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std::ranges::uninitialized_move, std::ranges::uninitialized_move_result

From cppreference.com
< cpp‎ | memory
 
 
Memory management library
(exposition only*)
Allocators
Uninitialized memory algorithms
Constrained uninitialized memory algorithms
Memory resources
Uninitialized storage (until C++20)
(until C++20*)
(until C++20*)

Garbage collector support (until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
 
Defined in header <memory>
Call signature
template< std::input_iterator I, std::sentinel_for<I> S1,

          no-throw-forward-iterator O, no-throw-sentinel-for<O> S2 >
    requires std::constructible_from<std::iter_value_t<O>,
                                     std::iter_rvalue_reference_t<I>>
uninitialized_move_result<I, O>

    uninitialized_move( I ifirst, S1 ilast, O ofirst, S2 olast );
(1) (since C++20)
(constexpr since C++26)
template< ranges::input_range IR, no-throw-forward-range OR >

    requires std::constructible_from
                 <ranges::range_value_t<OR>,
                  ranges::range_rvalue_reference_t<IR>>
uninitialized_move_result<ranges::borrowed_iterator_t<IR>,
                          ranges::borrowed_iterator_t<OR>>

    uninitialized_move( IR&& in_range, OR&& out_range );
(2) (since C++20)
(constexpr since C++26)
Helper types
template< class I, class O >
using uninitialized_move_result = ranges::in_out_result<I, O>;
(3) (since C++20)

Let N be ranges::min(ranges::distance(ifirst, ilast), ranges::distance(ofirst, olast)).

1) Copies N elements from [ifirstilast) (using move semantics if supported) to an uninitialized memory area [ofirstolast) as if by

for (; ifirst != ilast && ofirst != olast; ++ofirst, (void)++ifirst)
    ::new (voidify(*ofirst))
        std::remove_reference_t<std::iter_reference_t<O>>(ranges::iter_move(ifirst));
return {std::move(ifirst), ofirst};

If an exception is thrown during the initialization then the objects that already constructed in [ofirstolast) are destroyed in an unspecified order. Also, the objects in [ifirstilast) that were already moved, are left in a valid but unspecified state.
2) Equivalent to return ranges::uninitialized_move(ranges::begin(in_range), ranges::end(in_range),
                                  ranges::begin(out_range), ranges::end(out_range));
.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

Contents

[edit] Parameters

ifirst, ilast - the iterator-sentinel pair defining the input range of elements to move from
in_range - the input range of elements to move from
ofirst, olast - the iterator-sentinel pair defining the output range of elements to initialize
out_range - the output range to initialize

[edit] Return value

As described above.

[edit] Complexity

Linear in N.

[edit] Exceptions

Any exception thrown on construction of the elements in the destination range.

[edit] Notes

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

Feature-test macro Value Std Feature
__cpp_lib_raw_memory_algorithms 202411L (C++26) constexpr for specialized memory algorithms, (1,2)

[edit] Possible implementation

struct uninitialized_move_fn
{
    template<std::input_iterator I, std::sentinel_for<I> S1,
             no-throw-forward-iterator O, no-throw-sentinel-for<O> S2>
        requires std::constructible_from<std::iter_value_t<O>,
                                         std::iter_rvalue_reference_t<I>>
    constexpr ranges::uninitialized_move_result<I, O>
        operator()(I ifirst, S1 ilast, O ofirst, S2 olast) const
    {
        using ValueType = std::remove_reference_t<std::iter_reference_t<O>>;
        O current{ofirst};
        try
        {
            for (; !(ifirst == ilast or current == olast); ++ifirst, ++current)
                ::new (static_cast<void*>(std::addressof(*current))))
                    ValueType(ranges::iter_move(ifirst));
            return {std::move(ifirst), std::move(current)};
        }
        catch (...) // rollback: destroy constructed elements
        {
            for (; ofirst != current; ++ofirst)
                ranges::destroy_at(std::addressof(*ofirst));
            throw;
        }
    }
 
    template<ranges::input_range IR, no-throw-forward-range OR>
        requires std::constructible_from<ranges::range_value_t<OR>,
                                         ranges::range_rvalue_reference_t<IR>>
    constexpr ranges::uninitialized_move_result<ranges::borrowed_iterator_t<IR>,
                                                ranges::borrowed_iterator_t<OR>>
        operator()(IR&& in_range, OR&& out_range) const
    {
        return (*this)(ranges::begin(in_range), ranges::end(in_range),
                       ranges::begin(out_range), ranges::end(out_range));
    }
};
 
inline constexpr uninitialized_move_fn uninitialized_move{};

[edit] Example

#include <cstdlib>
#include <iomanip>
#include <iostream>
#include <memory>
#include <string>
 
void print(auto rem, auto first, auto last)
{
    for (std::cout << rem; first != last; ++first)
        std::cout << std::quoted(*first) << ' ';
    std::cout << '\n';
}
 
int main()
{
    std::string in[]{"Home", "World"};
    print("initially, in: ", std::begin(in), std::end(in));
 
    if (constexpr auto sz = std::size(in);
        void* out = std::aligned_alloc(alignof(std::string), sizeof(std::string) * sz))
    {
        try
        {
            auto first{static_cast<std::string*>(out)};
            auto last{first + sz};
            std::ranges::uninitialized_move(std::begin(in), std::end(in), first, last);
 
            print("after move, in: ", std::begin(in), std::end(in));
            print("after move, out: ", first, last);
 
            std::ranges::destroy(first, last);
        }
        catch (...)
        {
            std::cout << "Exception!\n";
        }
        std::free(out);
    }
}

Possible output:

initially, in: "Home" "World"
after move, in: "" ""
after move, out: "Home" "World"

[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 3870 C++20 this algorithm might create objects on a const storage kept disallowed

[edit] See also

moves a number of objects to an uninitialized area of memory
(algorithm function object)[edit]
moves a range of objects to an uninitialized area of memory
(function template) [edit]