# std::ranges::uninitialized_default_construct_n

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< cpp‎ | memory

Defined in header `<memory>` |  |
Call signature |  |
template< no-throw-forward-iterator I > requires std::default_initializable<std::iter_value_t<I>> I uninitialized_default_construct_n( I first, std::iter_difference_t<I> n ); |  | (since C++20)

Constructs n objects of type std::iter_value_t<I> in the uninitialized memory area starting at first by default-initialization, as if by

    for (; n-- > 0; ++first)
        ::new (static_cast<void*>(std::addressof(*first)))
            std::remove_reference_t<std::iter_reference_t<I>>;

If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.

The function-like entities described on this page are niebloids, that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

In practice, they may be implemented as function objects, or with special compiler extensions.

### Parameters

first | - | the beginning of the range of elements to initialize
n | - | the number of elements to construct

### Return value

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

### Complexity

Linear in n.

### Exceptions

The exception thrown on construction of the elements in the destination range, if any.

### Notes

An implementation may skip the objects construction (without changing the observable effect) if no non-trivial default constructor is called while default-initializing a std::iter_value_t<I> object, which can be detected by std::is_trivially_default_constructible_v.

### Possible implementation

    struct uninitialized_default_construct_n_fn
    {
        template<no-throw-forward-iterator I>
        requires std::default_initializable<std::iter_value_t<I>>
        I operator()(I first, std::iter_difference_t<I> n) const
        {
            using ValueType = std::remove_reference_t<std::iter_reference_t<I>>;
            if constexpr (std::is_trivially_default_constructible_v<ValueType>)
                return ranges::next(first, n); // skip initialization
            I rollback{first};
            try
            {
                for (; n-- > 0; ++first)
                    ::new (const_cast<void*>(static_cast<const volatile void*>
                        (std::addressof(*first)))) ValueType;
                return first;
            }
            catch (...) // rollback: destroy constructed elements
            {
                for (; rollback != first; ++rollback)
                    ranges::destroy_at(std::addressof(*rollback));
                throw;
            }
        }
    };

    inline constexpr uninitialized_default_construct_n_fn uninitialized_default_construct_n{};

### Example

Run this code

    #include <cstring>
    #include <iostream>
    #include <memory>
    #include <string>

    int main()
    {
        struct S { std::string m{ "█▓▒░ █▓▒░ " }; };

        constexpr int n{4};
        alignas(alignof(S)) char out[n * sizeof(S)];

        try
        {
            auto first{reinterpret_cast<S*>(out)};
            auto last = std::ranges::uninitialized_default_construct_n(first, n);

            auto count{1};
            for (auto it{first}; it != last; ++it)
                std::cout << count++ << ' ' << it->m << '\n';

            std::ranges::destroy(first, last);
        }
        catch(...) { std::cout << "Exception!\n"; }

        // Notice that for "trivial types" the uninitialized_default_construct_n
        // generally does not zero-fill the given uninitialized memory area.
        constexpr int etalon[]{1, 2, 3, 4, 5, 6};
        int v[]{1, 2, 3, 4, 5, 6};
        std::ranges::uninitialized_default_construct_n(std::begin(v), std::size(v));
        if (std::memcmp(v, etalon, sizeof(v)) == 0)
        {
            // Maybe undefined behavior, pending CWG 1997:
            // for (const int i : v) { std::cout << i << ' '; }
            for (const int i : etalon)
                std::cout << i << ' ';
        }
        else
            std::cout << "Unspecified!";
        std::cout << '\n';
    }

Possible output:

    1 █▓▒░ █▓▒░
    2 █▓▒░ █▓▒░
    3 █▓▒░ █▓▒░
    4 █▓▒░ █▓▒░
    1 2 3 4 5 6

### 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

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

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