# std::ranges::uninitialized_value_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_value_construct_n( I first, std::iter_difference_t <I> count ); |  | (since C++20) (constexpr since C++26)

Constructs objects of type std::iter_value_t <I> in the uninitialized memory area first`+` `[`​0​`,` count`)` by value-initialization, as if by return ranges::uninitialized_value_construct ( std::counted_iterator (first, count),
std::default_sentinel ).base();

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 algorithm function objects (informally known as 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.

### Parameters

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

### Return value

As described above.

### Complexity

Linear in count.

### Exceptions

Any exception thrown on construction of the elements in the destination range.

### Notes

An implementation may improve the efficiency of the `ranges::uninitialized_value_construct_n`, e.g. by using ranges::fill_n, if the value type of the range is a CopyAssignable TrivialType.

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_raw_memory_algorithms` | `202411L` | (C++26) | constexpr for specialized memory algorithms

### Possible implementation

    struct uninitialized_value_construct_n_fn
    {
        template<no-throw-forward-iterator I>
            requires std::default_initializable<std::iter_value_t<I>>
        constexpr I operator()(I first, std::iter_difference_t<I> count) const
        {
            auto iter = std::counted_iterator(first, count);
            return ranges::uninitialized_value_construct(iter, std::default_sentinel).base();
        }
    };

    inline constexpr uninitialized_value_construct_n_fn uninitialized_value_construct_n{};

### Example

Run this code

    #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_value_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";
        }

        // For scalar types, uninitialized_value_construct_n
        // zero-initializes the given uninitialized memory area.
        int v[]{1, 2, 3, 4, 5, 6, 7, 8};
        std::cout << ' ';
        for (const int i : v)
            std::cout << i << ' ';
        std::cout << "\n ";
        std::ranges::uninitialized_value_construct_n(std::begin(v), std::size(v));
        for (const int i : v)
            std::cout << i << ' ';
        std::cout << '\n';
    }

Output:

    1 █▓▒░ █▓▒░ █▓▒░
    2 █▓▒░ █▓▒░ █▓▒░
    3 █▓▒░ █▓▒░ █▓▒░
    4 █▓▒░ █▓▒░ █▓▒░
    1 2 3 4 5 6 7 8
    0 0 0 0 0 0 0 0

### 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_value_construct (C++20) | constructs objects by value-initialization in an uninitialized area of memory, defined by a range (algorithm function object)
ranges::uninitialized_default_construct (C++20) | constructs objects by default-initialization in an uninitialized area of memory, defined by a range (algorithm function object)
ranges::uninitialized_default_construct_n (C++20) | constructs objects by default-initialization in an uninitialized area of memory, defined by a start and count (algorithm function object)
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)

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