NAME¶

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

Synopsis¶

Defined in header <algorithm>
template< class ForwardIt, class T > (constexpr since
ForwardIt remove( ForwardIt first, C++20)
ForwardIt last, const T& value ); (until C++26)
template< class ForwardIt, class T =
typename std::iterator_traits

<ForwardIt>::value_type > (since C++26)
constexpr ForwardIt remove( ForwardIt
first, ForwardIt last,

const T& value
);
template< class ExecutionPolicy, class
ForwardIt, class T >

ForwardIt remove( ExecutionPolicy&& (since C++17)
policy, (until C++26)

ForwardIt first,
ForwardIt last, const T& value );
template< class ExecutionPolicy, class
ForwardIt, (1)

class T = typename
std::iterator_traits
(since C++26)
<ForwardIt>::value_type >
ForwardIt remove( ExecutionPolicy&& (2)
policy,

ForwardIt first,
ForwardIt last, const T& value );
template< class ForwardIt, class UnaryPred
> (3) (constexpr since
ForwardIt remove_if( ForwardIt first, C++20)
ForwardIt last, UnaryPred p );
template< class ExecutionPolicy, class
ForwardIt, class UnaryPred >

ForwardIt remove_if( ExecutionPolicy&& (4) (since C++17)
policy,

ForwardIt first,
ForwardIt last, UnaryPred p );

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

1) Removes all elements that are equal to value (using operator==).
3) Removes all elements for which predicate p returns true.
2,4) Same as (1,3), but executed according to policy.
These overloads participate in overload resolution only if

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. (until
C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. (since
C++20)

If the value type of ForwardIt is not CopyAssignable, the behavior is (until C++11)
undefined.
If the type of *first is not MoveAssignable, the behavior is (since C++11)
undefined.

Explanation¶

Removing is done by shifting 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.

* Shifting is done by
copy assignment
(until C++11)
move assignment
(since C++11).
* The removing operation is stable: the relative order of the elements not to be
removed stays the same.
* The underlying sequence of [first, last) is not shortened by the removing
operation. Given result as the returned iterator:

* All iterators in [result, last) are still dereferenceable.

* Each element of [result, last) has a valid but unspecified state,
because move assignment can eliminate elements by moving from (since C++11)
elements that were originally in that range.

Parameters¶

first, last - the range of elements to process
value - the value of elements to remove
policy - the execution policy to use. See execution policy for details.
unary predicate which returns true if the element should be removed.

The expression p(v) must be convertible to bool for every argument v
p - of type (possibly const) VT, where VT is the value type of ForwardIt,
regardless of value category, and must not modify v. Thus, a parameter
type of VT&is not allowed
, nor is VT unless for VT a move is equivalent to a copy
(since C++11).

Type requirements¶

-
ForwardIt must meet the requirements of LegacyForwardIterator.
-
UnaryPredicate must meet the requirements of Predicate.

Return value¶

Past-the-end iterator for the new range of values (if this is not end, then it
points to an unspecified value, and so do iterators to any values between this
iterator and end).

Complexity¶

Given \(\scriptsize N\)N as std::distance(first, last):

1,2) Exactly \(\scriptsize N\)N comparisons using operator==.
3,4) Exactly \(\scriptsize N\)N applications of the predicate p.

Exceptions¶

The overloads with a template parameter named ExecutionPolicy report errors as
follows:

* If execution of a function invoked as part of the algorithm throws an exception
and ExecutionPolicy is one of the standard policies, std::terminate is called.
For any other ExecutionPolicy, the behavior is implementation-defined.
* If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation¶

remove (1)
template<class ForwardIt, class T = typename std::iterator_traits<ForwardIt>::value_type>
ForwardIt remove(ForwardIt first, ForwardIt last, const T& value)
{
first = std::find(first, last, value);
if (first != last)
for (ForwardIt i = first; ++i != last;)
if (!(*i == value))
*first++ = std::move(*i);
return first;
}
remove_if (3)
template<class ForwardIt, class UnaryPred>
ForwardIt remove_if(ForwardIt first, ForwardIt last, UnaryPred p)
{
first = std::find_if(first, last, p);
if (first != last)
for (ForwardIt i = first; ++i != last;)
if (!p(*i))
*first++ = std::move(*i);
return first;
}

Notes¶

A call to remove is typically followed by a call to a container's erase member
function to actually remove elements from the container. These two invocations
together constitute a so-called Erase-remove idiom.

The same effect can also be achieved by the following non-member
functions:
(since C++20)
* std::erase, which has overloads for all standard sequence
containers.
* std::erase_if, which 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 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).

The standard library also defines an overload of std::remove in <cstdio>, which
takes a const char* and is used to delete files.

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

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

Example¶

The following code removes all spaces from a string by shifting all non-space
characters to the left and then erasing the extra. This is an example of
Erase-remove idiom.

// Run this code

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

int main()
{
std::string str1{"Text with some spaces"};

auto noSpaceEnd = std::remove(str1.begin(), str1.end(), ' ');

// The spaces are removed from the string only logically.
// Note, we use view, the original string is still not shrunk:
std::cout << std::string_view(str1.begin(), noSpaceEnd)
<< " size: " << str1.size() << '\n';

str1.erase(noSpaceEnd, str1.end());

// The spaces are removed from the string physically.
std::cout << str1 << " size: " << str1.size() << '\n';

std::string str2 = "Text\n with\tsome \t whitespaces\n\n";
str2.erase(std::remove_if(str2.begin(),
str2.end(),
[](unsigned char x) { return std::isspace(x); }),
str2.end());
std::cout << str2 << '\n';

std::vector<std::complex<double>> nums{{2, 2}, {1, 3}, {4, 8}};
#ifdef __cpp_lib_algorithm_default_value_type
nums.erase(std::remove(nums.begin(), nums.end(), {1, 3}), nums.end());
#else
nums.erase(std::remove(nums.begin(), nums.end(), std::complex<double>{1, 3}),
nums.end());
#endif
assert((nums == std::vector<std::complex<double>>{{2, 2}, {4, 8}}));
}

Output:¶

Textwithsomespaces size: 23
Textwithsomespaces size: 18
Textwithsomewhitespaces

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
T was required to be required the value type of
LWG 283 C++98 EqualityComparable, but ForwardIt
the value type of ForwardIt is not to be CopyAssignable instead
always T

See also¶

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