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std::copy, std::copy_if

From cppreference.com
< cpp‎ | algorithm
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
(C++17)
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
copycopy_if
(C++11)
(C++11)
(C++11)
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
(C++20)(C++20)
Sampling operations
(C++17)

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
 
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 UnaryPredicate >

OutputIt copy_if( InputIt first, InputIt last, OutputIt d_first,

                  UnaryPredicate pred );
(3) (since C++11)
(constexpr since C++20)
template< class ExecutionPolicy,

          class ForwardIt1, class ForwardIt2, class UnaryPredicate >
ForwardIt2 copy_if( ExecutionPolicy&& policy,
                    ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first,

                    UnaryPredicate pred );
(4) (since C++17)

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

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

Contents

[edit] 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.
-
UnaryPredicate must meet the requirements of Predicate.

[edit] Return value

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

[edit] Complexity

Given 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.

[edit] 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.

[edit] 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).

[edit] 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 UnaryPredicate>
OutputIt copy_if(InputIt first, InputIt last,
                 OutputIt d_first, UnaryPredicate pred)
{
    for (; first != last; ++first)
        if (pred(*first))
        {
            *d_first = *first;
            ++d_first;
        }
 
    return d_first;
}

[edit] 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.

#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

[edit] 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

[edit] See also

copies a range of elements in backwards order
(function template) [edit]
creates a copy of a range that is reversed
(function template) [edit]
(C++11)
copies a number of elements to a new location
(function template) [edit]
copy-assigns the given value to every element in a range
(function template) [edit]
copies a range of elements omitting those that satisfy specific criteria
(function template) [edit]
copies a range of elements to a new location
(niebloid)[edit]