std::uninitialized_copy(3)

✨ std::uninitialized_copy ✨

Okay bestie, welcome to the C++ Standard Library manual! This page is all about std::uninitialized_copy, and it's literally iconic. No cap, this function is the moment. Let's get into it. 💅

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💖 NAME 💖

std::uninitialized_copy — like, it copies elements into uninitialized memory. Slay.

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✨ Synopsis ✨

Defined in header <memory>

template< class InputIt, class NoThrowForwardIt >
NoThrowForwardIt uninitialized_copy( InputIt first, InputIt last,
                                     NoThrowForwardIt d_first );

template< class ExecutionPolicy, class ForwardIt, class NoThrowForwardIt >
NoThrowForwardIt uninitialized_copy( ExecutionPolicy&& policy,
                                     ForwardIt first, ForwardIt last,
                                     NoThrowForwardIt d_first );

(2) (since C++17)

1. Copies elements from the range [first, last) to an uninitialized memory area beginning at d_first as if by:

for (; first != last; ++d_first, (void) ++first)
    ::new (/* VOIDIFY */(*d_first))
        typename std::iterator_traits<NoThrowForwardIt>::value_type(*first);

where /* VOIDIFY */(e) is:

static_cast<void*>(&e)                (until C++11)
static_cast<void*>(std::addressof(e)) (since C++11)

If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order. Ngl, that's important.

If d_first + [0, std::distance(first, last)) overlaps with [first, last), the behavior is undefined. (since C++20) So don't do that, bestie.

2. Same as (1), 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) Or std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. (since C++20)

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🌸 Parameters 🌸

• first, last — the range of the elements to copy
• d_first — the beginning of the destination range
• policy — the execution policy to use. See execution policy for details.

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🦋 Type requirements 🦋

• InputIt must meet the requirements of LegacyInputIterator.
• ForwardIt must meet the requirements of LegacyForwardIterator.
• NoThrowForwardIt must meet the requirements of LegacyForwardIterator.
• No increment, assignment, comparison, or indirection through valid instances of NoThrowForwardIt may throw exceptions. Applying &* to a NoThrowForwardIt value must yield a pointer to its value type. (until C++11)

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🎀 Return value 🎀

Iterator to the element past the last element copied.

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🔥 Complexity 🔥

Linear in the distance between first and last.

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💅 Exceptions 💅

The overload with a template parameter named ExecutionPolicy reports 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.

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👑 Possible implementation 👑

template<class InputIt, class NoThrowForwardIt>
NoThrowForwardIt uninitialized_copy(InputIt first, InputIt last, NoThrowForwardIt d_first)
{
    using T = typename std::iterator_traits<NoThrowForwardIt>::value_type;
    NoThrowForwardIt current = d_first;
    try
    {
        for (; first != last; ++first, (void) ++current)
            ::new (static_cast<void*>(std::addressof(*current))) T(*first);
        return current;
    }
    catch (...)
    {
        for (; d_first != current; ++d_first)
            d_first->~T();
        throw;
    }
}

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🌈 Example 🌈

// Run this code

#include <cstdlib>
#include <iostream>
#include <memory>
#include <string>

int main()
{
    const char *v[] = {"This", "is", "an", "example"};

    auto sz = std::size(v);

    if (void *pbuf = std::aligned_alloc(alignof(std::string), sizeof(std::string) * sz))
    {
        try
        {
            auto first = static_cast<std::string*>(pbuf);
            auto last = std::uninitialized_copy(std::begin(v), std::end(v), first);

            for (auto it = first; it != last; ++it)
                std::cout << *it << '_';
            std::cout << '\n';

            std::destroy(first, last);
        }
        catch (...) {}
        std::free(pbuf);
    }
}

Output:

This_is_an_example_

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⭐ Defect reports ⭐

The following behavior-changing defect reports were applied retroactively to previously published C++ standards. So like, these are the receipts.

• LWG 866 (C++98): Given T as the value type of NoThrowForwardIt, if T::operator new exists, the program might be ill-formed. → Correct behavior: uses global replacement-new instead.
• LWG 2133 (C++98): The effect description used a for loop with the iteration expression ++d_first, ++first, which results in an argument-dependent lookup of operator,. → Correct behavior: discards the value of one operand to disable that ADL.
• LWG 2433 (C++11): This algorithm might be hijacked by overloaded operator&. → Correct behavior: uses std::addressof.
• LWG 3870 (C++20): This algorithm might create objects on a const storage. → Correct behavior: kept disallowed.

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🫶 See also 🫶

• uninitialized_copy_n (C++11) — copies a number of objects to an uninitialized area of memory (function template)
• ranges::uninitialized_copy (C++20) — copies a range of objects to an uninitialized area of memory (niebloid)

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This page is from the C++ Standard Library manual, version 2024.06.10, last updated on 2024.06.10. Check out http://cppreference.com for more. Love you, bestie! 💖✨