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)