P3016R2
Resolve inconsistencies in begin/end for valarray and braced initializer lists

1. Changelog

• R2:

  • Give resolutions for LWG3624 and LWG3625

  • Rebut concerns about breaking code; discuss how a deprecation period for std::begin(initializer_list) is basically unimplementable

  • Make valarray::iterator non-exposition-only

  • Remove R1’s Historical Background section, as basically distracting at this point

  • Mention [P2613] and [LWG4035] as further precedent for initializer_list.empty()

  • Add feature-test macros __cpp_lib_initializer_list and __cpp_lib_valarray

• R1:

  • Split [CWG2825] from this paper; this paper is now library-only

  • Add discussion of .data() and .empty() overloads

  • Add wording for ".data+.size cleanup"

2. Motivation and proposal

Casey Carter points out that the following program is supported by libstdc++ but not libc++ nor Microsoft (Godbolt):

#include <iterator>
#include <valarray>
int main() {
  std::valarray<int> v = {1,2,3};
  std::begin(v); // OK
  std::cbegin(v); // Error
}

This is because std::valarray defines its own non-member, non-hidden-friend overloads of std::begin and std::end. These overloads are found by the qualified call to std::begin here, but aren’t found by std::cbegin’s ADL because the primary template for std::cbegin happens to be defined before <valarray> is included. Swapping the order of #include <iterator> and #include <valarray> in this example doesn’t help, because the relevant parts of <iterator> are still transitively included by <valarray> before std::valarray’s own code.

Likewise, on all vendors (Godbolt):

#include <iterator>
int main() {
  std::begin({1,2,3}); // OK
  std::cbegin({1,2,3}); // Error
}

This is because {1,2,3} is a braced-initializer-list with no type; so it cannot bind to the deduced const C& in std::cbegin (defined in <iterator>). But it can bind to the initializer_list<E> in the non-member, non-hidden-friend overload std::begin(std::initializer_list<E>) (defined in <initializer_list>).

Notice that std::begin({1,2,3}) returns an iterator that will dangle at the end of the full-expression, and that the return values of std::begin({1,2,3}) and std::end({1,2,3}) do not form a range (because the two lists' backing arrays may be different). Therefore this overload is more harmful than helpful.

Note: Be careful to distinguish the scenario of calling std::begin on an object of type initializer_list (helpful!) from calling it on a braced-initializer-list (harmful).

We propose to resolve valarray’s begin/cbegin inconsistency in favor of "make it work" and to resolve braced-initializer-list’s begin/cbegin inconsistency in favor of "make it ill-formed."

2.1. data and empty

We also propose two more member functions for std::initializer_list: .data() and .empty().

Many places in the library clauses would like to operate on the contiguous data of an initializer_list using the "data + size" idiom, but since initializer_list lacks .data(), they’re forced to use an awkward "begin + size" approach instead. This (1) looks unnatural and (2) causes extra mental effort for library writers. Part of P3016’s proposed wording is to update these places in the library. For example:

constexpr basic_string& append(const basic_string& str);

1. Effects: Equivalent to return append(str.data(), str.size());

[...]

constexpr basic_string& append(initializer_list<charT> il);

16. Effects: Equivalent to return append(il.data(), il.size() il.begin(), il.size());

As for .empty(), it is generally recognized these days that ranges providing .size() should also provide .empty(). For example, iota_view’s missing .empty() was added as a DR by [LWG4001]. [LWG4035] adds single_view’s missing .empty(). [P2613], which added mdspan’s missing .empty(), claims that initializer_list and bitset remain the only two types with .size() and without .empty().

By making il.data() and il.empty() well-formed for std::initializer_list objects, we satisfy the SFINAE conditions of the primary templates for std::data(il) and std::empty(il), meaning that we can eliminate their special overloads for initializer_list arguments.

std::initializer_list<int> il = {1,2,3};
const int *p = std::data(il);
  // OK, calls the initializer_list overload
  // specially provided in <iterator>
bool b = std::empty(il);
  // OK, calls the initializer_list overload
  // specially provided in <iterator>

std::initializer_list<int> il = {1,2,3};
const int *p = std::data(il);
  // OK, calls the primary template
bool b = std::empty(il);
  // OK, calls the primary template

const int *p = std::data({1,2,3});
  // calls the initializer_list overload
  // specially provided in <iterator>,
  // and immediately dangles
const int *q = std::data({});
  // does not compile

const int *p = std::data({1,2,3});
  // does not compile
const int *q = std::data({});
  // does not compile

bool b = std::empty({1,2,3});
  // calls the initializer_list overload
  // specially provided in <iterator>,
  // invariably returns false
bool c = std::empty({});
  // does not compile

bool b = std::empty({1,2,3});
  // does not compile
bool c = std::empty({});
  // does not compile

2.2. Availability of begin/end

The following snippet tries to use std::begin(il) without first including <iterator> nor any of the 15 other headers listed in [iterator.range]/1 that declare the generic std::begin. This code works in C++23.

