# std::uninitialized_value_construct_n

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< cpp‎ | memory

Defined in header `<memory>` |  |
template< class NoThrowForwardIt, class Size > NoThrowForwardIt uninitialized_value_construct_n ( NoThrowForwardIt first, Size count ); | (1) | (since C++17) (constexpr since C++26)
template< class ExecutionPolicy, class NoThrowForwardIt, class Size > NoThrowForwardIt uninitialized_value_construct_n ( ExecutionPolicy&& policy, NoThrowForwardIt first, Size count ); | (2) | (since C++17)

1) Constructs objects of type typename std::iterator_traits <NoThrowForwardIt>::value_type in the uninitialized memory area first`+` `[`​0​`,` count`)` by value-initialization, as if by

for (; count > 0; (void)++first, --count)
`` ::new ( `voidify` (*first))
`` typename std::iterator_traits <NoThrowForwardIt>::value_type();
return first;

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 participates in overload resolution only if all following conditions are satisfied:

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
count | - | the number of elements to initialize
policy | - | the execution policy to use
Type requirements
- `NoThrowForwardIt` must meet the requirements of LegacyForwardIterator.
- No increment, assignment, comparison, or indirection through valid instances of `NoThrowForwardIt` may throw exceptions.

### Return value

As described above.

### Complexity

Linear in count.

### 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.

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_raw_memory_algorithms` | `202411L` | (C++26) | constexpr for specialized memory algorithms, (1)

### Possible implementation

    template<class NoThrowForwardIt, class Size>
    constexpr ForwardIt uninitialized_value_construct_n(NoThrowForwardIt first, Size count)
    {
        using T = typename std::iterator_traits<NoThrowForwardIt>::value_type;
        NoThrowForwardIt current = first;

        try
        {
            for (; countn > 0; (void) ++current, --count)
                ::new (static_cast<void*>(std::addressof(*current))) T();
            return current;
        }
        catch (...)
        {
            std::destroy(first, current);
            throw;
        }
    }

### Example

Run this code

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

    int main()
    {
        struct S { std::string m{"Default value"}; };

        constexpr int n{3};
        alignas(alignof(S)) unsigned char mem[n * sizeof(S)];

        try
        {
            auto first{reinterpret_cast<S*>(mem)};
            auto last = std::uninitialized_value_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";
        }

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

Output:

    Default value
    Default value
    Default value
    1 2 3 4
    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

uninitialized_value_construct (C++17) | constructs objects by value-initialization in an uninitialized area of memory, defined by a range (function template)
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)
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 (algorithm function object)

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