2510. Tag types should not be DefaultConstructible

Section: 17.6 [support.dynamic], 22.2 [utility], 22.3.5 [pair.piecewise], 20.2.2 [memory.syn], 20.2.7 [allocator.tag], 32.6 [thread.mutex] Status: C++17 Submitter: Ville Voutilainen Opened: 2015-06-13 Last modified: 2017-07-30

Priority: 2

View all other issues in [support.dynamic].

View all issues with C++17 status.

Discussion:

std::experimental::optional, for certain reasons, specifies its nullopt type to not be DefaultConstructible. It doesn't do so for its tag type in_place_t and neither does the standard proper for any of its tag types. That turns out to be very unfortunate, consider the following: 

#include <memory>
#include <array>

void f(std::array<int, 1>, int) {} // #1
void f(std::allocator_arg_t, int) {} // #2

int main()
{
  f({}, 666); // #3
}

The call at #3 is ambiguous. What's even worse is that if the overload #1 is removed, the call works just fine. The whole point of a tag type is that it either needs to mentioned in a call or it needs to be a forwarded argument, so being able to construct a tag type like that makes no sense.

Making the types have an explicit default constructor might have helped, but CWG 1518 is going against that idea.

[optional.nullopt]/3 solves this problem for nullopt:

Type nullopt_t shall not have a default constructor. It shall be a literal type. Constant nullopt shall be initialized with an argument of literal type.

[2015-06, Telecon]

Move to Tentatively Ready.

[2015-10, Kona Saturday afternoon]

Move back to Open

JW: The linked Core issue (CWG 1518) gives us a better tool to solve this (explicit default constructors). [The CWG Issue means that an explicit default constructor will no longer match "{}".] JW explains that it's important that tag types cannot be constructed from "{}" (e.g. the allocator tag in the tuple constructors).

WEB: Should we now go back and update our constructors? JW: For tag types, yes.

VV: The guideline is that anything that does not mention the type name explicitly should not invoke an explicit constructor.

Ville will provide wording.

Discussion about pair/tuple's default constructor - should they now be explicit?

[2016-01-31]

Ville provides revised wording.

Previous resolution [SUPERSEDED]:

This wording is relative to N4527.

  1. In 17.6 [support.dynamic]/1, change the header <new> synopsis:

    […]
struct nothrow_t {}; see below
extern const nothrow_t nothrow;
[…]

  2. Add a new paragraph after 17.6 [support.dynamic]/1 (following the header <new> synopsis):

    -?- Type nothrow_t shall not have a default constructor.

  3. In 22.2 [utility]/2, change the header <utility> synopsis:

    […]
// 20.3.5, pair piecewise construction
struct piecewise_construct_t { }; see below
constexpr piecewise_construct_t piecewise_construct{ unspecified };
[…]

  4. Add a new paragraph after 22.2 [utility]/2 (following the header <utility> synopsis):

    -?- Type piecewise_construct_t shall not have a default constructor. It shall be a literal type. Constant piecewise_construct shall be initialized with an argument of literal type.

  5. In 22.3.5 [pair.piecewise], apply the following edits:

    struct piecewise_construct_t { };
constexpr piecewise_construct_t piecewise_construct{ unspecified };

  6. In 20.2.2 [memory.syn]/1, change the header <memory> synopsis:

    […]
// 20.7.6, allocator argument tag
struct allocator_arg_t { }; see below
constexpr allocator_arg_t allocator_arg{ unspecified };
[…]

  7. Add a new paragraph after 20.2.2 [memory.syn]/1 (following the header <memory> synopsis):

    -?- Type allocator_arg_t shall not have a default constructor. It shall be a literal type. Constant allocator_arg shall be initialized with an argument of literal type.

  8. In 20.2.7 [allocator.tag], apply the following edits:

    namespace std {
  struct allocator_arg_t { };
  constexpr allocator_arg_t allocator_arg{ unspecified };
}

    Editorial drive-by: piecewise_construct_t is written, in 22.3.5 [pair.piecewise] like 

    struct piecewise_construct_t { };
constexpr piecewise_construct_t piecewise_construct{};

    whereas other tag types such as allocator_construct_t are, in e.g. 20.2.7 [allocator.tag], written like 

    namespace std {
  struct allocator_arg_t { };
  constexpr allocator_arg_t allocator_arg{};
}

    We should decide whether or not to write out the std namespace in such paragraphs. I would suggest not to write it out.

  9. In 32.6 [thread.mutex]/1, change the header <mutex> synopsis:

    […]
struct defer_lock_t { }; see below
struct try_to_lock_t { }; see below
struct adopt_lock_t { }; see below

constexpr defer_lock_t defer_lock { unspecified  };
constexpr try_to_lock_t try_to_lock { unspecified  };
constexpr adopt_lock_t adopt_lock { unspecified  };
[…]

  10. Add three new paragraphs after [thread.mutex]/1 (following the header <mutex> synopsis):

    -?- Type defer_lock_t shall not have a default constructor. It shall be a literal type. Constant defer_lock shall be initialized with an argument of literal type.

