deduction guides for std::basic_string

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< cpp‎ | string‎ | basic string
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. 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 { } )

Notes

Guides (2,3) are needed because the std::basic_string constructors for std::basic_string_view

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
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