# deduction guides for `std::basic_string`

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< cpp‎ | string‎ | 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 {}) 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 unsupported (exacerbated by LWG issue 2946) | deduction guides added

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