NAME¶

std::ranges::uninitialized_move_n,std::ranges::uninitialized_move_n_result - std::ranges::uninitialized_move_n,std::ranges::uninitialized_move_n_result

Synopsis¶

Defined in header <memory>
Call signature
template< std::input_iterator I, no-throw-forward-iterator O,

no-throw-sentinel-for<O> S >
requires std::constructible_from<std::iter_value_t<O>,
std::iter_rvalue_reference_t<I>> (1) (since C++20)
uninitialized_move_n_result<I, O>

uninitialized_move_n( I ifirst, std::iter_difference_t<I> n, O
ofirst, S olast );

Helper types¶

template< class I, class O > (2) (since C++20)
using uninitialized_move_n_result = ranges::in_out_result<I, O>;

Moves N elements from the input range beginning at ifirst to the uninitialized
storage designated by the range [ofirst, olast), where N is min(n,
ranges::distance(ofirst, olast)).

The effect is equivalent to:

for (; n-- > 0 && ofirst != olast; ++ifirst, ++ofirst)
::new (static_cast<void*>(std::addressof(*first)))
std::remove_reference_t<std::iter_reference_t<O>>(ranges::iter_move(ifirst));

If an exception is thrown during the initialization then the objects that already
constructed in [ofirst, olast) are destroyed in an unspecified order. Also, the
objects in the input range beginning at ifirst, that were already moved, are left in
a valid but unspecified state.

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¶

ifirst - the beginning of the input range of elements to move from
ofirst, olast - iterator-sentinel pair denoting the output range of elements to
initialize
n - the number of elements to move

Return value¶

{ifirst + N, ofirst + N}.

Complexity¶

Linear in 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_move_n,
e.g. by using ranges::copy_n, if the value type of the output range is TrivialType.

Possible implementation¶

struct uninitialized_move_n_fn
{
template<std::input_iterator I, no-throw-forward-iterator O,
no-throw-sentinel-for<O> S>
requires std::constructible_from<std::iter_value_t<O>,
std::iter_rvalue_reference_t<I>>
ranges::uninitialized_move_n_result<I, O>
operator()(I ifirst, std::iter_difference_t<I> n, O ofirst, S olast) const
{
O current{ofirst};
try
{
for (; n-- > 0 && current != olast; ++ifirst, ++current)
::new (const_cast<void*>(static_cast<const volatile void*>
(std::addressof(*current)))) std::remove_reference_t<
std::iter_reference_t<O>>(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;
}
}
};

inline constexpr uninitialized_move_n_fn uninitialized_move_n{};

Example¶

// Run this code

#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[]{ "No", "Diagnostic", "Required", };
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_n(std::begin(in), sz, 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: "No" "Diagnostic" "Required"
after move, in: "" "" ""
after move, out: "No" "Diagnostic" "Required"

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 kept disallowed
storage

See also¶

ranges::uninitialized_move moves a range of objects to an uninitialized area of
(C++20) memory
(niebloid)
uninitialized_move_n moves a number of objects to an uninitialized area of
(C++17) memory
(function template)