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deduction guides for std::forward_list

From cppreference.com
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>;
(since C++17)

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

Note: the extent to which the library determines that a type does not satisfy InputIterator 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.

[edit] Example

#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()}; 
}