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std::uninitialized_copy

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< cpp‎ | memory
 
 
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*)

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 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) Copies elements from the range [first, last) 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(*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​, std::distance (first, last)) overlaps with [first, last), the behavior is undefined.

(since C++20)

Contents

[edit] 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)

[edit] Return value

Iterator to the element past the last element copied.

[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

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_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;
 }
}

[edit] Example

Run this code
#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: 

This_is_an_example_

[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 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
LWG 3870 C++20 this algorithm might create objects on a const storage kept disallowed

[edit] See also

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

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