Difference between revisions of "cpp/memory/ranges/uninitialized copy n"

Revision as of 08:49, 14 August 2021

Let \(\scriptsize N\)N be ranges::min(count, ranges::distance(ofirst, olast)), constructs \(\scriptsize N\)N elements in the output range [ofirst, olast), which is an uninitialized memory area, from the elements in the input range beginning at ifirst.

The input range [ifirst, ifirst + count) must not overlap with the output range [ofirst, olast).

If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.

The function has the effect equivalent to:

auto ret = ranges::uninitialized_copy(std::counted_iterator(ifirst, count),
                                      std::default_sentinel, ofirst, olast);
return {std::move(ret.in).base(), ret.out};

The function-like entities described on this page are niebloids, that is:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

In practice, they may be implemented as function objects, or with special compiler extensions.

Parameters

Return value

{ifirst + N, ofirst + N}.

Complexity

\(\scriptsize\mathcal{O}(N)\)𝓞(N).

Exceptions

The exception thrown on construction of the elements in the destination range, if any.

Notes

An implementation may improve the efficiency of the ranges::uninitialized_copy_n, by using e.g. ranges::copy_n, if the value type of the output range is TrivialType.

Possible implementation

struct uninitialized_copy_n_fn {
    template <std::input_iterator I, no-throw-input-iterator O, no-throw-sentinel-for<O> S>
    requires std::constructible_from<std::iter_value_t<O>, std::iter_reference_t<I>>
    ranges::uninitialized_copy_n_result<I, O>
    operator()( I ifirst, std::iter_difference_t<I> count, O ofirst, S olast ) const {
        O current {ofirst};
        try {
            for (; count > 0 && current != olast; ++ifirst, ++current, --count)
                ranges::construct_at(std::addressof(*current), *ifirst);
            return {std::move(ifirst), std::move(current)};
        } catch (...) { // rollback: destroy constructed elements
            for (; ofirst != current; ++ofirst)
                ranges::destroy_at(std::addressof(*ofirst));
            throw;
        }
    }
};
 
inline constexpr uninitialized_copy_n_fn uninitialized_copy_n{};

Example

#include <iomanip>
#include <iostream>
#include <memory>
#include <string>
 
int main()
{
    const char* stars[] { "Procyon", "Spica", "Pollux", "Deneb", "Polaris", };
 
    constexpr int n {4};
    alignas(alignof(std::string)) char out[n * sizeof(std::string)];
 
    try
    {
        auto first {reinterpret_cast<std::string*>(out)};
        auto last {first + n};
        auto ret {std::ranges::uninitialized_copy_n(std::begin(stars), n, first, last)};
 
        std::cout << "{ ";
        for (auto it {first}; it != ret.out; ++it)
            std::cout << std::quoted(*it) << ", ";
        std::cout << "};\n";
 
        std::ranges::destroy(first, last);
    }
    catch(...)
    {
        std::cout << "uninitialized_copy_n exception\n";
    }
}

{ "Procyon", "Spica", "Pollux", "Deneb", };

See also