Issue 2583: There is no way to supply an allocator for basic_string(str, pos) (original) (raw)

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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, A> stringA; vector<stringA, scoped_allocator_adaptor<A>> 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 rlenof 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 rlenof 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 ofrlen consecutive elements of the string controlled by str beginning at position pos
size() rlen
capacity() a value at least as large as size()