deduction guides for std::basic_string
<string>
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>,
class Traits,
class Alloc = std::allocator <CharT> >
explicit basic_string( std::basic_string_view <CharT, Traits>,
const Alloc& = Alloc() )
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() )
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>,
InputIt satisfies LegacyInputIterator and Alloc satisfies Allocator.Alloc satisfies Allocator.size_type parameter type refers to the nested type size_type of the type deduced by the deduction guide.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
[edit] 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.
[edit] Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_containers_ranges |
202202L |
(C++23) | Ranges-aware construction and insertion; overload (4) |
[edit] 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 }
[edit] 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 |