}
explicit vector(const Allocator& = Allocator());
Change 24.3.11.2 [vector.cons] before p1 as indicated:
explicit vector(const Allocator& = Allocator());
Change class template specialization vector<bool> synopsis, 24.3.12 [vector.bool] p1 as indicated:
vector() : vector(Allocator()) {}
explicit vector(const Allocator& = Allocator());
Change class template map synopsis, 24.4.4.1 [map.overview] p2 as indicated:
map() : map(Compare()) {}
explicit map(const Compare& comp = Compare(),
             const Allocator& = Allocator());
Change 24.4.4.2 [map.cons] before p1 as indicated:
explicit map(const Compare& comp = Compare(),
             const Allocator& = Allocator());
Change class template multimap synopsis, 24.4.5.1 [multimap.overview] p2 as indicated:
multimap() : multimap(Compare()) {}
explicit multimap(const Compare& comp = Compare(),
                  const Allocator& = Allocator());
Change 24.4.5.2 [multimap.cons] before p1 as indicated:
explicit multimap(const Compare& comp = Compare(),
                  const Allocator& = Allocator());
Change class template set synopsis, 24.4.6.1 [set.overview] p2 as indicated:
set() : set(Compare()) {}
explicit set(const Compare& comp = Compare(),
             const Allocator& = Allocator());
Change 24.4.6.2 [set.cons] before p1 as indicated:
explicit set(const Compare& comp = Compare(),
             const Allocator& = Allocator());
Change class template multiset synopsis, 24.4.7.1 [multiset.overview] p2 as indicated:
multiset() : multiset(Compare()) {}
explicit multiset(const Compare& comp = Compare(),
                  const Allocator& = Allocator());
Change 24.4.7.2 [multiset.cons] before p1 as indicated:
explicit multiset(const Compare& comp = Compare(),
                  const Allocator& = Allocator());
Change class template unordered_map synopsis, 24.5.4.1 [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 24.5.4.2 [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, 24.5.5.1 [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 24.5.5.2 [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, 24.5.6.1 [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 24.5.6.2 [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, 24.5.7.1 [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 24.5.7.2 [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());

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

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, 23.4.3 [basic.string] p5 as indicated:

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

Change 23.4.3.3 [string.cons] before p1 as indicated:

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

Change class template deque synopsis, 24.3.8.1 [deque.overview] p2 as indicated:

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

Change 24.3.8.2 [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, 24.3.10.1 [list.overview] p2 as indicated:

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

Change 24.3.10.2 [list.cons] before p1 as indicated:
```
explicit list(const Allocator& = Allocator());
```

Change class template vector synopsis, 24.3.11.1 [vector.overview] p2 as indicated:

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

Change 24.3.11.2 [vector.cons] before p1 as indicated:
```
explicit vector(const Allocator& = Allocator());
```
Change class template specialization vector<bool> synopsis, 24.3.12 [vector.bool] p1 as indicated:
```
vector() : vector(Allocator()) { }
explicit vector(const Allocator& = Allocator());
```

Change class template map synopsis, 24.4.4.1 [map.overview] p2 as indicated:

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

Change 24.4.4.2 [map.cons] before p1 as indicated:

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

Change class template multimap synopsis, 24.4.5.1 [multimap.overview] p2 as indicated:

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

Change 24.4.5.2 [multimap.cons] before p1 as indicated:

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

Change class template set synopsis, 24.4.6.1 [set.overview] p2 as indicated:

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

Change 24.4.6.2 [set.cons] before p1 as indicated:

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

Change class template multiset synopsis, 24.4.7.1 [multiset.overview] p2 as indicated:

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

Change 24.4.7.2 [multiset.cons] before p1 as indicated:

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

Change class template unordered_map synopsis, 24.5.4.1 [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 24.5.4.2 [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, 24.5.5.1 [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 24.5.5.2 [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, 24.5.6.1 [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 24.5.6.2 [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, 24.5.7.1 [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 24.5.7.2 [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.