std::copy,std::copy_if(3)

std::copy,std::copy_if(3) 💅

okay so bestie this is from the C++ Standard Libary manual — yes, the header literally says “Libary” 😭 not my typo, I promise. It’s the page for std::copy and std::copy_if, and we’re gonna copy ranges like it’s our whole personality.

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🌸 NAME 🌸

std::copy,std::copy_if — std::copy,std::copy_if

no thoughts, just the same name twice, iconic 💖

✨ Synopsis ✨

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).

okay now the numbered tea:

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 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.

• LWG 2039 — applied to C++11 — behavior as published: the return value of std::copy_if was not specified; correct behavior: specified.
• LWG 2044 — applied to C++11 — behavior as published: the stability of std::copy_if was not defined; correct behavior: defined.

⭐ See also ⭐

• 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 — copies a number of elements to a new location (C++11) (function template)
• fill — copy-assigns the given value to every element in a range (function template)
• remove_copy, remove_copy_if — copies a range of elements omitting those that satisfy specific criteria (function template)
• ranges::copy, ranges::copy_if — copies a range of elements to a new location (C++20) (niebloid)

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