std::copy, std::copy_if

Defined in header `<algorithm>`
template< class InputIt, class OutputIt > OutputIt copy( InputIt first, InputIt last, OutputIt d_first );	(1)	(constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > ForwardIt2 copy( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first );	(2)	(since C++17)
template< class InputIt, class OutputIt, class UnaryPred > OutputIt copy_if( InputIt first, InputIt last, OutputIt d_first, UnaryPred pred );	(3)	(since C++11) (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class UnaryPred > ForwardIt2 copy_if( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first, UnaryPred pred );	(4)	(since C++17)

Copies the elements in the range, defined by [first, last), to another range beginning at d_first (copy destination range).

1) Copies all elements in the range [first, last) starting from first and proceeding to last.

If d_first is in [first, last), the behavior is undefined. In this case, std::copy_backward may be used instead.

2) Copies the elements, but executed according to policy.

This overload participates in overload resolution only if

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.	(until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.	(since C++20)

If [first, last) and the copy destination range overlaps, the behavior is undefined.

3) Only copies the elements for which the predicate pred returns true. This copy algorithm is stable: the relative order of the elements that are copied is preserved.

If [first, last) and the copy destination range overlaps, the behavior is undefined.

4) Same as (3), but executed according to policy.

This overload participates in overload resolution only if

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.	(until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.	(since C++20)

Parameters

first, last	-	the range of elements to copy
d_first	-	the beginning of the destination range
policy	-	the execution policy to use. See execution policy for details.
pred	-	unary predicate which returns true for the required elements. The expression pred(v) must be convertible to bool for every argument `v` of type (possibly const) `VT`, where `VT` is the value type of `InputIt`, 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).
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.
- `OutputIt` must meet the requirements of LegacyOutputIterator.
- `ForwardIt1, ForwardIt2` must meet the requirements of LegacyForwardIterator.
- `UnaryPred` must meet the requirements of Predicate.

Return value

Output iterator to the element in the destination range, one past the last element copied.

Complexity

Given $\scriptsize N$ $N$ as std::distance(first, last):

1,2) Exactly

N

assignments.

3,4) Exactly

N

applications of the predicate pred, and at most

N

assignments.

For the overloads with an ExecutionPolicy, there may be a performance cost if ForwardIt1's value type is not MoveConstructible.

Exceptions

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.

Possible implementation

copy (1)

template<class InputIt, class OutputIt>
OutputIt copy(InputIt first, InputIt last,
              OutputIt d_first)
{
    for (; first != last; (void)++first, (void)++d_first)
        *d_first = *first;
 
    return d_first;
}

copy_if (3)

template<class InputIt, class OutputIt, class UnaryPred>
OutputIt copy_if(InputIt first, InputIt last,
                 OutputIt d_first, UnaryPred pred)
{
    for (; first != last; ++first)
        if (pred(*first))
        {
            *d_first = *first;
            ++d_first;
        }
 
    return d_first;
}

Notes

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

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

Example

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

Run this code

#include <algorithm>
#include <iostream>
#include <iterator>
#include <numeric>
#include <vector>
 
int main()
{
    std::vector<int> from_vector(10);
    std::iota(from_vector.begin(), from_vector.end(), 0);
 
    std::vector<int> to_vector;
    std::copy(from_vector.begin(), from_vector.end(),
              std::back_inserter(to_vector));
// or, alternatively,
//  std::vector<int> to_vector(from_vector.size());
//  std::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: ";
 
    std::copy(to_vector.begin(), to_vector.end(),
              std::ostream_iterator<int>(std::cout, " "));
    std::cout << '\n';
 
    std::cout << "odd numbers in to_vector are: ";
 
    std::copy_if(to_vector.begin(), to_vector.end(),
                 std::ostream_iterator<int>(std::cout, " "),
                 [](int x) { return x % 2 != 0; });
    std::cout << '\n';
 
    std::cout << "to_vector contains these multiples of 3: ";
 
    to_vector.clear();
    std::copy_if(from_vector.begin(), from_vector.end(),
                 std::back_inserter(to_vector),
                 [](int x) { return x % 3 == 0; });
 
    for (const int x : to_vector)
        std::cout << x << ' ';
    std::cout << '\n';
}

Possible 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
to_vector contains these multiples of 3: 0 3 6 9

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
LWG 2039	C++11	the return value of `std::copy_if` was not specified	specified
LWG 2044	C++11	the stability of `std::copy_if` was not defined	defined

Compiler support
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Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
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Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
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Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
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Filesystem library (C++17)
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Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

copy_backward	copies a range of elements in backwards order (function template)
reverse_copy	creates a copy of a range that is reversed (function template)
copy_n (C++11)	copies a number of elements to a new location (function template)
fill	copy-assigns the given value to every element in a range (function template)
remove_copyremove_copy_if	copies a range of elements omitting those that satisfy specific criteria (function template)
ranges::copyranges::copy_if (C++20)(C++20)	copies a range of elements to a new location (niebloid)