std::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
If an exception is thrown during the initialization, some objects in [ first ,  last )
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 )

(since C++20)

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.

Return value

As described above.

Complexity

Linear in the distance between first and last.

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
  • If the algorithm fails to allocate memory, std::bad_alloc is thrown.

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)

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;
    }
}

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!"

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

See also

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