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std::uninitialized_copy - cppreference.com

From cppreference.com

Defined in header <memory> 

template< class InputIt, class NoThrowForwardIt >
NoThrowForwardIt uninitialized_copy( InputIt first, InputIt last,
                                     NoThrowForwardIt d_first );
(1) (constexpr since C++26) 
template< class ExecutionPolicy, class ForwardIt,
          class NoThrowForwardIt >
NoThrowForwardIt uninitialized_copy( ExecutionPolicy&& policy,
                                     ForwardIt first, ForwardIt last,
                                     NoThrowForwardIt d_first );
(2) (since C++17)

1) Constructs elements in the destination range beginning at d_first from elements in the source range [first, last) as if by

for (; first != last; ++d_first, (void) ++first)
    ::new (voidify(*d_first))
        typename std::iterator_traits<NoThrowForwardIt>::value_type(*first);
return d_first;

If an exception is thrown during the initialization, 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 the value of the following expression is true:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>

(until C++20)

std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>

(since C++20)

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

(since C++20)

Parameters

first, last - the pair of iterators defining the range of elements to copy
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. Applying &* to a NoThrowForwardIt value must yield a pointer to its value type.(until C++11)

Return value

As described above.

Exceptions

2) During the execution process:

  • If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
  • If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).

Notes

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_copy(InputIt first, InputIt last,
                                              NoThrowForwardIt d_first)
{
    using T = typename std::iterator_traits<NoThrowForwardIt>::value_type;
    NoThrowForwardIt current = d_first;
    try
    {
        for (; first != last; ++first, (void) ++current)
            ::new (static_cast<void*>(std::addressof(*current))) T(*first);
        return current;
    }
    catch (...)
    {
        for (; d_first != current; ++d_first)
            d_first->~T();
        throw;
    }
}

Example

#include <cstdlib>
#include <iostream>
#include <memory>
#include <string>
 
int main()
{
    const char* v[] = {"This", "is", "an", "example"};
    
    auto sz = std::size(v);
    
    if (void* pbuf = std::aligned_alloc(alignof(std::string), sizeof(std::string) * sz))
    {
        try
        {
            auto first = static_cast<std::string*>(pbuf);
            auto last = std::uninitialized_copy(std::begin(v), std::end(v), first);
            
            for (auto it = first; it != last; ++it)
                std::cout << *it << '_';
            std::cout << '\n';
            
            std::destroy(first, last);
        }
        catch (...) {}
        std::free(pbuf);
    }
}

Output:

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 866 C++98 given T as the value type of NoThrowForwardIt, if
T::operator new exists, the program might be ill-formed 		uses global replacement-
new instead
LWG 2133 C++98 the effect description used a for loop with the iteration
expression ++d_first, ++first, which results
in an argument-dependent lookup of operator, 		discards the value
of one operand
to disable that ADL
LWG 2433 C++11 this algorithm might be hijacked by overloaded operator& uses std::addressof

See also