std::uninitialized_default_construct_n

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Defined in header <memory>
template< class NoThrowForwardIt, class Size >

NoThrowForwardIt uninitialized_default_construct_n

    ( NoThrowForwardIt first, Size count );
 (1) (since C++17)
(constexpr since C++26)
template< class ExecutionPolicy, class NoThrowForwardIt, class Size >

NoThrowForwardIt uninitialized_default_construct_n

    ( ExecutionPolicy&& policy, NoThrowForwardIt first, Size count );
 (2) (since C++17)
1) Constructs objects of type typename std::iterator_traits<NoThrowForwardIt>::value_type in the uninitialized memory area first + [​0​, count) by default-initialization, as if by

for (; count > 0; (void)++first, --count)
    ::new (voidify(*first))
        typename std::iterator_traits<NoThrowForwardIt>::value_type;
return 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)

Parameters

first - the beginning of the range of elements to initialize
count - the number of elements to construct
policy - the execution policy to use
Type requirements
-
NoThrowForwardIt must meet the requirements of LegacyForwardIterator.
-
No increment, assignment, comparison, or indirection through valid instances of NoThrowForwardIt may throw exceptions.

Return value

As described above.

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.

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

Example

Run this code
#include <cstring>
#include <iostream>
#include <memory>
#include <string>
 
struct S
{
    std::string m{"default value"};
};
 
int main()
{
    constexpr int n{3};
    alignas(alignof(S)) unsigned char mem[n * sizeof(S)];
 
    try
    {
        auto first{reinterpret_cast<S*>(mem)};
        auto last = std::uninitialized_default_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";
    }
 
    // For scalar types, uninitialized_default_construct_n
    // generally does not zero-initialize the given uninitialized memory area.
    int v[]{1, 2, 3, 4};
    const int original[]{1, 2, 3, 4};
    std::uninitialized_default_construct_n(std::begin(v), std::size(v));
 
    // An attempt to access v might be an undefined behavior,
    // pending CWG 1997 to be resolved:
    // for (const int i : v)
    //     std::cout << i << ' ';
 
    // The result is unspecified:
    std::cout << (std::memcmp(v, original, sizeof(v)) == 0 ? "un" : "") << "modified\n";
}

Possible output: 

default value
default value
default value
unmodified

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