std::ranges::in_fun_result

template <class I, class F>
struct in_fun_result;

ranges::in_fun_result is a class template that provides a way to store an iterator and a function object as a single unit.

This class template has no base classes or declared members other than those shown below. Thus it is suitable for use with structured bindings.

All special member functions of this class template are implicitly declared, which makes specializations be aggregate classes, and propagate triviality, potentially-throwing-ness, and constexpr-ness of corresponding operations on data members.

Template parameters

Data members

[[no_unique_address]] I in;

[[no_unique_address]] F fun;

Member functions

template<class I2, class F2>
requires std::convertible_to<const I&, I2> && std::convertible_to<const F&, F2>
constexpr operator in_fun_result<I2, F2>() const &;

template<class I2, class F2>
requires std::convertible_to<I, I2> && std::convertible_to<F, F2>
constexpr operator in_fun_result<I2, F2>() &&;

Standard library

The following standard library functions use ranges::in_fun_result as the return type:

Synopsis

namespace std::ranges {
    template<class I, class F>
    struct in_fun_result {
        [[no_unique_address]] I in;
        [[no_unique_address]] F fun;
 
        template<class I2, class F2>
        requires std::convertible_to<const I&, I2> && std::convertible_to<const F&, F2>
        constexpr operator in_fun_result<I2, F2>() const & {
            return {in, fun};
        }
 
        template<class I2, class F2>
        requires std::convertible_to<I, I2> && std::convertible_to<F, F2>
        constexpr operator in_fun_result<I2, F2>() && {
            return {std::move(in), std::move(fun)};
        }
    };
}

Notes

Each standard library algorithm that uses this family of return types declares a new alias type, e.g. using merge_result = in_in_out_result<I1, I2, O>;.

The names for such aliases are formed by adding the suffix "_result" to the algorithm's name. So, the return type of std::ranges::merge can be named as std::ranges::merge_result.

Unlike std::pair and std::tuple, this class template has data members of meaningful names.

Example

#include <algorithm>
#include <iostream>
#include <iterator>
#include <ranges>
 
int main()
{
    int v[] { 1, 2, 3 };
 
    const auto [last, negate] = std::ranges::for_each_n(v, std::size(v),
        [](int& x) { return x = -x; });
 
    const auto result = std::ranges::for_each(std::cbegin(v), last,
        [](int x) { std::cout << x << ' '; });
    std::cout << "│ ";
 
    std::ranges::for_each(v, negate);
    std::ranges::for_each(v, result.fun);
    std::cout << '\n';
}

-1 -2 -3 │ 1 2 3

See also