std::ranges::remove, std::ranges::remove_if - cppreference.com (original) (raw)

Removes all elements satisfying specific criteria from the range [first, last) and returns a subrange [ret, last), where ret is a past-the-end iterator for the new end of the range.

1. Removes all elements that are equal to value, using std::invoke(proj, *i) == value to compare.

2,4) Same as (1,3), but uses r as the range, as if using ranges::begin(r) as first and ranges::end(r) as last.

Removing is done by shifting (by means of move assignment) the elements in the range in such a way that the elements that are not to be removed appear in the beginning of the range. Relative order of the elements that remain is preserved and the physical size of the container is unchanged. Iterators pointing to an element between the new logical end and the physical end of the range are still dereferenceable, but the elements themselves have unspecified values (as per MoveAssignable post-condition).

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 Notes
• 5 Possible implementation
• 6 Notes
• 7 Example
• 8 See also

[edit] Parameters

[edit] Return value

{ret, last}, where [first, ret) is the resulting subrange after removal, and the elements in subrange [ret, last) are all in valid but unspecified state, i.e. [ret, last) is the subrange to be erased.

[edit] Complexity

Exactly N applications of the corresponding predicate and any projection, where N = ranges::distance(first, last), and N - 1 move operations at worst.

[edit] Notes

A call to ranges::remove is typically followed by a call to a container's erase member function, which erases the unspecified values and reduces the physical size of the container to match its new logical size. These two invocations together constitute the so-called erase-remove idiom, which can be achieved by the free function std::erase that has overloads for all standard sequence containers, or std::erase_if that has overloads for all standard containers.

The similarly-named container member functions list::remove, list::remove_if, forward_list::remove, and forward_list::remove_if erase the removed elements.

These algorithms usually cannot be used with associative containers such as std::set and std::map because their iterator types do not dereference to MoveAssignable types (the keys in these containers are not modifiable).

Because ranges::remove takes value by reference, it can have unexpected behavior if it is a reference to an element of the range [first, last).

[edit] Possible implementation

remove (1,2)
struct remove_fn { template<std::permutable I, std::sentinel_for<I> S, class Proj = std::identity, class T = std::projected_value_t<I, Proj>> requires std::indirect_binary_predicate <ranges::equal_to, std::projected<I, Proj>, const T*> constexpr ranges::subrange<I> operator()(I first, S last, const T& value, Proj proj = {}) const { first = ranges::find(std::move(first), last, value, proj); if (first != last) { for (I i{std::next(first)}; i != last; ++i) if (value != std::invoke(proj, *i)) { *first = ranges::iter_move(i); ++first; } } return {first, last}; }   template<ranges::forward_range R, class Proj = std::identity, class T = std::projected_value_t<ranges::iterator_t<R>, Proj>> requires std::permutable<ranges::iterator_t<R>> && std::indirect_binary_predicate <ranges::equal_to, std::projected<ranges::iterator_t<R>, Proj>, const T*> constexpr ranges::borrowed_subrange_t<R> operator()(R&& r, const T& value, Proj proj = {}) const { return (*this)(ranges::begin(r), ranges::end(r), value, std::move(proj)); } };   inline constexpr remove_fn remove {};
remove_if (3,4)
struct remove_if_fn { template<std::permutable I, std::sentinel_for<I> S, class Proj = std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred> constexpr ranges::subrange<I> operator()(I first, S last, Pred pred, Proj proj = {}) const { first = ranges::find_if(std::move(first), last, pred, proj); if (first != last) { for (I i{std::next(first)}; i != last; ++i) if ((pred, std::invoke(proj, *i))) { *first = ranges::iter_move(i); ++first; } } return {first, last}; }   template<ranges::forward_range R, class Proj = std::identity, std::indirect_unary_predicate <std::projected<ranges::iterator_t<R>, Proj>> Pred> requires std::permutable<ranges::iterator_t<R>> constexpr ranges::borrowed_subrange_t<R> operator()(R&& r, Pred pred, Proj proj = {}) const { return (*this)(ranges::begin(r), ranges::end(r), pred, std::move(proj)); } };   inline constexpr remove_if_fn remove_if {};

[edit] Notes

[edit] Example

#include #include #include #include #include #include #include #include #include   int main() { std::string v1{"No - Diagnostic - Required"}; std::cout << std::quoted(v1) << " (v1, size: " << v1.size() << ")\n"; const auto ret = std::ranges::remove(v1, ' '); std::cout << std::quoted(v1) << " (v1 after remove, size: " << v1.size() << ")\n"; std::cout << ' ' << std::string(std::distance(v1.begin(), ret.begin()), '^') << '\n'; v1.erase(ret.begin(), ret.end()); std::cout << std::quoted(v1) << " (v1 after erase, size: " << v1.size() << ")\n\n";   // remove_if with custom unary predicate: auto rm = [](char c) { return (c); }; std::string v2{"Substitution Failure Is Not An Error"}; std::cout << std::quoted(v2) << " (v2, size: " << v2.size() << ")\n"; const auto [first, last] = std::ranges::remove_if(v2, rm); std::cout << std::quoted(v2) << " (v2 after remove_if, size: " << v2.size() << ")\n"; std::cout << ' ' << std::string(std::distance(v2.begin(), first), '^') << '\n'; v2.erase(first, last); std::cout << std::quoted(v2) << " (v2 after erase, size: " << v2.size() << ")\n\n";   // creating a view into a container that is modified by remove_if: for (std::string s : {"Small Object Optimization", "Non-Type Template Parameter"}) std::cout << std::quoted(s) << " => " << std::string_view{begin(s), std::ranges::remove_if(s, rm).begin()} << '\n';   std::vector<std::complex> nums{{2, 2}, {1, 3}, {4, 8}}; #ifdef __cpp_lib_algorithm_default_value_type auto e = std::ranges::remove(nums, {1, 3}); // T gets deduced #else auto e = std::ranges::remove(nums, std::complex{1, 3}); #endif nums.erase(e.begin(), e.end()); assert((nums == std::vector<std::complex>{{2, 2}, {4, 8}})); }

Possible output:

"No _ Diagnostic _ Required" (v1, size: 26) "No_Diagnostic_Requiredired" (v1 after remove, size: 26) ^^^^^^^^^^^^^^^^^^^^^^ "No_Diagnostic_Required" (v1 after erase, size: 22)   "Substitution Failure Is Not An Error" (v2, size: 36) "SFINAEtution Failure Is Not An Error" (v2 after remove_if, size: 36) ^^^^^^ "SFINAE" (v2 after erase, size: 6)   "Small Object Optimization" => SOO "Non-Type Template Parameter" => NTTP

[edit] See also