std::remove, std::remove_if

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Removing operations
removeremove_if
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(until C++17)(C++11)
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Defined in header <algorithm>
(1)
template< class ForwardIt, class T >
ForwardIt remove( ForwardIt first, ForwardIt last, const T& value );
 (constexpr since C++20)
(until C++26)
template< class ForwardIt, class T = typename std::iterator_traits

                                         <ForwardIt>::value_type >
constexpr ForwardIt remove( ForwardIt first, ForwardIt last,

                            const T& value );
 (since C++26)
(2)
template< class ExecutionPolicy, class ForwardIt, class T >

ForwardIt remove( ExecutionPolicy&& policy,

                  ForwardIt first, ForwardIt last, const T& value );
 (since C++17)
(until C++26)
template< class ExecutionPolicy, class ForwardIt,

          class T = typename std::iterator_traits
                        <ForwardIt>::value_type >
ForwardIt remove( ExecutionPolicy&& policy,

                  ForwardIt first, ForwardIt last, const T& value );
 (since C++26)
template< class ForwardIt, class UnaryPred >
ForwardIt remove_if( ForwardIt first, ForwardIt last, UnaryPred p );
 (3) (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt, class UnaryPred >

ForwardIt remove_if( ExecutionPolicy&& policy,

                     ForwardIt first, ForwardIt last, UnaryPred p );
 (4) (since C++17)

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 undefined.

(until C++11)

If the type of *first is not MoveAssignable, the behavior is undefined.

(since C++11)

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

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.
p - unary predicate which returns ​true if the element should be removed.

The expression p(v) must be convertible to bool for every argument v 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 N as std::distance(first, last):

1,2) Exactly N comparisons using operator==.
3,4) Exactly 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)

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
LWG 283 C++98 T was required to be EqualityComparable, but
the value type of ForwardIt is not always T 		required the value type of ForwardIt
to be CopyAssignable instead

See also

 copies a range of elements omitting those that satisfy specific criteria
(function template)
removes consecutive duplicate elements in a range
(function template)
(C++20)(C++20)
 removes elements satisfying specific criteria
(niebloid)
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