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

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

Priority:0

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Discussion:

Container andstring 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 constructorexcept the copy constructor for which an allocator is not provided uses a default-constructed allocator.

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

The current standard allows constructing astring asbasic_string(str, pos) but notbasic_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 templatebasic_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 ifpos > str.size().

   -4-Effects: Constructs an object of classbasic_string and determines the effective lengthrlen of the initial string value asthe smaller ofn andstr.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 ifpos > str.size().

   -?-Effects: Constructs an object of classbasic_string and determines the effective lengthrlen of the initial string value as the smaller ofn andstr.size() - pos, as indicated in Table 65.

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

   Element	Value
   data()	points at the first element of an allocated copy ofrlen consecutive elements of the string controlledbystr beginning at positionpos
   size()	rlen
   capacity()	a value at least as large assize()