std::ranges::copy_if() algorithm
- od C++20
- Simplified
- Detailed
// (1)
constexpr copy_if_result<I, O>
copy_if( I first, S last, O result, Pred pred, Proj proj = {} );
// (2)
constexpr copy_if_result<ranges::borrowed_iterator_t<R>, O>
copy_if( R&& r, O result, Pred pred, Proj proj = {} );
The type of arguments are generic and have following constraints:
I
-std::input_iterator
S
-std::sentinel_for<I>
O
-std::weakly_incrementable
Proj
- (none)Pred
:- (1) -
std::indirect_unary_predicate<std::projected<I, Proj>>
- (2) -
std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>>
- (1) -
The Proj
template argument has a default type of std::identity
for all overloads.
Additionally, each overload has the following constraints:
- (1) -
std::indirectly_copyable<I, O>
- (2) -
std::indirectly_copyable<ranges::iterator_t<R>, O>
// (1)
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 = {} );
// (2)
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 = {} );
With the helper types defined as follows:
template< class I, class O >
using copy_if_result = ranges::in_out_result<I, O>;
Copies the elements in the range, defined by [first
; last
), to another range beginning at result
.
-
(1) Only copies the elements for which the predicate
pred
returnstrue
. The relative order of the elements that are copied is preserved.Undefined BehaviourThe behavior is undefined if
result
is within the range [first
;last
). -
(2) Same as (1), but uses
r
as the source range, as if usingranges::begin(r)
asfirst
andranges::end(r)
aslast
.
The function-like entities described on this page are niebloids.
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
Exactly last - first
assignments.
Exactly last - first
applications of the predicate and projection, between 0
and last - first
assignments (assignment for every element for which predicate returns true
).
Exceptions
(none)
Possible implementation
copy_if(1) and copy_if(2)
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;
Notes
In practice, implementations of ranges::copy_if
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
.
Examples
The following code uses ranges::copy_if
to both copy the contents of one std::vector
to another and to display the resulting std::vector
:
#include <algorithm>
#include <iostream>
#include <iterator>
#include <numeric>
#include <vector>
int main()
{
std::vector<int> source(10);
std::iota(source.begin(), source.end(), 0);
std::vector<int> destination;
std::ranges::copy_if(source.begin(), source.end(),
std::back_inserter(destination));
// or, alternatively,
// std::vector<int> destination(source.size());
// std::ranges::copy_if(source.begin(), source.end(), destination.begin());
// either way is equivalent to
// std::vector<int> destination = source;
std::cout << "destination contains: ";
std::ranges::copy_if(destination, std::ostream_iterator<int>(std::cout, " "));
std::cout << '\n';
std::cout << "odd numbers in destination are: ";
std::ranges::copy_if_if(destination, std::ostream_iterator<int>(std::cout, " "),
[](int x) { return (x % 2) == 1; });
std::cout << '\n';
}
destination contains: 0 1 2 3 4 5 6 7 8 9
odd numbers in destination are: 1 3 5 7 9
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