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3075. basic_string needs deduction guides from basic_string_view

Section: 27.4.3 [basic.string], 27.4.3.3 [string.cons] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2018-03-03 Last modified: 2021-02-25

Priority: Not Prioritized

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Discussion:

The Proposed Resolution for LWG 2946⁽ⁱ⁾ appears to be correct and we've implemented it in MSVC, but it worsens a pre-existing problem with basic_string class template argument deduction.

The following s1 and s2 compiled in C++17 before LWG 2946's PR, fail to compile after LWG 2946's PR, and are fixed by my PR:

basic_string s1("cat"sv);
basic_string s2("cat"sv, alloc);

The following s4 failed to compile in C++17, and is fixed by my PR:

// basic_string s3("cat"sv, 1, 1);
basic_string s4("cat"sv, 1, 1, alloc);

(s3 failed to compile in C++17, and would be fixed by my PR, but it is affected by a pre-existing and unrelated ambiguity which I am not attempting to fix here.)

As C++17 and LWG 2946's PR introduced templated constructors for basic_string from basic_string_view, we need to add corresponding deduction guides.

The constructors take const T& that's convertible to basic_string_view (the additional constraint about not converting to const charT* is irrelevant here). However, CTAD can't deduce charT and traits from arbitrary user-defined types, so the deduction guides need T to be exactly basic_string_view.

Additionally, we need to handle the size_type parameters in the same way that the unordered containers do. This PR has been implemented in MSVC.

[2018-14: Wednesday night issues processing: both this and 2946⁽ⁱ⁾ to status "Immediate".]

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4727.

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

   […]
   
   template<class InputIterator,
            class Allocator = allocator<typename iterator_traits<InputIterator>::value_type>>
     basic_string(InputIterator, InputIterator, Allocator = Allocator())
       -> basic_string<typename iterator_traits<InputIterator>::value_type,
                       char_traits<typename iterator_traits<InputIterator>::value_type>,
                        Allocator>;
   
   template<class charT,
            class traits,
            class Allocator = allocator<charT>>
     explicit basic_string(basic_string_view<charT, traits>, const Allocator& = Allocator())
       -> basic_string<charT, traits, Allocator>;
   
   template<class charT,
            class traits,
            class Allocator = allocator<charT>>
     basic_string(basic_string_view<charT, traits>, typename see below::size_type, typename see below::size_type, 
                  const Allocator& = Allocator())
       -> basic_string<charT, traits, Allocator>;
   
   }                     
   

   -?- A size_type parameter type in a basic_string deduction guide refers to the size_type member type of the type deduced by the deduction guide.

2. Edit 27.4.3.3 [string.cons] as indicated:

   template<class InputIterator,
            class Allocator = allocator<typename iterator_traits<InputIterator>::value_type>>
     basic_string(InputIterator, InputIterator, Allocator = Allocator())
       -> basic_string<typename iterator_traits<InputIterator>::value_type,
                       char_traits<typename iterator_traits<InputIterator>::value_type>,
                        Allocator>;
   

   -25- Remarks: Shall not participate in overload resolution if InputIterator is a type that does not qualify as an input iterator, or if Allocator is a type that does not qualify as an allocator (23.2.2 [container.requirements.general]).

   template<class charT,
            class traits,
            class Allocator = allocator<charT>>
     explicit basic_string(basic_string_view<charT, traits>, const Allocator& = Allocator())
       -> basic_string<charT, traits, Allocator>;
   
   template<class charT,
            class traits,
            class Allocator = allocator<charT>>
     basic_string(basic_string_view<charT, traits>, typename see below::size_type, typename see below::size_type, 
                  const Allocator& = Allocator())
       -> basic_string<charT, traits, Allocator>;                                          
   

   -?- Remarks: Shall not participate in overload resolution if Allocator is a type that does not qualify as an allocator (23.2.2 [container.requirements.general]).