std::ranges::next_permutation, std::ranges::next_permutation_result

Since C++20

Header: <algorithm>

  1. Transforms the range [first,last) into the next permutation, where the set of all permutations is ordered lexicographically with respect to binary comparison function object comp and projection function object proj. Returns {last, true} if such a “next permutation” exists; otherwise transforms the range into the lexicographically first permutation as if by ranges::sort(first, last, comp, proj), and returns {last, false}.

# Declarations

Call signature

template< std::bidirectional_iterator I, std::sentinel_for<I> S,
class Comp = ranges::less, class Proj = std::identity >
requires std::sortable<I, Comp, Proj>
constexpr next_permutation_result<I>
next_permutation( I first, S last, Comp comp = {}, Proj proj = {} );

(since C++20)

template< ranges::bidirectional_range R, class Comp = ranges::less,
class Proj = std::identity >
requires std::sortable<ranges::iterator_t<R>, Comp, Proj>
constexpr next_permutation_result<ranges::borrowed_iterator_t<R>>
next_permutation( R&& r, Comp comp = {}, Proj proj = {} );

(since C++20)

Helper type

template< class I >
using next_permutation_result = ranges::in_found_result<I>;

(since C++20)

# Parameters

# Notes

Implementations (e.g. MSVC STL) may enable vectorization when the iterator type models contiguous_iterator and swapping its value type calls neither non-trivial special member function nor ADL-found swap.

# Example

#include <algorithm>
#include <array>
#include <compare>
#include <functional>
#include <iostream>
#include <string>
 
struct S
{
    char c;
    int i;
    auto operator<=>(const S&) const = default;
    friend std::ostream& operator<<(std::ostream& os, const S& s)
    {
        return os << "{'" << s.c << "', " << s.i << "}";
    }
};
 
auto print = [](auto const& v, char term = ' ')
{
    std::cout << "{ ";
    for (const auto& e : v)
        std::cout << e << ' ';
    std::cout << '}' << term;
};
 
int main()
{
    std::cout << "Generate all permutations (iterators case):\n";
    std::string s{"abc"};
    do
    {
        print(s);
    }
    while (std::ranges::next_permutation(s.begin(), s.end()).found);
 
    std::cout << "\n" "Generate all permutations (range case):\n";
    std::array a{'a', 'b', 'c'};
    do
    {
        print(a);
    }
    while (std::ranges::next_permutation(a).found);
 
    std::cout << "\n" "Generate all permutations using comparator:\n";
    using namespace std::literals;
    std::array z{"█"s, "▄"s, "▁"s};
    do
    {
        print(z);
    }
    while (std::ranges::next_permutation(z, std::greater()).found);
 
    std::cout << "\n" "Generate all permutations using projection:\n";
    std::array<S, 3> r{S{'A',3}, S{'B',2}, S{'C',1}};
    do
    {
        print(r, '\n');
    }
    while (std::ranges::next_permutation(r, {}, &S::c).found);
}

# See also