std::ranges:: remove_copy, std::ranges:: remove_copy_if, std::ranges:: remove_copy_result, std::ranges:: remove_copy_if_result

From cppreference.com

Defined in header `<algorithm>`
Call signature
	(1)
template < std:: input_iterator I, std:: sentinel_for < I > S, std:: weakly_incrementable O, class T, class Proj = std:: identity > requires std:: indirectly_copyable < I, O > && std:: indirect_binary_predicate < ranges:: equal_to , std :: projected < I, Proj > , const T * > constexpr remove_copy_result < I, O > remove_copy ( I first, S last, O result, const T & value, Proj proj = { } ) ;			(since C++20) (until C++26)
template < std:: input_iterator I, std:: sentinel_for < I > S, std:: weakly_incrementable O, class Proj = std:: identity , class T = std :: projected_value_t < I, Proj > > requires std:: indirectly_copyable < I, O > && std:: indirect_binary_predicate < ranges:: equal_to , std :: projected < I, Proj > , const T * > constexpr remove_copy_result < I, O > remove_copy ( I first, S last, O result, const T & value, Proj proj = { } ) ;			(since C++26)
		(2)
template < ranges:: input_range R, std:: weakly_incrementable O, class T, class Proj = std:: identity > requires std:: indirectly_copyable < ranges:: iterator_t < R > , O > && std:: indirect_binary_predicate < ranges:: equal_to , std :: projected < ranges:: iterator_t < R > , Proj > , const T * > constexpr remove_copy_result < ranges:: borrowed_iterator_t < R > , O > remove_copy ( R && r, O result, const T & value, Proj proj = { } ) ;				(since C++20) (until C++26)
template < ranges:: input_range R, std:: weakly_incrementable O, class Proj = std:: identity , class T = std :: projected_value_t < ranges:: iterator_t < R > , Proj > > requires std:: indirectly_copyable < ranges:: iterator_t < R > , O > && std:: indirect_binary_predicate < ranges:: equal_to , std :: projected < ranges:: iterator_t < R > , Proj > , const T * > constexpr remove_copy_result < ranges:: borrowed_iterator_t < R > , O > remove_copy ( R && r, O result, const T & value, Proj proj = { } ) ;				(since C++26)
template < std:: input_iterator I, std:: sentinel_for < I > S, std:: weakly_incrementable O, class Proj = std:: identity , std:: indirect_unary_predicate < std :: projected < I, Proj >> Pred > requires std:: indirectly_copyable < I, O > constexpr remove_copy_if_result < I, O > remove_copy_if ( I first, S last, O result, Pred pred, Proj proj = { } ) ;			(3)	(since C++20)
template < ranges:: input_range R, std:: weakly_incrementable O, class Proj = std:: identity , std:: indirect_unary_predicate < std :: projected < ranges:: iterator_t < R > , Proj >> Pred > requires std:: indirectly_copyable < ranges:: iterator_t < R > , O > constexpr remove_copy_if_result < ranges:: borrowed_iterator_t < R > , O > remove_copy_if ( R && r, O result, Pred pred, Proj proj = { } ) ;			(4)	(since C++20)
Helper types
template < class I, class O > using remove_copy_result = ranges:: in_out_result < I, O > ;			(5)	(since C++20)
template < class I, class O > using remove_copy_if_result = ranges:: in_out_result < I, O > ;			(6)	(since C++20)

Copies elements from the source range [ first , last ) , to the destination range beginning at result , omitting the elements which (after being projected by proj ) satisfy specific criteria. The behavior is undefined if the source and destination ranges overlap.

1) Ignores all elements that are equal to value .

3) Ignores all elements for which predicate pred returns true .

2,4) Same as (1,3) , but uses r as the source range, as if using ranges:: begin ( r ) as first , and ranges:: end ( r ) as last .

The function-like entities described on this page are algorithm function objects (informally known as niebloids ), that is:

Explicit template argument lists cannot be specified when calling any of them.
None of them are visible to argument-dependent lookup .
When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

Parameters

first, last	-	the source range of elements
r	-	the source range of elements
result	-	the beginning of the destination range
value	-	the value of the elements not to copy
comp	-	the binary predicate to compare the projected elements
proj	-	the projection to apply to the elements

Return value

{ last, result + N } , where N is the number of elements copied.

Complexity

Exactly ranges:: distance ( first, last ) applications of the corresponding predicate comp and any projection proj .

Notes

The algorithm is stable, i.e. preserves the relative order of the copied elements.

