deduction guides for std::basic_string

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Defined in header <string>
template< class InputIt, class Alloc = std::allocator<

                         typename std::iterator_traits<InputIt>::value_type> >
basic_string( InputIt, InputIt, Alloc = Alloc() )
  -> basic_string<typename std::iterator_traits<InputIt>::value_type,
                  std::char_traits<typename std::iterator_traits<InputIt>::value_type>,

                  Alloc>;
 (1) (since C++17)
template< class CharT,

          class Traits,
          class Alloc = std::allocator<CharT> >
explicit basic_string( std::basic_string_view<CharT, Traits>, const Alloc& = Alloc() )

  -> basic_string<CharT, Traits, Alloc>;
 (2) (since C++17)
template< class CharT,

          class Traits,
          class Alloc = std::allocator<CharT>> >
basic_string( std::basic_string_view<CharT, Traits>, typename /*see below*/::size_type,
              typename /*see below*/::size_type, const Alloc& = Alloc() )

  -> basic_string<CharT, Traits, Alloc>;
 (3) (since C++17)
template< ranges::input_range R,

          class Alloc = std::allocator<ranges::range_value_t<R>> >
basic_string( std::from_range_t, R&&, Alloc = Alloc() )
  -> basic_string<ranges::range_value_t<R>, std::char_traits<ranges::range_value_t<R>>,

                  Alloc>;
 (4) (since C++23)
1) This deduction guide is provided for std::basic_string to allow deduction from an iterator range. This overload participates in overload resolution only if InputIt satisfies LegacyInputIterator and Alloc satisfies Allocator.
2-3) These deduction guides are provided for std::basic_string to allow deduction from a std::basic_string_view. The size_type parameter type in (3) refers to the size_type member type of the type deduced by the deduction guide. These overloads participate in overload resolution only if Alloc satisfies Allocator.
4) This deduction guide is provided for std::basic_string to allow deduction from a std::from_range_t tag and an input_range.

Note: the extent to which the library determines that a type does not satisfy LegacyInputIterator is unspecified, except that as a minimum integral types do not qualify as input iterators. Likewise, the extent to which it determines that a type does not satisfy Allocator is unspecified, except that as a minimum the member type Alloc::value_type must exist and the expression std::declval<Alloc&>().allocate(std::size_t{}) must be well-formed when treated as an unevaluated operand.

Notes

Guides (2-3) are needed because the std::basic_string constructors for std::basic_string_views are made templates to avoid causing ambiguities in existing code, and those templates do not support class template argument deduction.

Notes

Feature-test macro Value Std Comment
__cpp_lib_containers_ranges 202202L (C++23) Ranges-aware construction and insertion; overloads (4)

Example

Run this code
#include <cassert>
#include <string>
#include <vector>
 
int main()
{
    std::vector<char> v = {'a', 'b', 'c'};
    std::basic_string s1(v.begin(), v.end()); // uses deduction guide (1)
    assert(s1 == "abc");
 
#if __cpp_lib_containers_ranges >= 202202L
    std::vector<wchar_t> v4{0x43, 43, 053, 0x32, 0x33};
    std::basic_string s4(std::from_range, v4); // uses deduction guide (4)
    assert(s4 == L"C++23");
#endif
}

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3075 C++17 deduction from basic_string_view was unsupported (exacerbated by LWG issue 2946) deduction guides added
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