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std::remove, std::remove_if - cppreference.com

template< class ForwardIt, class T >
ForwardIt remove( ForwardIt first, ForwardIt last, const T& value );

template< class ForwardIt, class UnaryPred >
ForwardIt remove_if( 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 all following conditions are satisfied:

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 elements that were originally in that range.

The expression p(v) must be convertible to bool for every argument v 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). ​

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).

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.

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.

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;
}

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:

• 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).

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 <cassert>
#include <cctype>
#include <complex>
#include <iomanip>
#include <iostream>
#include <string>
#include <string_view>
#include <vector>
 
int main()
{
    std::string str1{"Quick  Red  Dog"};
    std::cout << "1) " << std::quoted(str1) << '\n';
    const auto noSpaceEnd = std::remove(str1.begin(), str1.end(), ' ');
    std::cout << "2) " << std::quoted(str1) << '\n';
 
    // The spaces are removed from the string only logically.
    // Note, we use view, the original string is still not shrunk:
    std::cout << "3) " << std::quoted(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 << "4) " << std::quoted(str1) << ", size: " << str1.size() << '\n';
 
    std::string str2 = "Jumped\n Over\tA\vLazy \t  Fox\r\n";
    str2.erase(std::remove_if(str2.begin(), 
                              str2.end(),
                              [](unsigned char x) { return std::isspace(x); }),
               str2.end());
    std::cout << "5) " << std::quoted(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:

1) "Quick  Red  Dog"
2) "QuickRedDog Dog"
3) "QuickRedDog", size: 15
4) "QuickRedDog", size: 11
5) "JumpedOverALazyFox"

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

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