    -?- Type try_to_lock_t shall not have a default constructor. It shall be a literal type. Constant try_to_lock shall be initialized with an argument of literal type.

    -?- Type adopt_lock_t shall not have a default constructor. It shall be a literal type. Constant adopt_lock shall be initialized with an argument of literal type.

[2016-03 Jacksonville]

AM: should have note about compatibility in Annex C
HH: like this idiom well enough that I've started using it in my own code
AM: why are pair and tuple involved here?
GR: they are the only types which forward explicitness with EXPLICIT
AM: British spelling of behaviour
AM: happy to drop my issue about Annex C

[2016-06 Oulu]

This is waiting on Core issue 1518

Saturday: Core 1518 was resolved in Oulu

[2016-07 Chicago]

This is related to 2736

Monday PM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4567.

  1. In 17.6 [support.dynamic]/1, change the header <new> synopsis:

    […]
struct nothrow_t { explicit nothrow_t() = default; };
extern const nothrow_t nothrow;
[…]

  2. In 22.2 [utility]/2, change the header <utility> synopsis:

    […]
// 20.3.5, pair piecewise construction
struct piecewise_construct_t { explicit piecewise_construct_t() = default; };
constexpr piecewise_construct_t piecewise_construct{};
[…]

  3. In 22.3.2 [pairs.pair], change the class template pair synopsis:

    […]
pair(pair&&) = default;
EXPLICIT constexpr pair();
EXPLICIT constexpr pair(const T1& x, const T2& y);
[…]

  4. Around 22.3.2 [pairs.pair] p3, apply the following edits:

    EXPLICIT constexpr pair();

    -3- Effects: Value-initializes first and second.

    -4- Remarks: This constructor shall not participate in overload resolution unless is_default_constructible<first_type>::value is true and is_default_constructible<second_type>::value is true. [Note: This behaviour can be implemented by a constructor template with default template arguments. — end note] The constructor is explicit if and only if either first_type or second_type is not implicitly default-constructible. [Note: This behaviour can be implemented with a trait that checks whether a const first_type& or a const second_type& can be initialized with {}. — end note]

  5. In 22.3.5 [pair.piecewise], apply the following edits:

    struct piecewise_construct_t { explicit piecewise_construct_t() = default; };
constexpr piecewise_construct_t piecewise_construct{};

  6. In 22.4.4 [tuple.tuple], change the class template tuple synopsis:

    […]
// 20.4.2.1, tuple construction
EXPLICIT constexpr tuple();
EXPLICIT constexpr tuple(const Types&...); // only if sizeof...(Types) >= 1
[…]

  7. Around 22.4.4.2 [tuple.cnstr] p4, apply the following edits:

    EXPLICIT constexpr tuple();

    -4- Effects: Value initializes each element.

    -5- Remarks: This constructor shall not participate in overload resolution unless is_default_constructible<Ti>::value is true for all i. [Note: This behaviour can be implemented by a constructor template with default template arguments. — end note] The constructor is explicit if and only if Ti is not implicitly default-constructible for at least one i. [Note: This behaviour can be implemented with a trait that checks whether a const Ti& can be initialized with {}. — end note]

  8. In 20.2.2 [memory.syn]/1, change the header <memory> synopsis:

    […]
// 20.7.6, allocator argument tag
struct allocator_arg_t { explicit allocator_arg_t() = default; };
constexpr allocator_arg_t allocator_arg{};
[…]

  9. In 20.2.7 [allocator.tag], apply the following edits:

    namespace std {
  struct allocator_arg_t { explicit allocator_arg_t() = default; };
  constexpr allocator_arg_t allocator_arg{};
}

    Editorial drive-by: piecewise_construct_t is written, in 22.3.5 [pair.piecewise] like 

    struct piecewise_construct_t { };
constexpr piecewise_construct_t piecewise_construct{};

    whereas other tag types such as allocator_construct_t are, in e.g. 20.2.7 [allocator.tag], written like 

    namespace std {
  struct allocator_arg_t { };
  constexpr allocator_arg_t allocator_arg{};
}

    We should decide whether or not to write out the std namespace in such paragraphs. I would suggest not to write it out.

  10. In 32.6 [thread.mutex]/1, change the header <mutex> synopsis:

    […]
struct defer_lock_t { explicit defer_lock_t() = default; };
struct try_to_lock_t { explicit try_to_lock_t() = default; };
struct adopt_lock_t { explicit adopt_lock_t() = default; };

constexpr defer_lock_t defer_lock { };
constexpr try_to_lock_t try_to_lock { };
constexpr adopt_lock_t adopt_lock { };
[…]