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 >
   ForwardIt remove( ForwardIt first, ForwardIt (until C++20)
   last, const T& value );
   template< class ForwardIt, class T >
   constexpr ForwardIt remove( ForwardIt first, (since C++20)
   ForwardIt last, const T& value );
   template< class ExecutionPolicy, class
   ForwardIt, class T >
   ForwardIt remove( ExecutionPolicy&& policy, (2) (since C++17)
   ForwardIt first, ForwardIt last,
   const T& value );
   template< class ForwardIt, class UnaryPredicate (1)
   > (until C++20)
   ForwardIt remove_if( ForwardIt first, ForwardIt
   last, UnaryPredicate p );
   template< class ForwardIt, class UnaryPredicate
   > (since C++20)
   constexpr ForwardIt remove_if( ForwardIt first,
   ForwardIt last, UnaryPredicate p ); (3)
   template< class ExecutionPolicy, class
   ForwardIt, class UnaryPredicate >
   ForwardIt remove_if( ExecutionPolicy&& policy, (4) (since C++17)
   ForwardIt first, ForwardIt last,
   UnaryPredicate 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== to compare them.
   3) Removes all elements for which predicate p returns true.
   2,4) Same as (1,3), but executed according to policy. These overloads do not
   participate in overload resolution unless
   std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
   (until C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
   (since C++20) is true.
   Removing is done by shifting (by means of
   copy assignment
   (until C++11)
   move assignment
   (since C++11)) 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).

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.
   -
   The type of dereferenced ForwardIt must meet the requirements of MoveAssignable.
   -
   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
   Exactly std::distance(first, last) applications of the predicate.

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.

Notes
   A call to remove is typically followed by a call to a container's erase method,
   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
   (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).

Possible implementation

First version
   template< class ForwardIt, class T >
   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;
   }

Second version
   template<class ForwardIt, class UnaryPredicate>
   ForwardIt remove_if(ForwardIt first, ForwardIt last, UnaryPredicate 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;
   }

Examples
   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 <string>
 #include <string_view>
 #include <iostream>
 #include <cctype>
 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';
 }

Output:
 Textwithsomespaces size: 23
 Textwithsomespaces size: 18
 Textwithsomewhitespaces

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)