std::ranges::for_each_n, std::ranges::for_each_n_result - cppreference.com (original) (raw)

1. Applies the given function object f to the projected result by proj of dereferencing each iterator in the range [first, first + n), in order.

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. If n is less than zero, the behavior is undefined.

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

An object {first + n, std::move(f)}, where first + n may be evaluated as std::ranges::next(std::move(first), n) depending on iterator category.

[edit] Complexity

Exactly n applications of f and proj.

[edit] Possible implementation

struct for_each_n_fn { template<std::input_iterator I, class Proj = std::identity, std::indirectly_unary_invocable<std::projected<I, Proj>> Fun> constexpr for_each_n_result<I, Fun> operator()(I first, std::iter_difference_t n, Fun fun, Proj proj = Proj{}) const { for (; n-- > 0; ++first) std::invoke(fun, std::invoke(proj, *first)); return {std::move(first), std::move(fun)}; } };   inline constexpr for_each_n_fn for_each_n {};

[edit] Example

#include #include #include #include #include   struct P { int first; char second; friend std::ostream& operator<<(std::ostream& os, const P& p) { return os << '{' << p.first << ",'" << p.second << "'}"; } };   auto print = name, auto const& v) { std::cout << name << ": "; for (auto n = v.size(); const auto& e : v) std::cout << e << (--n ? ", " : "\n"); };   int main() { std::array a {1, 2, 3, 4, 5}; print("a", a); // Negate first three numbers: std::ranges::for_each_n(a.begin(), 3, [](auto& n) { n *= -1; }); print("a", a);   std::array s { P{1,'a'}, P{2, 'b'}, P{3, 'c'}, P{4, 'd'} }; print("s", s); // Negate data members 'P::first' using projection: std::ranges::for_each_n(s.begin(), 2, [](auto& x) { x *= -1; }, &P::first); print("s", s); // Capitalize data members 'P::second' using projection: std::ranges::for_each_n(s.begin(), 3, [](auto& c) { c -= 'a'-'A'; }, &P::second); print("s", s); }

Output:

a: 1, 2, 3, 4, 5 a: -1, -2, -3, 4, 5 s: {1,'a'}, {2,'b'}, {3,'c'}, {4,'d'} s: {-1,'a'}, {-2,'b'}, {3,'c'}, {4,'d'} s: {-1,'A'}, {-2,'B'}, {3,'C'}, {4,'d'}

[edit] See also