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deduction guides for std::priority_queue

Defined in header <queue> 
template <class Comp, class Container>
priority_queue(Comp, Container)
 -> priority_queue<typename Container::value_type, Container, Comp>;
 (1) (since C++17) 
template<class InputIt, 
         class Comp = std::less</*iter-value-type*/<InputIt>>,
         class Container = std::vector</*iter-value-type*/<InputIt>>
priority_queue(InputIt, InputIt, Comp = Comp(), Container = Container())
  -> priority_queue</*iter-value-type*/<InputIt>, Container, Comp>;
 (2) (since C++17) 
template<class Comp, class Container, class Alloc>
priority_queue(Comp, Container, Alloc)
  -> priority_queue<typename Container::value_type, Container, Comp>;
 (3) (since C++17) 
template<class InputIt, class Alloc>
priority_queue(InputIt, InputIt, Alloc)
  -> priority_queue</*iter-value-type*/<InputIt>,
                    std::vector</*iter-value-type*/<InputIt>, Alloc>,
                    std::less</*iter-value-type*/<InputIt>>>;
 (4) (since C++17) 
template<class InputIt, class Comp, class Alloc>
priority_queue(InputIt, InputIt, Comp, Alloc)
  -> priority_queue</*iter-value-type*/<InputIt>,
                    std::vector</*iter-value-type*/<InputIt>, Alloc>, Comp>;
 (5) (since C++17) 
template<class InputIt, class Comp, class Container, class Alloc>
priority_queue(InputIt, InputIt, Comp, Container, Alloc)
  -> priority_queue<typename Container::value_type, Container, Comp>;
 (6) (since C++17)

These deduction guides are provided for std::priority_queue to allow deduction from underlying container type and from an iterator range. /*iter-value-type*/<It> denotes typename std::iterator_traits<It>::value_type for any type It.

These overloads participate in overload resolution only if.

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

#include <queue>
#include <vector>
#include <iostream>
#include <functional>
 
int main() {
    const std::vector<int> v = {1,2,3,4};
    std::priority_queue pq1{std::greater<int>{}, v}; // deduces std::priority_queue<
                                                     //   int, std::vector<int>,
                                                     //   std::greater<int>>
    for (; !pq1.empty(); pq1.pop())
        std::cout << pq1.top() << ' ';
    std::cout << '\n';
 
    std::priority_queue pq2{v.begin(), v.end()}; // deduces std::priority_queue<int>
 
    for (; !pq2.empty(); pq2.pop())
        std::cout << pq2.top() << ' ';
    std::cout << '\n';
}

Output:

1 2 3 4
4 3 2 1

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 3506 C++17 deduction guides from iterator and allocator were missing added

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Last Updated 2022-06-02T22:02:33Z