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This page is a snapshot from the LWG issues list, see theLibrary Active Issues List for more information and the meaning ofC++14 status.

2193. Default constructors for standard library containers are explicit

Section: 23[containers]Status:C++14Submitter: Richard SmithOpened: 2012-10-04Last modified: 2017-07-05

Priority:1

View otheractive issues in [containers].

View all otherissues in [containers].

View all issues withC++14 status.

Discussion:

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

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

Note that Clang + libc++ rejects the declaration ofs3 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 theybelieve 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 whenthe 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());

by

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

(and similarly for other container and container-like classes) might introduce a potentialbackwards-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 ofstd::basic_string) will break ifstd::basic_string ctors are modified as proposed by this issue, sincemy_allocator doesn't havea default ctor.

[2013-10-06, Daniel comments:]

Issue2303(i) describes the more general problem related to explicit instantiation requests in the currentlibrary 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.

  1. Change class templatebasic_string synopsis, 27.4.3[basic.string] p5 as indicated:

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

  2. Change 27.4.3.3[string.cons] before p1 as indicated:

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

  3. Change class templatedeque synopsis, 23.3.5.1[deque.overview] p2 as indicated:

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

  4. Change 23.3.5.2[deque.cons] before p1 as indicated:

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

  5. Change class templateforward_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 templatelist synopsis, 23.3.11.1[list.overview] p2 as indicated:

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

  8. Change 23.3.11.2[list.cons] before p1 as indicated:

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

  9. Change class templatevector synopsis, 23.3.13.1[vector.overview] p2 as indicated:

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

  10. Change 23.3.13.2[vector.cons] before p1 as indicated:

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

  11. Change class template specializationvector<bool> synopsis, 23.3.14[vector.bool] p1 as indicated:

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

  12. Change class templatemap synopsis, 23.4.3.1[map.overview] p2 as indicated:

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

  13. Change 23.4.3.2[map.cons] before p1 as indicated:

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

  14. Change class templatemultimap synopsis, 23.4.4.1[multimap.overview] p2 as indicated:

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

  15. Change 23.4.4.2[multimap.cons] before p1 as indicated:

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

  16. Change class templateset synopsis, 23.4.6.1[set.overview] p2 as indicated:

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

  17. Change 23.4.6.2[set.cons] before p1 as indicated:

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

  18. Change class templatemultiset synopsis, 23.4.7.1[multiset.overview] p2 as indicated:

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

  19. Change 23.4.7.2[multiset.cons] before p1 as indicated:

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

  20. Change class templateunordered_map synopsis, 23.5.3.1[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());

  21. Change 23.5.3.2[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());

  22. Change class templateunordered_multimap synopsis, 23.5.4.1[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());

  23. Change 23.5.4.2[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());

  24. Change class templateunordered_set synopsis, 23.5.6.1[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());

  25. Change 23.5.6.2[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());

  26. Change class templateunordered_multiset synopsis, 23.5.7.1[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());

  27. Change 23.5.7.2[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.

  1. Change class templatebasic_string synopsis, 27.4.3[basic.string] p5 as indicated:

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

  2. Change 27.4.3.3[string.cons] before p1 as indicated:

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

  3. Change class templatedeque synopsis, 23.3.5.1[deque.overview] p2 as indicated:

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

  4. Change 23.3.5.2[deque.cons] before p1 as indicated:

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

  5. Change class templateforward_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 templatelist synopsis, 23.3.11.1[list.overview] p2 as indicated:

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

  8. Change 23.3.11.2[list.cons] before p1 as indicated:

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

  9. Change class templatevector synopsis, 23.3.13.1[vector.overview] p2 as indicated:

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

  10. Change 23.3.13.2[vector.cons] before p1 as indicated:

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

  11. Change class template specializationvector<bool> synopsis, 23.3.14[vector.bool] p1 as indicated:

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

  12. Change class templatemap synopsis, 23.4.3.1[map.overview] p2 as indicated:

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

  13. Change 23.4.3.2[map.cons] before p1 as indicated:

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

  14. Change class templatemultimap synopsis, 23.4.4.1[multimap.overview] p2 as indicated:

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

  15. Change 23.4.4.2[multimap.cons] before p1 as indicated:

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

  16. Change class templateset synopsis, 23.4.6.1[set.overview] p2 as indicated:

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

  17. Change 23.4.6.2[set.cons] before p1 as indicated:

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

  18. Change class templatemultiset synopsis, 23.4.7.1[multiset.overview] p2 as indicated:

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

  19. Change 23.4.7.2[multiset.cons] before p1 as indicated:

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

  20. Change class templateunordered_map synopsis, 23.5.3.1[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());

  21. Change 23.5.3.2[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 emptyunordered_map using the specified hash function, key equality func-
    tion, and allocator, and using at leastn buckets.Ifn is not provided,For the default constructor
    the number of buckets is implementation-defined.max_load_factor() returns 1.0.

  22. Change class templateunordered_multimap synopsis, 23.5.4.1[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());

  23. Change 23.5.4.2[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 emptyunordered_multimap using the specified hash function, key equality
    function, and allocator, and using at leastn buckets.Ifn is not provided,For the default constructor
    the number of buckets is implementation-defined.max_load_factor() returns 1.0.

  24. Change class templateunordered_set synopsis, 23.5.6.1[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());

  25. Change 23.5.6.2[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 emptyunordered_set using the specified hash function, key equality func-
    tion, and allocator, and using at leastn buckets.Ifn is not provided,For the default constructor
    the number of buckets is implementation-defined.max_load_factor() returns 1.0.

  26. Change class templateunordered_multiset synopsis, 23.5.7.1[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());

  27. Change 23.5.7.2[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 emptyunordered_multiset using the specified hash function, key equality
    function, and allocator, and using at leastn buckets.Ifn is not provided,For the default constructor
    the number of buckets is implementation-defined.max_load_factor() returns 1.0.
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