Difference between revisions of "cpp/algorithm/ranges/remove copy"

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Latest revision as of 04:07, 12 February 2025

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.

1 Parameters
2 Return value
3 Complexity
4 Notes
5 Possible implementation
6 Example
7 See also

[edit] Parameters

first, last	-	the iterator-sentinel pair defining the source range of elements to process
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

[edit] Return value

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

[edit] Complexity

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

[edit] 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)

[edit] Possible implementation

remove_copy (1,2)

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 (3,4)

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 {};

[edit] 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)

[edit] See also

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

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