std::ranges::copy, std::ranges::copy_if, std::ranges::copy_result, std::ranges::copy_if_result

Defined in header `<algorithm>`
Call signature
template< std::input_iterator I, std::sentinel_for<I> S, std::weakly_incrementable O > requires std::indirectly_copyable<I, O> constexpr copy_result<I, O> copy( I first, S last, O result );	(1)	(since C++20)
template< ranges::input_range R, std::weakly_incrementable O > requires std::indirectly_copyable<ranges::iterator_t<R>, O> constexpr copy_result<ranges::borrowed_iterator_t<R>, O> copy( R&& r, O result );	(2)	(since C++20)
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 copy_if_result<I, O> 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 copy_if_result<ranges::borrowed_iterator_t<R>, O> copy_if( R&& r, O result, Pred pred, Proj proj = {} );	(4)	(since C++20)
Helper types
template< class I, class O > using copy_result = ranges::in_out_result<I, O>;	(5)	(since C++20)
template< class I, class O > using copy_if_result = ranges::in_out_result<I, O>;	(6)	(since C++20)

Copies the elements in the range, defined by [first, last), to another range beginning at result.

1) Copies all elements in the range [first, last) starting from first and proceeding to last - 1. The behavior is undefined if result is within the range [first, last). In this case, ranges::copy_backward may be used instead.

3) Only copies the elements for which the predicate pred returns true. The relative order of the elements that are copied is preserved. The behavior is undefined if the source and the destination ranges overlap.

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 niebloids, that is:

Explicit template argument lists may not be specified when calling any of them.
None of them is visible to argument-dependent lookup.
When one of them is found by normal unqualified lookup for the name to the left of the function-call operator, it inhibits argument-dependent lookup.

In practice, they may be implemented as function objects, or with special compiler extensions.

Parameters

first, last	-	the range of elements to copy
r	-	the range of elements to copy
result	-	the beginning of the destination range.
pred	-	predicate to apply to the projected elements
proj	-	projection to apply to the elements

Return value

A ranges::in_out_result containing an input iterator equal to last and an output iterator past the last element copied.

Complexity

1-2) Exactly (last - first) assignments

3-4) Exactly (last - first) applications of the predicate and projection, between 0 and (last - first) assignments (assignment for every element for which predicate returns true, dependent on predicate and input data)

Notes

In practice, implementations of std::ranges::copy avoid multiple assignments and use bulk copy functions such as std::memmove if the value type is TriviallyCopyable and the iterator types satisfy contiguous_iterator.

When copying overlapping ranges, std::ranges::copy is appropriate when copying to the left (beginning of the destination range is outside the source range) while std::ranges::copy_backward is appropriate when copying to the right (end of the destination range is outside the source range).

Possible implementation

First version

struct copy_fn {
  template< std::input_iterator I, std::sentinel_for<I> S, std::weakly_incrementable O >
  requires std::indirectly_copyable<I, O>
  constexpr ranges::copy_result<I, O> operator()( I first, S last, O result ) const
  {
      for (; first != last; ++first, (void)++result) {
          *result = *first;
      }
      return {std::move(first), std::move(result)};
  }
 
  template< ranges::input_range R, std::weakly_incrementable O >
  requires std::indirectly_copyable<ranges::iterator_t<R>, O>
  constexpr ranges::copy_result<ranges::borrowed_iterator_t<R>, O>
  operator()( R&& r, O result ) const
  {
      return (*this)(ranges::begin(r), ranges::end(r), std::move(result));
  }
};
 
inline constexpr copy_fn copy;

Second version

struct 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::copy_if_result<I, O>
  operator()( I first, S last, O result, Pred pred, Proj proj = {} ) const
  {
      for (; first != last; ++first) {
          if (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::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::ref(pred), std::ref(proj));
  }
};
 
inline constexpr copy_if_fn copy_if;

Example

The following code uses copy to both copy the contents of one vector to another and to display the resulting vector:

Run this code

#include <algorithm>
#include <iostream>
#include <vector>
#include <iterator>
#include <numeric>
 
int main()
{
    std::vector<int> from_vector(10);
    std::iota(from_vector.begin(), from_vector.end(), 0);
 
    std::vector<int> to_vector;
 
    namespace ranges = std::ranges;
    ranges::copy(from_vector.begin(), from_vector.end(),
                 std::back_inserter(to_vector));
// or, alternatively,
//  std::vector<int> to_vector(from_vector.size());
//  ranges::copy(from_vector.begin(), from_vector.end(), to_vector.begin());
// either way is equivalent to
//  std::vector<int> to_vector = from_vector;
 
    std::cout << "to_vector contains: ";
 
    ranges::copy(to_vector, std::ostream_iterator<int>(std::cout, " "));
    std::cout << '\n';
 
    std::cout << "odd numbers in to_vector are: ";
 
    ranges::copy_if(to_vector, std::ostream_iterator<int>(std::cout, " "),
                 [](int x) { return (x % 2) == 1; });
    std::cout << '\n';
}

Output:

to_vector contains: 0 1 2 3 4 5 6 7 8 9 
odd numbers in to_vector are: 1 3 5 7 9

Compiler support
Freestanding and hosted
Language
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)
Technical specifications
Symbols index
External libraries

ranges::copy_backward (C++20)	copies a range of elements in backwards order (niebloid)
ranges::reverse_copy (C++20)	creates a copy of a range that is reversed (niebloid)
ranges::copy_n (C++20)	copies a number of elements to a new location (niebloid)
ranges::fill (C++20)	assigns a range of elements a certain value (niebloid)
ranges::remove_copyranges::remove_copy_if (C++20)(C++20)	copies a range of elements omitting those that satisfy specific criteria (niebloid)
copycopy_if (C++11)	copies a range of elements to a new location (function template)