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std::ranges::for_each, std::ranges::for_each_result

From cppreference.com
< cpp‎ | algorithm‎ | ranges
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms: std::ranges::copy, std::ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
C library
 
Constrained algorithms
Non-modifying sequence operations
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Permutations
 
Defined in header <algorithm>
Call signature
template< std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity,

          std::indirectly_unary_invocable<std::projected<I, Proj>> Fun >

constexpr ranges::for_each_result<I, Fun> for_each( I first, S last, Fun f, Proj proj = {} );
(1) (since C++20)
template< ranges::input_range R, class Proj = std::identity,

          std::indirectly_unary_invocable<std::projected<ranges::iterator_t<R>, Proj>> Fun >
constexpr ranges::for_each_result<ranges::borrowed_iterator_t<R>, Fun>

for_each( R&& r, Fun f, Proj proj = {} );
(2) (since C++20)
Helper types
template< class I, class F >
using for_each_result = ranges::in_fun_result<I, F>;
(3) (since C++20)
1) Applies the given function object f to the result of the value projected by each iterator in the range [first, last), in order.
2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

For both overloads, if the iterator type is mutable, f may modify the elements of the range through the dereferenced iterator. If f returns a result, the result is ignored.

The function-like entities described on this page are niebloids, that is:

In practice, they may be implemented as function objects, or with special compiler extensions.

Contents

[edit] Parameters

first, last - iterator-sentinel pair denoting the range to apply the function to
r - the range of elements to apply the function to
f - the function to apply to the projected range
proj - projection to apply to the elements

[edit] Return value

{std::ranges::next(std::move(first), last), std::move(f)}

[edit] Complexity

Exactly last - first applications of f and proj

[edit] Possible implementation

struct for_each_fn {
  template<std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
           std::indirectly_unary_invocable<std::projected<I, Proj>> Fun>
  constexpr ranges::for_each_result<I, Fun>
  operator()(I first, S last, Fun f, Proj proj = {}) const
  {
    for (; first != last; ++first) {
      std::invoke(f, std::invoke(proj, *first));
    }
    return {std::move(first), std::move(f)};
  }
 
  template<ranges::input_range R, class Proj = std::identity,
           std::indirectly_unary_invocable<std::projected<ranges::iterator_t<R>, Proj>> Fun>
  constexpr ranges::for_each_result<ranges::borrowed_iterator_t<R>, Fun>
  operator()(R&& r, Fun f, Proj proj = {}) const
  {
    return (*this)(ranges::begin(r), ranges::end(r), std::move(f), std::ref(proj));
  }
};
 
inline constexpr for_each_fn for_each;

[edit] Example

The following example uses a lambda expression to increment all of the elements of a vector and then uses an overloaded operator() in a functor to compute their sum. Note that to compute the sum, it is recommended to use the dedicated algorithm std::accumulate.

#include <algorithm>
#include <cassert>
#include <iostream>
#include <string>
#include <utility>
#include <vector>
 
struct Sum
{
    void operator()(int n) { sum += n; }
    int sum{0};
};
 
int main()
{
    std::vector<int> nums{3, 4, 2, 8, 15, 267};
 
    auto print = [](const auto& n) { std::cout << ' ' << n; };
 
    namespace ranges = std::ranges;
    std::cout << "before:";
    ranges::for_each(std::as_const(nums), print);
    print('\n');
 
    ranges::for_each(nums, [](int& n){ ++n; });
 
    // calls Sum::operator() for each number
    auto [i, s] = ranges::for_each(nums.begin(), nums.end(), Sum());
    assert(i == nums.end());
 
    std::cout << "after: ";
    ranges::for_each(nums.cbegin(), nums.cend(), print);
 
    std::cout << "\n" "sum: " << s.sum << '\n';
 
    using pair = std::pair<int, std::string>; 
    std::vector<pair> pairs{{1,"one"}, {2,"two"}, {3,"tree"}};
 
    std::cout << "project the pair::first: ";
    ranges::for_each(pairs, print, [](const pair& p) { return p.first; });
 
    std::cout << "\n" "project the pair::second:";
    ranges::for_each(pairs, print, &pair::second);
    print('\n');
}

Output:

before: 3 4 2 8 15 267 
after:  4 5 3 9 16 268
sum: 305
project the pair::first:  1 2 3
project the pair::second: one two tree

[edit] See also

range-for loop(C++11) executes loop over range[edit]
applies a function to a range of elements
(niebloid) [edit]
applies a function object to the first n elements of a sequence
(niebloid) [edit]
applies a function to a range of elements
(function template) [edit]