std::ranges::for_each, std::ranges::for_each_result - cppreference.com (original) (raw)

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 algorithm function objects (informally known as niebloids), that is:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

Contents

• 1 Parameters
• 2 Return value
• 3 Complexity
• 4 Possible implementation
• 5 Example
• 6 See also

[edit] Parameters

[edit] Return value

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

[edit] Complexity

Exactly ranges::distance(first, last) applications of f and proj.

[edit] Possible implementation

struct for_each_fn { template<std::input_iterator I, std::sentinel_for 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, Proj>> Fun> constexpr ranges::for_each_result<ranges::borrowed_iterator_t, 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 #include #include #include #include #include   struct Sum { void operator()(int n) { sum += n; } int sum {0}; };   int main() { std::vector 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 pairs {{1,"one"}, {2,"two"}, {3,"tree"}};   std::cout << "project the pair:🥇 "; ranges::for_each(pairs, print, [](const pair& p) { return p.first; });   std::cout << "\n" "project the pair:🥈"; 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:🥇 1 2 3 project the pair:🥈 one two tree

[edit] See also