# 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)

1) This deduction guide is provided for std::basic_string to allow deduction from an iterator range. This overload only participates in overload resolution 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 only participate in overload resolution if `Alloc` satisfies Allocator.

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.

### 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

### Example

Run this code

    #include <string>
    #include <vector>
    int main() {
       std::vector<char> v = {'a', 'b', 'c'};
       std::basic_string s(v.begin(), v.end()); // uses explicit deduction guide
    }

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