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

From cppreference.com
< cpp‎ | memory
m (→‎Top: line wrap: de-supersizing.)
(LWG 3552. Parallel specialized memory algorithms should require forward iterators)
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{{dcl header | memory }}
 
{{dcl header | memory }}
 
{{dcl | since=c++11 | num= 1 |1=  
 
{{dcl | since=c++11 | num= 1 |1=  
−
template< class InputIt, class Size, class ForwardIt >
+
template< class InputIt, class Size, class NoThrowForwardIt >
−
ForwardIt uninitialized_copy_n( InputIt first, Size count, ForwardIt d_first );
+
NoThrowForwardIt uninitialized_copy_n( InputIt first, Size count, NoThrowForwardIt d_first );
 
}}
 
}}
 
{{dcl | since=c++17 | num= 2 |1=
 
{{dcl | since=c++17 | num= 2 |1=
−
template< class ExecutionPolicy, class InputIt, class Size, class ForwardIt >
+
template< class ExecutionPolicy, class ForwardIt, class Size, class NoThrowForwardIt >
−
ForwardIt uninitialized_copy_n( ExecutionPolicy&& policy, InputIt first, Size count,
+
NoThrowForwardIt uninitialized_copy_n( ExecutionPolicy&& policy, ForwardIt first, Size count,
−
                                ForwardIt d_first );}}
+
                                      NoThrowForwardIt d_first );}}
 
{{dcl end}}
 
{{dcl end}}
 
                                      
 
                                      
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for ( ; n > 0; ++d_first, (void) ++first, --n)
 
for ( ; n > 0; ++d_first, (void) ++first, --n)
 
   ::new (/*VOIDIFY*/(*d_first))
 
   ::new (/*VOIDIFY*/(*d_first))
−
       typename std::iterator_traits<ForwardIt>::value_type(*first);
+
       typename std::iterator_traits<NoThrowForwardIt>::value_type(*first);
 
}}
 
}}
 
where {{c|/*VOIDIFY*/(e)}} is:
 
where {{c|/*VOIDIFY*/(e)}} is:
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{{par req named | InputIt | InputIterator}}
 
{{par req named | InputIt | InputIterator}}
 
{{par req named | ForwardIt | ForwardIterator}}
 
{{par req named | ForwardIt | ForwardIterator}}
−
{{par req | No increment, assignment, comparison, or indirection through valid instances of {{tt|ForwardIt}} may throw exceptions.}}
+
{{par req named | NoThrowForwardIt | ForwardIterator}}
 +
{{par req | No increment, assignment, comparison, or indirection through valid instances of {{tt|NoThrowForwardIt}} may throw exceptions.}}
 
{{par end}}
 
{{par end}}
  
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===Possible implementation===
 
===Possible implementation===
 
{{eq fun | 1=
 
{{eq fun | 1=
−
template<class InputIt, class Size, class ForwardIt>
+
template<class InputIt, class Size, class NoThrowForwardIt>
−
ForwardIt uninitialized_copy_n(InputIt first, Size count, ForwardIt d_first)
+
NoThrowForwardIt uninitialized_copy_n(InputIt first, Size count, NoThrowForwardIt d_first)
 
{
 
{
−
     using T = typename std::iterator_traits<ForwardIt>::value_type;
+
     using T = typename std::iterator_traits<NoThrowForwardIt>::value_type;
−
     ForwardIt current = d_first;
+
     NoThrowForwardIt current = d_first;
 
     try {
 
     try {
 
         for (; count > 0; ++first, (void) ++current, --count) {
 
         for (; count > 0; ++first, (void) ++current, --count) {

Revision as of 19:11, 12 June 2021

 
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 (/*VOIDIFY*/(*d_first))
      typename std::iterator_traits<NoThrowForwardIt>::value_type(*first);

where /*VOIDIFY*/(e) is:

static_cast<void*>(std::addressof(e))
(until C++20)
const_cast<void*>(static_cast<const volatile void*>(std::addressof(e)))
(since C++20)
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

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

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.

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.

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 (const_cast<void*>(static_cast<const volatile 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::return_temporary_buffer(p);
}

Output: 

This is an example

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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