We propose simply to break the left-hand snippet, and force the programmer to write the right-hand snippet instead. One LEWG reviewer expressed concern that we were breaking valid code without a deprecation period.

#include <initializer_list>
std::initializer_list<int> il = {1,2,3};
const int *b = std::begin(il);
  // OK, calls the initializer_list overload
  // specially provided in <initializer_list>
const auto r = std::rbegin(il);
  // Error, not declared in <initializer_list>
const int *c = std::cbegin(il);
  // Error, not declared in <initializer_list>
size_t s = std::size(il);
  // Error, not declared in <initializer_list>

#include <iterator>
std::initializer_list<int> il = {1,2,3};
const int *b = std::begin(il);
  // OK, calls the primary template
const auto r = std::rbegin(il);
  // OK, calls the initializer_list overload
  // specially provided in <iterator>
const int *c = std::cbegin(il);
  // OK, calls the primary template
size_t s = std::size(il);
  // OK, calls the primary template

At first, I was amenable to the idea that the declarations of std::begin and std::end in <initializer_list> should merely be deprecated, not fully removed in C++26, so that the behavior of the code on the left would be (temporarily) preserved. To do that, we’d modify [iterator.range] like this:

template<class C> constexpr auto begin(C& c) -> decltype(c.begin());
template<class C> constexpr auto begin(const C& c) -> decltype(c.begin());

2. Returns: c.begin().

x. Remarks: In addition to being available via inclusion of the <iterator> header, these function templates are available when <initializer_list> is included. This availability is deprecated.

But library vendors can’t implement that wording! We have [[deprecated]] to mark an entity as deprecated, but we have no way to mark a single declaration as deprecated. (I.e., "If, hypothetically, removing this declaration would make the call ill-formed, then give a deprecation warning; otherwise don’t.") Three years from now, we’d come back asking "Can we remove this deprecated feature yet?" and the answer would be "No, vendors haven’t started giving a warning for it yet," because it’s physically impossible to give a warning for it. This would be a silly situation to get into. And the feature itself is so obscure (using std::begin specifically on an initializer_list without including any of more than a dozen STL headers) and so specific (as shown, it already doesn’t work for cbegin, rbegin, or size), and the fix in code is so surgical (include <iterator> or <span>) that I don’t think the lack of deprecation period will matter.

[LWG3624] and [LWG3625] somewhat intersect this proposal, but do not conflict with it. We expect these issues to be resolved along the lines given in § 4.9 Resolutions for LWG3624 and LWG3625.

3. Implementation experience

Arthur has implemented § 4 Proposed wording in his fork of libc++ (source), and used it to compile both LLVM/Clang/libc++ and another large C++17 codebase. Naturally, it caused no problems except in this single test from libc++'s own test suite:

#include <initializer_list> // but not <iterator>
std::initializer_list<int> il;
static_assert(noexcept(std::begin(il)));

This test now fails first because <iterator> was not included, and second because today’s begin(initializer_list<E>) is noexcept but the primary template begin(C&) is non-noexcept (per P0884 guidance).

il.begin() and ranges::begin(il) remain noexcept.

std::begin(valarray) remains non-noexcept.

3.1. Tony Table

#include <initializer_list>
void f(std::initializer_list<int> il) {
  auto it = std::begin(il);
}

#include <initializer_list>
#include <iterator> // for std::begin
void f(std::initializer_list<int> il) {
  auto it = std::begin(il);
}

S(std::initializer_list<int> il) :
  S(il.begin(), il.size()) {}

S(std::initializer_list<int> il) :
  S(il.data(), il.size()) {}

auto dangle = std::begin({1,2,3});
  // calls the initializer_list overload
  // specially provided in <initializer_list>,
  // and immediately dangles

// no longer compiles

bool b = std::empty({1,2,3});
  // calls the initializer_list overload
  // specially provided in <iterator>,
  // yielding false

// no longer compiles

#include <valarray>
#include <utility>
std::valarray<int> va;
auto it = std::begin(std::as_const(va));

#include <valarray>
#include <iterator>
std::valarray<int> va;
auto it = std::cbegin(va);

4. Proposed wording

Note: Vendors should provide __cpp_lib_initializer_list if il.data() and il.empty() are well-formed. Vendors should provide __cpp_lib_valarray if va.begin() and va.end() are well-formed and the free-function begin and end have been removed from <valarray>.

