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2206. Inaccuracy in initializer_list constructor requirements

Section: 23.2.4 [sequence.reqmts], 23.2.7 [associative.reqmts], 23.2.8 [unord.req], 29.5.3.2 [rand.req.seedseq] Status: Open Submitter: Jeffrey Yasskin Opened: 2012-10-21 Last modified: 2020-09-06

Priority: 3

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Discussion:

In 23.2.4 [sequence.reqmts] p3, we have "il designates an object of type initializer_list<value_type>", and then several functions that take 'il' as an argument. However, an expression like {1, 2, 'a'} is not an object of type initializer_list<int> unless it's used to initialize an explicitly-typed variable of that type. I believe we want:

std::vector<int> v;
v = {1, 2, 'a'};

to compile portably, so we should say something different when defining 'il'. The same phrasing happens in 23.2.7 [associative.reqmts], 23.2.8 [unord.req], and 29.5.3.2 [rand.req.seedseq].

This may just be an editorial issue because the actual class synopses declare the functions to take initializer_list<exact_type>.

[2013-03-15 Issues Teleconference]

Moved to Open.

This is definitely not NAD

Should copy the suggested wording as the proposed resolution.

[2019-03-26; Daniel comments and provides wording]

The 2013-03-15 comment is confusing, since it recommends to "copy the suggested wording as the proposed resolution". I couldn't find such wording in the issue nor in the associated wiki, so I provided that wording out of myself. The tricky part is to define which kind of braced-init-list we want to allow. As Tim Song pointed out, we still need the existing support for std::initializer_list<value_type> as well, because otherwise existing semantics based on expressions such as li.begin() won't work anymore. The below suggested wording restricts supported braced-init-lists to every initializer list that can be used to copy-list-initialize an object of type std::initializer_list<value_type> by saying:

"bil designates any braced-init-list suitable to copy-list-initialize an object of type initializer_list<value_type> (9.4.5 [dcl.init.list])"

As a drive-by fix, the provided wording adds another initialization "expression" that makes the construction of the form

std::vector<int> v = {1, 2, 'a'};

valid (We just miss a copy-initialization case).

Proposed resolution:

This wording is relative to N4810.

[Drafting note: We need to special-case the "expression" X u = bil; below, because for empty braced-init-list the effects are those of calling the default constructor. — end drafting note]

1. Modify 23.2.4 [sequence.reqmts] as indicated:

   -3- In Tables 66 and 67, […] il designates an objectvalue of type initializer_list<value_type>, bil designates any braced-init-list suitable to copy-list-initialize an object of type initializer_list<value_type> (9.4.5 [dcl.init.list]), […]

2. Modify Table 66 — "Sequence container requirements (in addition to container)" as indicated:

   Table 66 — Sequence container requirements (in addition to container)

   Expression	Return type	Assertion/note pre-/post-condition
   […]
   X(il) X u = il;		Equivalent to X(il.begin(), il.end()) or X u(il.begin(), il.end());, respectively
   X(bil)		Equivalent to X(initializer_list<value_type>(bil))
   X u = bil;		If bil is empty, equivalent to X u;, otherwise equivalent to X u = initializer_list<value_type>(bil);
   a = il	X&	[…]
   a = bil	X&	Equivalent to a = initializer_list<value_type>(bil)
   […]
   a.insert(p, il)	iterator	[…]
   a.insert(p, bil)	iterator	Equivalent to a.insert(p, initializer_list<value_type>(bil))
   […]
   a.assign(il)	void	[…]
   a.assign(bil)	void	Equivalent to a.assign(initializer_list<value_type>(bil))
   […]

3. Modify 23.2.7 [associative.reqmts] as indicated:

   -8- In Table 69, […] il designates an objectvalue of type initializer_list<value_type>, bil designates any braced-init-list suitable to copy-list-initialize an object of type initializer_list<value_type> (9.4.5 [dcl.init.list]), […]

4. Modify Table 69 — "Associative container requirements (in addition to container)" as indicated:

   Table 69 — Associative container requirements (in addition to container)

