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std::remove, std::remove_if

Defined in header `<algorithm>`
template< class ForwardIt, class T > ForwardIt remove( ForwardIt first, ForwardIt last, const T& value );	(1)
template< class ExecutionPolicy, class ForwardIt, class T > ForwardIt remove( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, const T& value );	(2)	(since C++17)
template< class ForwardIt, class UnaryPredicate > ForwardIt remove_if( ForwardIt first, ForwardIt last, UnaryPredicate p );	(3)
template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate > ForwardIt remove_if( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryPredicate 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.

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>> is true

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). 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.

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 signature of the predicate function should be equivalent to the following: `bool pred(const Type &a);` The signature does not need to have `const &`, but the function must not modify the objects passed to it. The type `Type` must be such that an object of type `ForwardIt` can be dereferenced and then implicitly converted to `Type`.
Type requirements
-`ForwardIt` must meet the requirements of `ForwardIterator`.
-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 three 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

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 ForwardIt does not dereference to a MoveAssignable type (the keys in these containers are not modifiable).

The standard library also defines an overload of std::remove taking const char*, used to delete files: std::remove.

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; }

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.

#include <algorithm>
#include <string>
#include <iostream>
#include <cctype>
 
int main()
{
    std::string str1 = "Text with some   spaces";
    str1.erase(std::remove(str1.begin(), str1.end(), ' '),
               str1.end());
    std::cout << str1 << '\n';
 
    std::string str2 = "Text\n with\tsome \t  whitespaces\n\n";
    str2.erase(std::remove_if(str2.begin(), 
                              str2.end(),
                              [](char x){return std::isspace(x);}),
               str2.end());
    std::cout << str2 << '\n';
}

Output:

Textwithsomespaces
Textwithsomewhitespaces

remove_copyremove_copy_if	copies a range of elements omitting those that satisfy specific criteria (function template)
unique	removes consecutive duplicate elements in a range (function template)