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std::ranges::for_each,std::ranges::for_each_result(3) | C++ Standard Libary | std::ranges::for_each,std::ranges::for_each_result(3) |
NAME¶
std::ranges::for_each,std::ranges::for_each_result - std::ranges::for_each,std::ranges::for_each_result
Synopsis¶
Defined in header <algorithm>
Call signature
template< std::input_iterator I, std::sentinel_for<I> S, class Proj
=
std::identity,
(since
std::indirectly_unary_invocable<std::projected<I, Proj>> Fun >
(1) C++20)
constexpr for_each_result<I, Fun>
for_each( I first, S last, Fun f, Proj proj = {} );
template< ranges::input_range R, class Proj = std::identity,
std::indirectly_unary_invocable<std::projected<ranges::iterator_t<R>,
(since
Proj>> Fun > (2) C++20)
constexpr for_each_result<ranges::borrowed_iterator_t<R>,
Fun>
for_each( R&& r, Fun f, Proj proj = {} );
Helper types¶
template< class I, class F > (3) (since
using for_each_result = ranges::in_fun_result<I, F>; 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:
* Explicit template argument lists may not be specified when calling any of
them.
* None of them is visible to argument-dependent lookup.
* When one of them is found by normal unqualified lookup for the name to the
left
of the function-call operator, it inhibits argument-dependent lookup.
In practice, they may be implemented as function objects, or with special
compiler
extensions.
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
Return value¶
{std::ranges::next(std::move(first), last), std::move(f)}
Complexity¶
Exactly last - first 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;
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) executes loop over range
ranges::transform applies a function to a range of elements
(C++20) (niebloid)
ranges::for_each_n applies a function object to the first n elements of a
(C++20) sequence
(niebloid)
for_each applies a function to a range of elements
(function template)
2022.07.31 | http://cppreference.com |