std::uninitialized_default_construct_n() algorithm

since C++17

// (1)
template< class ForwardIt, class Size >
ForwardIt uninitialized_default_construct_n( ForwardIt first, Size n );

// (2)
template< class ExecutionPolicy, class ForwardIt, class Size >
ForwardIt uninitialized_default_construct_n( ExecutionPolicy&& policy, ForwardIt first, Size n );

• (1) Constructs n objects of type typename std::iterator_traits<ForwardIt>::value_type in the uninitialized storage starting at first by default-initialization, as if by:

  for (; n > 0; (void) ++first, --n)
      ::new (static_cast<void*>(std::addressof(*first)))
          typename std::iterator_traits<ForwardIt>::value_type;
  caution

  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.

Overload Resolution

These overloads participate in overload resolution only if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>  (until C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>  (since C++20) is true.

Parameters

first	The beginning of the range of elements to initialize.
n	The number of elements to construct.
policy	The execution policy to use. See execution policy for details.

Type requirements

ForwardIt

LegacyForwardIterator

No increment, assignment, comparison, or indirection through valid instances of NoThrowForwardIt may throw exceptions.

Return value

(none)

Complexity

Linear in n.

Exceptions

The overloads with a template parameter named ExecutionPolicy report 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 none other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

uninitialized_default_construct_n(1)

template<class ForwardIt, class Size>
ForwardIt uninitialized_default_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;
    }
}

Examples

Main.cpp

#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";
    }

    // Notice that for "trivial types" the 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:
    // for (const int i : v)
    //     std::cout << i << ' ';

    // The result is unspecified:
    std::cout << (std::memcmp(v, original, sizeof(v)) == 0 ? "un" : "") << "modified\n";
}

Output

default value
default value
default value
unmodified