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std:: uninitialized_copy_n

From cppreference.net
C++
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* )
( 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>
template < class InputIt, class Size, class NoThrowForwardIt >

NoThrowForwardIt uninitialized_copy_n ( InputIt first, Size count,

NoThrowForwardIt d_first ) ;
(1) (since C++11)
(constexpr since C++26)
template < class ExecutionPolicy, class ForwardIt,

class Size, class NoThrowForwardIt >
NoThrowForwardIt uninitialized_copy_n ( ExecutionPolicy && policy,
ForwardIt first, Size count,

NoThrowForwardIt d_first ) ;
(2) (since C++17)
1) Copies count elements from a range beginning at first to an uninitialized memory area beginning at d_first as if by

for ( ; count > 0 ; ++ d_first, ( void ) ++ first, -- count )
:: new ( voidify ( * d_first ) )
typename std:: iterator_traits < NoThrowForwardIt > :: value_type ( * 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 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 ​ , count ) overlaps with first + [ ​ 0 ​ , count ) , the behavior is undefined.

(since C++20)

Contents

Parameters

first - the beginning of the range of the elements to copy
count - the number 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.

Return value

Iterator to the element past the last element copied.

Complexity

Linear in count .

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.

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 Size, class NoThrowForwardIt>
constexpr NoThrowForwardIt uninitialized_copy_n(InputIt first, Size count,
                                                NoThrowForwardIt d_first)
{
    using T = typename std::iterator_traits<NoThrowForwardIt>::value_type;
    NoThrowForwardIt current = d_first;
    try
    {
        for (; count > 0; ++first, (void) ++current, --count)
            ::new (static_cast<void*>(std::addressof(*current))) T(*first);
    }
    catch (...)
    {
        for (; d_first != current; ++d_first)
            d_first->~T();
        throw;
    }
    return current;
}

Example

Run this code 
#include <algorithm>
#include <iostream>
#include <memory>
#include <string>
#include <tuple>
#include <vector>
int main()
{
    std::vector<std::string> v = {"This", "is", "an", "example"};
    std::string* p;
    std::size_t sz;
    std::tie(p, sz) = std::get_temporary_buffer<std::string>(v.size());
    sz = std::min(sz, v.size());
    std::uninitialized_copy_n(v.begin(), sz, p);
    for (std::string* i = p; i != p + sz; ++i)
    {
        std::cout << *i << ' ';
        i->~basic_string<char>();
    }
    std::cout << '\n';
    std::return_temporary_buffer(p);
}

Possible output: 

This is an example

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 2133 C++98 the effect description used a for loop with the iteration
expression ++ d_first, ++ first, -- count , which
results in argument-dependent lookups of operator, 		discards the value
of one operand to
disable those ADLs
LWG 2433 C++11 this algorithm might be hijacked by overloaded operator & uses std::addressof
LWG 3870 C++20 this algorithm might create objects on a const storage kept disallowed

See also

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