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Difference between revisions of "cpp/memory/uninitialized value construct n"

From cppreference.com
< cpp‎ | memory
m (→‎Synopsis: fmt)
(LWG3870)
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@1@ Constructs {{c|n}} objects of type {{c|typename iterator_traits<ForwardIt>::value_type}} in the uninitialized storage starting at {{c|first}} by [[cpp/language/value_initialization|value-initialization]], as if by
 
@1@ Constructs {{c|n}} objects of type {{c|typename iterator_traits<ForwardIt>::value_type}} in the uninitialized storage starting at {{c|first}} by [[cpp/language/value_initialization|value-initialization]], as if by
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{{source|1=
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{{c multi|
−
for (; n > 0; (void) ++first, --n)
+
for (; n > 0; (void) ++first, --n)|
−
     ::new (/*VOIDIFY*/(*first))
+
     ::new (static_cast<void*>(std::addressof(e)))|
 
         typename std::iterator_traits<ForwardIt>::value_type();
 
         typename std::iterator_traits<ForwardIt>::value_type();
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}}
 
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where {{c|/*VOIDIFY*/(e)}} is:
 
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{{rrev multi
 
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|rev1=
 
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:{{c|static_cast<void*>(std::addressof(e))}}
 
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|since2=c++20|rev2=
 
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:{{c|const_cast<void*>(static_cast<const volatile void*>(std::addressof(e)))}}
 
 
}}
 
}}
  
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0 0 0 0
 
0 0 0 0
 
}}
 
}}
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===Defect reports===
 +
{{dr list begin}}
 +
{{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 end}}
  
 
===See also===
 
===See also===

Revision as of 18:57, 25 July 2023

 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
Dynamic memory management
Uninitialized memory algorithms
(C++11)
(C++17)
(C++17)
uninitialized_value_construct_n
(C++17)
(C++17)
(C++20)

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 ForwardIt, class Size >
ForwardIt uninitialized_value_construct_n( ForwardIt first, Size n );
 (1) (since C++17)
template< class ExecutionPolicy, class ForwardIt, class Size >

ForwardIt uninitialized_value_construct_n( ExecutionPolicy&& policy,

                                           ForwardIt first, Size n );
 (2) (since C++17)
1) Constructs n objects of type typename iterator_traits<ForwardIt>::value_type in the uninitialized storage starting at first by value-initialization, as if by for (; n > 0; (void) ++first, --n)
    ::new (static_cast<void*>(std::addressof(e)))
        typename std::iterator_traits<ForwardIt>::value_type();
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 elements to initialize
n - the number of elements to initialize
policy - the execution policy to use. See execution policy for details.
Type requirements
-
ForwardIt must meet the requirements of LegacyForwardIterator.
-
No increment, assignment, comparison, or indirection through valid instances of ForwardIt may throw exceptions.

Return value

The end of the range of objects (i.e., std::next(first, n)).

Complexity

Linear in n.

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 ForwardIt, class Size>
ForwardIt uninitialized_value_construct_n(ForwardIt first, Size n)
{
    using T = typename std::iterator_traits<ForwardIt>::value_type;
    ForwardIt current = first;
    try
    {
        for (; n > 0 ; (void) ++current, --n)
            ::new (const_cast<void*>(static_cast<const volatile void*>(
                std::addressof(*current)))) T();
        return current;
    }
    catch (...)
    {
        std::destroy(first, current);
        throw;
    }
}

Example

Run this code
#include <iostream>
#include <memory>
#include <string>
 
int main()
{
    struct S { std::string m{"Default value"}; };
 
    constexpr int n{3};
    alignas(alignof(S)) unsigned char mem[n * sizeof(S)];
 
    try
    {
        auto first{reinterpret_cast<S*>(mem)};
        auto last = std::uninitialized_value_construct_n(first, n);
 
        for (auto it{first}; it != last; ++it)
            std::cout << it->m << '\n';
 
        std::destroy(first, last);
    }
    catch (...)
    {
        std::cout << "Exception!\n";
    }
 
    // Notice that for "trivial types" the uninitialized_value_construct_n
    // zero-initializes the given uninitialized memory area.
    int v[]{1, 2, 3, 4};
    for (const int i : v)
        std::cout << i << ' ';
    std::cout << '\n';
    std::uninitialized_value_construct_n(std::begin(v), std::size(v));
    for (const int i : v)
        std::cout << i << ' ';
    std::cout << '\n';
}

Output: 

Default value
Default value
Default value
1 2 3 4
0 0 0 0

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

See also

(C++17)
 constructs objects by value-initialization in an uninitialized area of memory, defined by a range
(function template) [edit]
(C++17)
 constructs objects by default-initialization in an uninitialized area of memory, defined by a start and a count
(function template) [edit]
(C++20)
 constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count
(niebloid)[edit]
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