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deduction guides for std::basic_string

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)
1) This deduction guide is provided for std::basic_string to allow deduction from an iterator range. This overload only participates in overload resolution if InputIt satisfies InputIterator 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 only participate in overload resolution if Alloc satisfies Allocator.

Note: the extent to which the library determines that a type does not satisfy InputIterator 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.

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

Example

#include <string>
#include <vector>
int main() {
   std::vector<char> v = {'a', 'b', 'c'};
   std::basic_string s(v.begin(), v.end()); // uses explicit deduction guide
}

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