# deduction guides for `std::forward_list`

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< cpp‎ | container‎ | forward list

Defined in header `<forward_list>` |  |
template< class InputIt, class Alloc = std::allocator< typename std::iterator_traits<InputIt>::value_type> > forward_list( InputIt, InputIt, Alloc = Alloc() ) -> forward_list<typename std::iterator_traits<InputIt>::value_type, Alloc>; | (1) | (since C++17)
template< ranges::input_range R, class Alloc = std::allocator<ranges::range_value_t<R>> > forward_list( std::from_range_t, R&&, Alloc = Alloc() ) -> forward_list<ranges::range_value_t<R>, Alloc>; | (2) | (since C++23)

1) This deduction guide is provided for forward_list to allow deduction from an iterator range. This overload participates in overload resolution only if `InputIt` satisfies LegacyInputIterator and `Alloc` satisfies Allocator.
2) This deduction guide is provided for forward_list 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

Feature-test macro | Value | Std | Comment
--- | --- | --- | ---
`__cpp_lib_containers_ranges` | 202202L | (C++23) | Ranges-aware construction and insertion; overload (2)

### Example

Run this code

    #include <forward_list>
    #include <vector>

    int main()
    {
        std::vector<int> v = {1, 2, 3, 4};

        // uses explicit deduction guide to deduce std::forward_list<int>
        std::forward_list x(v.begin(), v.end());

        // deduces std::forward_list<std::vector<int>::iterator>
        // first phase of overload resolution for list-initialization selects the candidate
        // synthesized from the initializer-list constructor; second phase is not performed
        // and deduction guide has no effect
        std::forward_list y{v.begin(), v.end()};
    }

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