4.1. [version.syn]

Add two feature-test macros to [version.syn]/2:

#define __cpp_lib_incomplete_container_elements     201505L // also in <forward_list>, <list>, <vector>
#define __cpp_lib_initializer_list                  YYYYMML // also in <initializer_list>
#define __cpp_lib_int_pow2                          202002L // freestanding, also in <bit>
[...]
#define __cpp_lib_unwrap_ref                        201811L // freestanding, also in <type_traits>
#define __cpp_lib_valarray                          YYYYMML // also in <valarray>
#define __cpp_lib_variant                           202306L // also in <variant>

4.2. [valarray.syn]

Modify [valarray.syn] as follows:

[...]

  template<class T> valarray<T> tan  (const valarray<T>&);
  template<class T> valarray<T> tanh (const valarray<T>&);

  template<class T> unspecified1 begin(valarray<T>& v);
  template<class T> unspecified2 begin(const valarray<T>& v);
  template<class T> unspecified1 end(valarray<T>& v);
  template<class T> unspecified2 end(const valarray<T>& v);
}

[...]

3․ Any function returning a valarray<T> is permitted to return an object of another type, provided all the const member functions of valarray<T> other than begin and end are also applicable to this type. This return type shall not add more than two levels of template nesting over the most deeply nested argument type.

4․ Implementations introducing such replacement types shall provide additional functions and operators as follows:

• (4.1) for every function taking a const valarray<T>& other than begin and end, identical functions taking the replacement types shall be added;

• (4.2) for every function taking two const valarray<T>& arguments, identical functions taking every combination of const valarray<T>& and replacement types shall be added.

5․ In particular, an implementation shall allow a valarray<T> to be constructed from such replacement types and shall allow assignments and compound assignments of such types to valarray<T>, slice_array<T>, gslice_array<T>, mask_array<T> and indirect_array<T> objects.

[...]

4.3. [template.valarray.overview]

Note: R1 proposed the iterator and const_iterator typedefs as exposition-only, but since LEWG didn’t seem to object, R2 makes them non-exposition-only.

Note: We propose that valarray’s .begin() should be non-noexcept, for consistency with .size(). Adding noexcept consistently throughout <valarray> would be cool, but is out of scope.

Modify [template.valarray.overview] as follows:

namespace std {
  template<class T> class valarray {
  public:
    using value_type = T;
    using iterator = unspecified;
    using const_iterator = unspecified;

    // [valarray.cons], construct/destroy
    valarray();
    explicit valarray(size_t);

[...]

  // [valarray.range], range access

  iterator begin();
  iterator end();
  const_iterator begin() const;
  const_iterator end() const;

  // [valarray.members], member functions
  void swap(valarray&) noexcept;

  size_t size() const;

  T sum() const;
  T min() const;
  T max() const;

  valarray shift (int) const;
  valarray cshift(int) const;
  valarray apply(T func(T)) const;
  valarray apply(T func(const T&)) const;
  void resize(size_t sz, T c = T());
};

4.4. [valarray.members]

Move the existing section [valarray.range] from its current location to make it a sibling of [valarray.members]; then modify it as follows:

28.6.10 28.6.2.x valarray range access [valarray.range]

1․ In the begin and end function templates that follow, unspecified1 is a type that The iterator type meets the requirements of a mutable Cpp17RandomAccessIterator ([random.access.iterators]) and models contiguous_iterator ([iterator.concept.contiguous]), whose . Its value_type is the template parameter T and whose its reference type is T&. unspecified2 is a type that The const_iterator type meets the requirements of a constant Cpp17RandomAccessIterator and models contiguous_iterator, whose . Its value_type is the template parameter T and whose its reference type is const T&.

2․ The iterators returned by begin and end for an array are guaranteed to be valid until the member function resize(size_t, T) is called for that array or until the lifetime of that array ends, whichever happens first.

template<class T> unspecified1 begin(valarray<T>& v);
template<class T> unspecified2 begin(const valarray<T>& v);
iterator begin();
const_iterator begin() const;

3․ Returns: An iterator referencing the first value in the array.

template<class T> unspecified1 end(valarray<T>& v);
template<class T> unspecified2 end(const valarray<T>& v);
iterator end();
const_iterator end() const;

4․ Returns: An iterator referencing one past the last value in the array.

28.6.2.8 Member functions [valarray.members]

void swap(valarray& v) noexcept;

1․ Effects: *this obtains the value of v. v obtains the value of *this.

2․ Complexity: Constant.

