std::ranges::for_each, std::ranges::for_each_result
From cppreference.com
| Defined in header <algorithm>
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| Call signature |
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template< std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity, std::indirectly_unary_invocable<std::projected<I, Proj>> Fun > constexpr 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 for_each_result<ranges::borrowed_iterator_t<R>, Fun> for_each( R&& r, Fun f, Proj proj = {} ); |
(2) | (since C++20) |
| Helper types |
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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 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.
Parameters
| first, last | - | the iterator-sentinel pair defining the range of elements 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 |
Return value
{ranges::next(std::move(first), last), std::move(f)}
Complexity
Exactly ranges::distance(first, last) applications of f and proj.
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;
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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.
Run this code
#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
See also
range-for loop(C++11)
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executes loop over range |
(C++20) |
applies a function to a range of elements (algorithm function object) |
(C++20) |
applies a function object to the first N elements of a sequence (algorithm function object) |
| applies a unary function object to elements from a range (function template) |