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2583. There is no way to supply an allocator for basic_string(str, pos)

Section: 27.4.3.3 [string.cons] Status: C++17 Submitter: Pablo Halpern Opened: 2016-01-05 Last modified: 2017-07-30

Priority: 0

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Discussion:

Container and string constructors in the standard follow two general rules:

1. Every constructor needs a version with and without an allocator argument (possibly through the use of default arguments).

2. Every constructor except the copy constructor for which an allocator is not provided uses a default-constructed allocator.

The first rule ensures emplacing a string into a container that uses a scoped allocator will correctly propagate the container's allocator to the new element.

The current standard allows constructing a string as basic_string(str, pos) but not basic_string(str, pos, alloc). This omission breaks the first rule and causes something like the following to fail:

typedef basic_string<char, char_traits<char>, A<char>> stringA;
vector<stringA, scoped_allocator_adaptor<A<stringA>>> vs;
stringA s;

vs.emplace_back(s, 2); // Ill-formed

[2016-02, Issues Telecon]

P0; move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4567.

1. Change 27.4.3 [basic.string], class template basic_string synopsis, as indicated

   basic_string(const basic_string& str, size_type pos, size_type n = npos,
                const Allocator& a = Allocator());
   basic_string(const basic_string& str, size_type pos, size_type n,
                const Allocator& a = Allocator());             
   

2. Change 27.4.3.3 [string.cons] as indicated

   basic_string(const basic_string& str,
                size_type pos, size_type n = npos,
                const Allocator& a = Allocator());
   

   -3- Throws: out_of_range if pos > str.size().

   -4- Effects: Constructs an object of class basic_string and determines the effective length rlen of the initial string value as the smaller of n and str.size() - pos, as indicated in Table 65.

   basic_string(const basic_string& str, size_type pos, size_type n,
                const Allocator& a = Allocator());             
   

   -?- Throws: out_of_range if pos > str.size().

   -?- Effects: Constructs an object of class basic_string and determines the effective length rlen of the initial string value as the smaller of n and str.size() - pos, as indicated in Table 65.

   Table 65 — basic_string(const basic_string&, size_type, size_type, const Allocator&) and basic_string(const basic_string&, size_type, size_type, const Allocator&) effects

   Element	Value
   data()	points at the first element of an allocated copy of rlen consecutive elements of the string controlled by str beginning at position pos
   size()	rlen
   capacity()	a value at least as large as size()