std:: remove, std:: remove_if

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.

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 .

Parameters

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 ) :

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

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;
}

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 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 ) .

Example

#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}}));
}

Textwithsomespaces size: 23
Textwithsomespaces size: 18
Textwithsomewhitespaces

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also