# std::ranges::uninitialized_value_construct

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< cpp‎ | memory

Defined in header `<memory>` |  |
Call signature |  |
template< no-throw-forward-iterator I, no-throw-sentinel-for<I> S > requires std::default_initializable<std::iter_value_t<I>> I uninitialized_value_construct( I first, S last ); | (1) | (since C++20)
template< no-throw-forward-range R > requires std::default_initializable<ranges::range_value_t<R>> ranges::borrowed_iterator_t<R> uninitialized_value_construct( R&& r ); | (2) | (since C++20)

1) Constructs objects of type std::iter_value_t<I> in the uninitialized storage designated by the range `[`first`,` last`)` by value-initialization, as if by

    for (; first != last; ++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.
2) Same as (1), but uses r as the range, as if using ranges::begin(r) as first, and ranges::end(r) as last.

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, last | - | iterator-sentinel pair denoting the range of elements to value-initialize
r | - | the range of the elements to value-initialize

### Return value

An iterator equal to last.

### Complexity

Linear in the distance between first and last.

### Exceptions

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

### Notes

An implementation may improve the efficiency of the `ranges::uninitialized_value_construct`, e.g. by using ranges::fill, if the value type of the range is TrivialType and CopyAssignable.

### Possible implementation

    struct uninitialized_value_construct_fn
    {
        template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S>
        requires std::default_initializable<std::iter_value_t<I>>
        I operator()(I first, S last) const
        {
            using T = std::remove_reference_t<std::iter_reference_t<I>>;
            if constexpr (std::is_trivial_v<T> && std::is_copy_assignable_v<T>)
                return ranges::fill(first, last, T());
            I rollback{first};
            try
            {
                for (; !(first == last); ++first)
                    ::new (const_cast<void*>(static_cast<const volatile void*>
                        (std::addressof(*first)))) T();
                return first;
            }
            catch (...) // rollback: destroy constructed elements
            {
                for (; rollback != first; ++rollback)
                    ranges::destroy_at(std::addressof(*rollback));
                throw;
            }
        }

        template<no-throw-forward-range R>
        requires std::default_initializable<ranges::range_value_t<R>>
        ranges::borrowed_iterator_t<R>
        operator()(R&& r) const
        {
            return (*this)(ranges::begin(r), ranges::end(r));
        }
    };

    inline constexpr uninitialized_value_construct_fn uninitialized_value_construct{};

### 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{first + n};

            std::ranges::uninitialized_value_construct(first, last);

            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_value_construct
        // zero-fills the given uninitialized memory area.
        int v[]{0, 1, 2, 3};
        std::cout << ' ';
        for (const int i : v)
            std::cout << ' ' << static_cast<char>(i + 'A');
        std::cout << "\n ";
        std::ranges::uninitialized_value_construct(std::begin(v), std::end(v));
        for (const int i : v)
            std::cout << ' ' << static_cast<char>(i + 'A');
        std::cout << '\n';
    }

Output:

    1 ▄▀▄▀▄▀▄▀
    2 ▄▀▄▀▄▀▄▀
    3 ▄▀▄▀▄▀▄▀
    4 ▄▀▄▀▄▀▄▀
      A B C D
      A A A A

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

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