deductionguidesforstd::deque(3)

NAME
deductionguidesforstd::deque - deductionguidesforstd::deque

Synopsis
   Defined in header <deque>
   template< class InputIt,
   class Alloc = std::allocator<typename
   std::iterator_traits<InputIt>::value_type>> (since C++17)
   deque(InputIt, InputIt, Alloc = Alloc())
   -> deque<typename std::iterator_traits<InputIt>::value_type, Alloc>;
   This deduction guide is provided for deque to allow deduction from an iterator
   range. This overload participates in overload resolution only if InputIt satisfies
   LegacyInputIterator and 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.

Example
// Run this code
 #include <deque>
 #include <vector>
 int main() {
    std::vector<int> v = {1, 2, 3, 4};
    // uses explicit deduction guide to deduce std::deque<int>
    std::deque x(v.begin(), v.end());
    // deduces std::deque<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::deque y{v.begin(), v.end()};
 }