   Expression	Return type	Assertion/note pre-/post-condition	Complexity
   […]
   X(il) X u = il;		same as X(il.begin(), il.end()) or X u(il.begin(), il.end());, respectively	same as X(il.begin(), il.end()) or X u(il.begin(), il.end());, respectively
   X(bil)		Equivalent to X(initializer_list<value_type>(bil))
   X u = bil;		If bil is empty, equivalent to X u;, otherwise equivalent to X u = initializer_list<value_type>(bil);
   X(il,c)		same as X(il.begin(), il.end(), c)	same as X(il.begin(), il.end(), c)
   X(bil, c)		Equivalent to X(initializer_list<value_type>(bil), c)
   a = il	X&	[…]	[…]
   a = bil	X&	Equivalent to a = initializer_list<value_type>(bil)
   […]
   a.insert(il)	void	equivalent to a.insert(il.begin(), il.end())
   a.insert(bil)	void	Equivalent to a.insert(initializer_list<value_type>(bil))
   […]
   a.assign(il)	void	[…]
   a.assign(bil)	void	Equivalent to a.assign(initializer_list<value_type>(bil))
   […]

5. Modify 23.2.8 [unord.req] p11's bullet list as indicated:

   -11- In Table 70:

   1. (11.1) — […]

   2. […]

   3. (11.14) — il denotes a value of type initializer_list<value_type>,

   4. (11.?) — bil denotes any braced-init-list suitable to copy-list-initialize an object of type initializer_list<value_type> (9.4.5 [dcl.init.list]),

   5. […]

6. Modify Table 70 — "Unordered associative container requirements (in addition to container)" as indicated:

   [Drafting note: There is a preexisting issue with Table 70, that there is no symbol u specified ("u denotes the name of a variable being declared"), so existing initialization forms with a named variable are currently always written as "X a[…]" where a is defined as "a denotes a value of type X", the wording below follows this existing practice but the author of this wording would like to kindly ask the Project Editor to introduce said symbol u and apply it to all existing and new such named initialization forms instead. — end drafting note]

   Table 70 — Unordered associative container requirements (in addition to container)

   Expression	Return type	Assertion/note pre-/post-condition	Complexity
   […]
   X(il) X a = il;	X	Same as X(il.begin(), il.end()) or X a(il.begin(), il.end());, respectively	Same as X(il.begin(), il.end()) or X a(il.begin(), il.end());, respectively
   X(bil)	X	Equivalent to X(initializer_list<value_type>(bil))
   X a = bil;	X	If bil is empty, equivalent to X a;, otherwise equivalent to X a = initializer_list<value_type>(bil);
   X(il, n)	X	Same as X(il.begin(), il.end(), n)	Same as X(il.begin(), il.end(), n)
   X(bil, n)	X	Equivalent to X(initializer_list<value_type>(bil), n)
   X(il, n, hf)	X	Same as X(il.begin(), il.end(), n, hf)	Same as X(il.begin(), il.end(), n, hf)
   X(bil, n, hf)	X	Equivalent to X(initializer_list<value_type>(bil), n, hf)
   X(il, n, hf, eq)	X	Same as X(il.begin(), il.end(), n, hf, eq)	Same as X(il.begin(), il.end(), n, hf, eq)
   X(bil, n, hf, eq)	X	Equivalent to X(initializer_list<value_type>(bil), n, hf, eq)
   […]
   a = il	X&	[…]	[…]
   a = bil	X&	Equivalent to a = initializer_list<value_type>(bil)
   […]
   a.insert(il)	void	Same as a.insert(il.begin(), il.end()).	Same as a.insert(il.begin(), il.end()).
   a.insert(bil)	void	Equivalent to a.insert(initializer_list<value_type>(bil))
   […]

7. Modify 29.5.3.2 [rand.req.seedseq] p2's bullet list as indicated:

   -2- A class S satisfies the requirements of a seed sequence if the expressions shown in Table 82 are valid and have the indicated semantics, and […] In that Table and throughout this subclause:

   1. (2.1) — […]

   2. (2.?) — u denotes the name of a variable being declared,

   3. […]

   4. (2.6) — il is a value of initializer_list<T>.;

   5. (2.?) — bil denotes any braced-init-list suitable to copy-list-initialize an object of type initializer_list<T> (9.4.5 [dcl.init.list]).

8. Modify Table 82 — "Seed sequence requirements" as indicated:

   Table 82 — Seed sequence requirements

   Expression	Return type	Pre/post-condition	Complexity
   […]
   S(il) S u = il;		Same as S(il.begin(), il.end()) or S u(il.begin(), il.end());, respectively	same as S(il.begin(), il.end()) or S u(il.begin(), il.end());, respectively
   S(bil)		Equivalent to S(initializer_list<T>(bil))
   S u = bil;		If bil is empty, equivalent to S u;, otherwise equivalent to S u = initializer_list<T>(bil);
   […]