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deduction guides for std::basic_string

From cppreference.net
C++
Strings library
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)
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 . These overloads participate in overload resolution only if Alloc satisfies Allocator .
3) The size_type parameter type refers to the nested type size_type of the type deduced by the deduction guide.
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.

Contents

Notes

Guides ( 2,3 ) are needed because the std::basic_string constructors for std::basic_string_view s 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 Feature
__cpp_lib_containers_ranges 202202L (C++23) Ranges-aware construction and insertion; overload ( 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