# std::uninitialized_default_construct_n

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< cpp‎ | memory

Defined in header `<memory>` |  |
template< class ForwardIt, class Size > ForwardIt uninitialized_default_construct_n( ForwardIt first, Size n ); | (1) | (since C++17)
template< class ExecutionPolicy, class ForwardIt, class Size > ForwardIt uninitialized_default_construct_n( ExecutionPolicy&& policy, ForwardIt first, Size n ); | (2) | (since C++17)

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;
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 does not participate in overload resolution unless

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
n | - | the number of elements to construct
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_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;
        }
    }

### 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";
        }

        // 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";
    }

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

uninitialized_default_construct (C++17) | constructs objects by default-initialization in an uninitialized area of memory, defined by a range (function template)
uninitialized_value_construct_n (C++17) | constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count (function template)
ranges::uninitialized_default_construct_n (C++20) | constructs objects by default-initialization in an uninitialized area of memory, defined by a start and count (niebloid)

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