4.5. [support.initlist]

Modify [support.initlist] as follows:

[...]

17.10.2 Header <initializer_list> synopsis [initializer.list.syn]

namespace std {
  template<class E> class initializer_list {
  public:
    using value_type      = E;
    using reference       = const E&;
    using const_reference = const E&;
    using size_type       = size_t;

    using iterator        = const E*;
    using const_iterator  = const E*;

    constexpr initializer_list() noexcept;

    constexpr const E* data() const noexcept;
    constexpr size_t size() const noexcept;     // number of elements
    [[nodiscard]] constexpr bool empty() const noexcept;
    constexpr const E* begin() const noexcept;  // first element
    constexpr const E* end() const noexcept;    // one past the last element
  };

  // [support.initlist.range], initializer list range access
  template<class E> constexpr const E* begin(initializer_list<E> il) noexcept;
  template<class E> constexpr const E* end(initializer_list<E> il) noexcept;
}

1․ An object of type initializer_list<E> provides access to an array of objects of type const E.

[Note: A pair of pointers or a pointer plus a length would be obvious representations for initializer_list. initializer_list is used to implement initializer lists as specified in [dcl.init.list]. Copying an initializer_list does not copy the underlying elements. — end note]

2․ If an explicit specialization or partial specialization of initializer_list is declared, the program is ill-formed.

17.10.3 Initializer list constructors [support.initlist.cons]

constexpr initializer_list() noexcept;

1․ Postconditions: size() == 0.

17.10.4 Initializer list access [support.initlist.access]

constexpr const E* begin() const noexcept;

1․ Returns: A pointer to the beginning of the array. If size() == 0 the values of begin() and end() are unspecified but they shall be identical.

constexpr const E* end() const noexcept;

2․ Returns: begin() + size().

constexpr const E* data() const noexcept;

x․ Returns: begin().

constexpr size_t size() const noexcept;

3․ Returns: The number of elements in the array.

4․ Complexity: Constant time.

[[nodiscard]] constexpr bool empty() const noexcept;

x․ Returns: size() == 0.

17.10.5 Initializer list range access [support.initlist.range]

template<class E> constexpr const E* begin(initializer_list<E> il) noexcept;

1․ Returns: il.begin().

template<class E> constexpr const E* end(initializer_list<E> il) noexcept;

2․ Returns: il.end().

4.6. [iterator.synopsis]

Modify [iterator.synopsis] as follows:

25.2 Header synopsis [iterator.synopsis]

#include <compare>              // see [compare.syn]
#include <concepts>             // see [concepts.syn]
#include <initializer_list>     // see [initializer.list.syn]

namespace std {

[...]

  // [iterator.range], range access
  template<class C> constexpr auto begin(C& c) -> decltype(c.begin());
  template<class C> constexpr auto begin(const C& c) -> decltype(c.begin());
  template<class C> constexpr auto end(C& c) -> decltype(c.end());
  template<class C> constexpr auto end(const C& c) -> decltype(c.end());
  template<class T, size_t N> constexpr T* begin(T (&array)[N]) noexcept;
  template<class T, size_t N> constexpr T* end(T (&array)[N]) noexcept;
  template<class C> constexpr auto cbegin(const C& c)
    noexcept(noexcept(std::begin(c))) -> decltype(std::begin(c));
  template<class C> constexpr auto cend(const C& c)
    noexcept(noexcept(std::end(c))) -> decltype(std::end(c));
  template<class C> constexpr auto rbegin(C& c) -> decltype(c.rbegin());
  template<class C> constexpr auto rbegin(const C& c) -> decltype(c.rbegin());
  template<class C> constexpr auto rend(C& c) -> decltype(c.rend());
  template<class C> constexpr auto rend(const C& c) -> decltype(c.rend());
  template<class T, size_t N> constexpr reverse_iterator<T*> rbegin(T (&array)[N])
  template<class T, size_t N> constexpr reverse_iterator<T*> rend(T (&array)[N]);
  template<class E> constexpr reverse_iterator<const E*>
    rbegin(initializer_list<E> il);
  template<class E> constexpr reverse_iterator<const E*>
    rend(initializer_list<E> il);
  template<class C> constexpr auto
    crbegin(const C& c) -> decltype(std::rbegin(c));
  template<class C> constexpr auto
    crend(const C& c) -> decltype(std::rend(c));

  template<class C> constexpr auto
    size(const C& c) -> decltype(c.size());
  template<class T, size_t N> constexpr size_t
    size(const T (&array)[N]) noexcept;

