deduction guides for std::basic_string

Since C++17

Header: <string>

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.

# Declarations

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>;

(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>;

(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>;

(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>;

(since C++23)

# 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.

# Example

#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

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