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std::uninitialized_move

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< cpp‎ | memory
 
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uninitialized_move
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Defined in header <memory>
template< class InputIt, class NoThrowForwardIt >

NoThrowForwardIt uninitialized_move( InputIt first, InputIt last,

                                     NoThrowForwardIt d_first );
 (1) (since C++17)
(constexpr since C++26)
template< class ExecutionPolicy,

          class ForwardIt, class NoThrowForwardIt >
NoThrowForwardIt uninitialized_move( ExecutionPolicy&& policy,
                                     ForwardIt first, ForwardIt last,

                                     NoThrowForwardIt d_first );
 (2) (since C++17)
1) Copies the elements from [first, last) (using move semantics if supported) 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(/* value */);
return d_first;

where /* value */ is std::move(*first) if *first is of an lvalue reference type, or *first otherwise.
If an exception is thrown during the initialization, some objects in [first, last) are left in a valid but unspecified state, and the objects already constructed are destroyed in an unspecified order.
2) Same as (1), but executed according to policy.
This overload participates in overload resolution only if all following conditions are satisfied:

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 d_first + [​0​, std::distance(first, last)) overlaps with [first, last), the behavior is undefined.

(since C++20)

Contents

[edit] Parameters

first, last - the range of the elements to move
d_first - the beginning of the destination range
policy - the execution policy to use
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.

[edit] Return value

As described above.

[edit] Complexity

Linear in the distance between first and last.

[edit] 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.

[edit] Notes

When the input iterator deferences to an rvalue, the behavior of std::uninitialized_move is same as std::uninitialized_copy.

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

[edit] Possible implementation

template<class InputIt, class NoThrowForwardIt>
constexpr NoThrowForwardIt uninitialized_move(InputIt first, InputIt last,
                                              NoThrowForwardIt d_first)
{
    using ValueType = typename std::iterator_traits<NoThrowForwardIt>::value_type;
    auto current = d_first;
    try
    {
        for (; first != last; ++first, (void) ++current)
        {
            auto addr = static_cast<void*>(std::addressof(*current));
            if constexpr (std::is_lvalue_reference_v<decltype(*first)>)
                ::new (addr) ValueType(std::move(*first));
            else
                ::new (addr) ValueType(*first);
        }
        return current;
    }
    catch (...)
    {
        std::destroy(d_first, current);
        throw;
    }
}

[edit] 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[]{"Home", "Work!"};
    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::uninitialized_move(std::begin(in), std::end(in), first);
 
            print("after move, in: ", std::begin(in), std::end(in));
            print("after move, out: ", first, last);
 
            std::destroy(first, last);
        }
        catch (...)
        {
            std::cout << "Exception!\n";
        }
        std::free(out);
    }
}

Possible output: 

initially, in: "Home" "Work!"
after move, in: "" ""
after move, out: "Home" "Work!"

[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
LWG 3918 C++17 additional temporary materialization was required
when the input iterator deferences to a prvalue 		copies the element in this case

[edit] See also

 copies a range of objects to an uninitialized area of memory
(function template) [edit]
(C++17)
 moves a number of objects to an uninitialized area of memory
(function template) [edit]
(C++20)
 moves a range of objects to an uninitialized area of memory
(algorithm function object)[edit]
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