std::uninitialized_default_construct() algorithm

since C++17

// (1)
template< class ForwardIt >
void uninitialized_default_construct( ForwardIt first, ForwardIt last );

// (2)
template< class ExecutionPolicy, class ForwardIt >
void uninitialized_default_construct( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last );

• (1) Constructs objects of type typename std::iterator_traits<ForwardIt>::value_type in the uninitialized storage designated by the range [first; last) by default-initialization, as if by:

  for (; first != last; ++d_first, (void) ++first)
      ::new (/* VOIDIFY */(*d_first))
          typename std::iterator_traits<NoThrowForwardIt>::value_type(*first);
  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 last	The range of elements to initialize.
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 the distance between first and last.

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(1)

template<class ForwardIt>
void uninitialized_default_construct(ForwardIt first, ForwardIt last)
{
    using Value = typename std::iterator_traits<ForwardIt>::value_type;
    ForwardIt current = first;
    try
    {
        for (; current != last; ++current)
        {
            ::new (const_cast<void*>(static_cast<const volatile void*>(
                std::addressof(*current)))) Value;
        }
    }
    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{first + n};

        std::uninitialized_default_construct(first, last);

        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
    // generally does not zero-fill the given uninitialized memory area.
    int v[]{1, 2, 3, 4};
    const int original[]{1, 2, 3, 4};
    std::uninitialized_default_construct(std::begin(v), std::end(v));

    // Maybe 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 ? "Unmodified\n" : "Modified\n");
}

Output

Default value
Default value
Default value
Unmodified