NAME¶

deductionguidesforstd::basic_string - deductionguidesforstd::basic_string

Synopsis¶

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 (1) (since C++17)
std::iterator_traits<InputIt>::value_type,
std::char_traits<

typename
std::iterator_traits<InputIt>::value_type>, Alloc>;
template< class CharT,

class Traits,
class Alloc = std::allocator<CharT> > (2) (since C++17)
explicit basic_string( std::basic_string_view<CharT, Traits>,
const Alloc& = Alloc() )

-> basic_string<CharT, Traits, Alloc>;
template< class CharT,

class Traits,
class Alloc = std::allocator<CharT>> >
basic_string( std::basic_string_view<CharT, Traits>, (3) (since C++17)
typename /* see below */::size_type,
typename /* see below */::size_type,
const Alloc& = Alloc() )

-> basic_string<CharT, Traits, Alloc>;
template< ranges::input_range R,

class Alloc = std::allocator<ranges::range_value_t<R>> >
basic_string( std::from_range_t, R&&, Alloc = Alloc() ) (4) (since C++23)
-> basic_string<ranges::range_value_t<R>,

std::char_traits<ranges::range_value_t<R>>,
Alloc>;

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 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 deduction guides
unsupported (exacerbated by LWG issue 2946) added