std::remove,std::remove_if(3)

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)