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Difference between revisions of "cpp/memory/uninitialized copy n"

From cppreference.com
< cpp‎ | memory
(Avoid regression)
(Added LWG issue #2133 DR (part 2/2).)
 
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     std::return_temporary_buffer(p);
 
     std::return_temporary_buffer(p);
 
}
 
}
 +
|p=true
 
|output=
 
|output=
 
This is an example
 
This is an example
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===Defect reports===
 
===Defect reports===
 
{{dr list begin}}
 
{{dr list begin}}
 +
{{dr list item|wg=lwg|dr=2133|std=C++98|before=the effect description used a {{c/core|for}} loop with the iteration<br>expression {{c|++d_first, ++first, --n}}, which results<br>in argument-dependent lookups of {{c/core|operator,}}|after=discards the value<br>of one operand to<br>disable those ADLs}}
 
{{dr list item|wg=lwg|dr=2433|std=C++11|before=this algorithm might be hijacked by overloaded {{c/core|operator&}}|after=uses {{lc|std::addressof}}}}
 
{{dr list item|wg=lwg|dr=2433|std=C++11|before=this algorithm might be hijacked by overloaded {{c/core|operator&}}|after=uses {{lc|std::addressof}}}}
 
{{dr list item|wg=lwg|dr=3870|std=C++20|before=this algorithm might create objects on a {{c/core|const}} storage|after=kept disallowed}}
 
{{dr list item|wg=lwg|dr=3870|std=C++20|before=this algorithm might create objects on a {{c/core|const}} storage|after=kept disallowed}}

Latest revision as of 21:58, 4 March 2024

 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
Dynamic memory management
Uninitialized memory algorithms
Constrained uninitialized memory algorithms
Allocators
Garbage collection support
(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)
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 (; n > 0; ++d_first, (void) ++first, --n)
     ::new (static_cast<void*>(std::addressof(*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.

If d_first + [​0​, n) overlaps with first + [​0​, n), the behavior is undefined.

(since C++20)
2) Same as (1), but executed according to policy. This overload participates in overload resolution only if

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)

Contents

[edit] 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. See execution policy for details.
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

Iterator to the element past the last element copied.

[edit] Complexity

Linear in count.

[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] Possible implementation

template<class InputIt, class Size, class NoThrowForwardIt>
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;
}

[edit] 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

[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 2133 C++98 the effect description used a for loop with the iteration
expression ++d_first, ++first, --n, 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

[edit] See also

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