NAME¶

std::ranges::remove,std::ranges::remove_if - std::ranges::remove,std::ranges::remove_if

Synopsis¶

Defined in header <algorithm>
Call signature
template< std::permutable I, std::sentinel_for<I> S,

class T, class Proj = std::identity >
requires std::indirect_binary_predicate (since
<ranges::equal_to, std::projected<I, Proj>, C++20)
const T*> (until
constexpr ranges::subrange<I> C++26)

remove( I first, S last, const T& value, Proj proj = {}
);
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 (since
<ranges::equal_to, std::projected<I, Proj>, C++26)
const T*>
constexpr ranges::subrange<I>

remove( I first, S last, const T& value, Proj proj = {}
);
template< ranges::forward_range R,

class T, class Proj = std::identity >
requires std::permutable<ranges::iterator_t<R>> && (since
std::indirect_binary_predicate C++20)
<ranges::equal_to, (until
std::projected<ranges::iterator_t<R>, Proj>, C++26)
const T*>
constexpr ranges::borrowed_subrange_t<R>

remove( R&& r, const T& value, Proj proj = {} ); (1)
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>> && (since
std::indirect_binary_predicate C++26)
<ranges::equal_to,
std::projected<ranges::iterator_t<R>, Proj>,
const T*> (2)
constexpr ranges::borrowed_subrange_t<R>

remove( R&& r, const T& value, Proj proj = {} );
template< std::permutable I, std::sentinel_for<I> S,

class Proj = std::identity,
std::indirect_unary_predicate<std::projected<I, (3) (since
Proj>> Pred > C++20)
constexpr ranges::subrange<I>

remove_if( I first, S last, Pred pred, Proj proj = {} );
template< ranges::forward_range R,

class Proj = std::identity,
std::indirect_unary_predicate
<std::projected<ranges::iterator_t<R>, Proj>> (4) (since
Pred > C++20)
requires std::permutable<ranges::iterator_t<R>>
constexpr ranges::borrowed_subrange_t<R>

remove_if( R&& r, Pred pred, Proj proj = {} );

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.
3) Removes all elements for which std::invoke(pred, std::invoke(proj, *i)) returns
true.
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 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.

In practice, they may be implemented as function objects, or with special compiler
extensions.

Parameters¶

first, last - the range of elements to process
r - the range of elements to process
value - the value of elements to remove
pred - predicate to apply to the projected elements
proj - projection to apply to the elements

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.

Complexity¶

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

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 a 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).

Possible implementation¶

remove
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
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 (!std::invoke(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 {};

Notes¶

Feature-test macro Value Std Feature
__cpp_lib_algorithm_default_value_type 202403 (C++26) List-initialization for
algorithms (1,2)

Example¶

// Run this code

#include <algorithm>
#include <cassert>
#include <complex>
#include <cctype>
#include <iomanip>
#include <iostream>
#include <string>
#include <string_view>
#include <vector>

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 !std::isupper(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<double>> 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<double>{1, 3});
#endif
nums.erase(e.begin(), e.end());
assert((nums == std::vector<std::complex<double>>{{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

See also¶

ranges::remove_copy copies a range of elements omitting those that satisfy
ranges::remove_copy_if specific criteria
(C++20) (niebloid)
(C++20)
ranges::unique removes consecutive duplicate elements in a range
(C++20) (niebloid)
remove removes elements satisfying specific criteria
remove_if (function template)