Feature-test macro	Value	Std	Feature
`__cpp_lib_algorithm_default_value_type`	202403	(C++26)	List-initialization for algorithms ( 1,2 )

Possible implementation

remove_copy

struct remove_copy_fn
{
    template<std::input_iterator I, std::sentinel_for<I> S,
             std::weakly_incrementable O, class Proj = std::identity,
             class T = std::projected_value_t<I, Proj>>
    requires std::indirectly_copyable<I, O> &&
             std::indirect_binary_predicate<ranges::equal_to,
                                            std::projected<I, Proj>, const T*>
    constexpr ranges::remove_copy_result<I, O>
        operator()(I first, S last, O result, const T& value, Proj proj = {}) const
    {
        for (; !(first == last); ++first)
            if (value != std::invoke(proj, *first))
            {
                *result = *first;
                ++result;
            }
        return {std::move(first), std::move(result)};
    }
 
    template<ranges::input_range R, 
             std::weakly_incrementable O, class Proj = std::identity,
             class T = std::projected_value_t<ranges::iterator_t<R>, Proj>>
    requires std::indirectly_copyable<ranges::iterator_t<R>, O> &&
             std::indirect_binary_predicate<ranges::equal_to,
             std::projected<ranges::iterator_t<R>, Proj>, const T*>
    constexpr ranges::remove_copy_result<ranges::borrowed_iterator_t<R>, O>
        operator()(R&& r, O result, const T& value, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::move(result), value,
                       std::move(proj));
    }
};
 
inline constexpr remove_copy_fn remove_copy {};

remove_copy_if

struct remove_copy_if_fn
{
    template<std::input_iterator I, std::sentinel_for<I> S, std::weakly_incrementable O,
             class Proj = std::identity,
             std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
    requires std::indirectly_copyable<I, O>
    constexpr ranges::remove_copy_if_result<I, O>
        operator()(I first, S last, O result, Pred pred, Proj proj = {}) const
    {
        for (; first != last; ++first)
            if (false == std::invoke(pred, std::invoke(proj, *first)))
            {
                *result = *first;
                ++result;
            }
        return {std::move(first), std::move(result)};
    }
 
    template<ranges::input_range R, std::weakly_incrementable O,
             class Proj = std::identity,
             std::indirect_unary_predicate<
                 std::projected<ranges::iterator_t<R>, Proj>> Pred>
    requires std::indirectly_copyable<ranges::iterator_t<R>, O>
    constexpr ranges::remove_copy_if_result<ranges::borrowed_iterator_t<R>, O>
        operator()(R&& r, O result, Pred pred, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::move(result),
                       std::move(pred), std::move(proj));
    }
};
 
inline constexpr remove_copy_if_fn remove_copy_if {};

Example

Run this code

#include <algorithm>
#include <array>
#include <complex>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <string_view>
#include <vector>
 
void println(const auto rem, const auto& v)
{
    std::cout << rem << ' ';
    for (const auto& e : v)
        std::cout << e << ' ';
    std::cout << '\n';
}
 
int main()
{
    // Filter out the hash symbol from the given string.
    const std::string_view str{"#Small #Buffer #Optimization"};
    std::cout << "before: " << std::quoted(str) << '\n';
 
    std::cout << "after:  \"";
    std::ranges::remove_copy(str.begin(), str.end(),
                             std::ostream_iterator<char>(std::cout), '#');
    std::cout << "\"\n";
 
    // Copy only the complex numbers with positive imaginary part.
    using Ci = std::complex<int>;
    constexpr std::array<Ci, 5> source
    {
        Ci{1, 0}, Ci{0, 1}, Ci{2, -1}, Ci{3, 2}, Ci{4, -3}
    };
    std::vector<std::complex<int>> target;
 
    std::ranges::remove_copy_if
    (
        source,
        std::back_inserter(target),
        [](int imag) { return imag <= 0; },
        [](Ci z) { return z.imag(); }
    );
 
    println("source:", source);
    println("target:", target);
 
    std::vector<std::complex<float>> nums{{2, 2}, {1, 3}, {4, 8}, {1, 3}};
    std::vector<std::complex<double>> outs;
    #ifdef __cpp_lib_algorithm_default_value_type
        std::remove_copy(nums.cbegin(), nums.cend(), std::back_inserter(outs),
                         {1, 3}); // T gets deduced to std::complex<float>
    #else
        std::remove_copy(nums.cbegin(), nums.cend(), std::back_inserter(outs),
                         std::complex<float>{1, 3});
    #endif
    println("nums:  ", nums);
    println("outs:  ", outs);
}

Output:

before: "#Small #Buffer #Optimization"
after:  "Small Buffer Optimization"
source: (1,0) (0,1) (2,-1) (3,2) (4,-3)
target: (0,1) (3,2)
nums:   (2,2) (1,3) (4,8) (1,3)
outs:   (2,2) (4,8)

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

ranges::remove ranges::remove_if (C++20) (C++20)	removes elements satisfying specific criteria (algorithm function object)
ranges::copy ranges::copy_if (C++20) (C++20)	copies a range of elements to a new location (algorithm function object)
ranges::copy_n (C++20)	copies a number of elements to a new location (algorithm function object)
ranges::copy_backward (C++20)	copies a range of elements in backwards order (algorithm function object)
ranges::replace_copy ranges::replace_copy_if (C++20) (C++20)	copies a range, replacing elements satisfying specific criteria with another value (algorithm function object)
ranges::reverse_copy (C++20)	creates a copy of a range that is reversed (algorithm function object)
ranges::rotate_copy (C++20)	copies and rotate a range of elements (algorithm function object)
ranges::unique_copy (C++20)	creates a copy of some range of elements that contains no consecutive duplicates (algorithm function object)
remove_copy remove_copy_if	copies a range of elements omitting those that satisfy specific criteria (function template)