  template<class C> constexpr auto
    ssize(const C& c)
      -> common_type_t<ptrdiff_t, make_signed_t<decltype(c.size())>>;
  template<class T, ptrdiff_t N> constexpr ptrdiff_t
    ssize(const T (&array)[N]) noexcept;

  template<class C> [[nodiscard]] constexpr auto
    empty(const C& c) -> decltype(c.empty());
  template<class T, size_t N> [[nodiscard]] constexpr bool
    empty(const T (&array)[N]) noexcept;
  template<class E> [[nodiscard]] constexpr bool
    empty(initializer_list<E> il) noexcept;

  template<class C> constexpr auto data(C& c) -> decltype(c.data());
  template<class C> constexpr auto data(const C& c) -> decltype(c.data());
  template<class T, size_t N> constexpr T* data(T (&array)[N]) noexcept;
  template<class E> constexpr const E* data(initializer_list<E> il) noexcept;
}

4.7. [iterator.range]

Modify [iterator.range] as follows:

1. In addition to being available via inclusion of the <iterator> header, the function templates in [iterator.range] are available when any of the following headers are included: <array>, <deque>, <flat_map>, <flat_set>, <forward_list>, <list>, <map>, <regex>, <set>, <span>, <string>, <string_view>, <unordered_map>, <unordered_set>, <valarray>, and <vector>.

[...]

template<class E> [[nodiscard]] constexpr bool empty(initializer_list<E> il) noexcept;

22․ Returns: il.size() == 0.

[...]

template<class E> constexpr const E* data(initializer_list<E> il) noexcept;

25․ Returns: il.begin().

4.8. .data+.size cleanup

4.8.1. [string.cons]

Modify [string.cons] as follows:

constexpr basic_string& operator=(initializer_list<charT> il);

36․ Effects: Equivalent to:

return *this = basic_string_view<charT, traits>(il.begin() il.data(), il.size());

4.8.2. [string.append]

Modify [string.append] as follows:

constexpr basic_string& append(initializer_list<charT> il);

16․ Effects: Equivalent to: return append(il.begin() il.data(), il.size());

4.8.3. [string.assign]

Modify [string.assign] as follows:

constexpr basic_string& assign(initializer_list<charT> il);

12․ Effects: Equivalent to: return assign(il.begin() il.data(), il.size());

4.8.4. [string.replace]

Modify [string.replace] as follows:

constexpr basic_string& replace(const_iterator i1, const_iterator i2, initializer_list<charT> il);

12․ Effects: Equivalent to: return replace(i1, i2, il.begin() il.data(), il.size());

4.8.5. [span.cons]

Modify [span.cons] as follows:

constexpr explicit(extent != dynamic_extent) span(std::initializer_list il);

18․ Constraints: is_const_v<element_type> is true.

19․ Preconditions: If extent is not equal to dynamic_extent, then il.size() is equal to extent.

20․ Effects: Initializes data_ with il.begin() il.data() and size_ with il.size().

4.8.6. [valarray.cons]

Modify [valarray.cons] as follows:

valarray(initializer_list<T> il);

9․ Effects: Equivalent to valarray(il.begin() il.data(), il.size()).

4.9. Resolutions for LWG3624 and LWG3625

Modify [iterator.range] as follows:

1. In addition to being available via inclusion of the <iterator> header, the function templates in [iterator.range] are available when any of the following headers are included: <array>, <deque>, <flat_map>, <flat_set>, <forward_list>, <list>, <map>, <regex>, <set>, <span>, <stacktrace>, <string>, <string_view>, <unordered_map>, <unordered_set>, <valarray>, and <vector>.

Modify [iterator.synopsis] as follows:

#include <compare>              // see [compare.syn]
#include <concepts>             // see [concepts.syn]
#include <initializer_list>     // see [initializer.list.syn]

Modify [any.synop] as follows (since [any.class] requires both std::initializer_list and std::type_info):

#include <initializer_list>     // see [initializer.list.syn]
#include <typeinfo>             // see [typeinfo.syn]

namespace std {

Modify [functional.syn] as follows (since [func.wrap.move] requires std::initializer_list and [func.wrap.func] requires std::type_info):

#include <initializer_list>     // see [initializer.list.syn]
#include <typeinfo>             // see [typeinfo.syn]

namespace std {

Contra LWG3624, do not modify [type.index.synopsis]; its only use of std::type_info is exposition-only.

Contra LWG3624, do not modify [stacktrace.syn]. It uses std::initializer_list indirectly, via [iterator.range]/1 —​but it uses std::reverse_iterator indirectly in the same way, and we certainly don’t expect it to include <iterator>! The exact mechanism by which the library vendor satisfies [dcl.init.list]/2 should be left unspecified in this case.