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    std::uninitialized_value_construct

    From cppreference.com
     
    C++
     
    Algorithm library
    Constrained algorithms and algorithms on ranges (C++20)
    Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
    Non-modifying sequence operations    
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    (C++17)
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    (C++11)
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    (C++11)
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    (until C++17)(C++11)
    (C++20)(C++20)
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    (C++17)

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    (C++11)    
    (C++11)

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    (on partitioned ranges)
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    (C++11)
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    (C++20)
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    Specialized <memory> algorithms
    Helper templates (exposition only*)
    (C++17)
    Object (batch) construction
    Object (batch) destruction
    (C++17)
    (C++17)
    (C++17)
     
    Defined in header <memory> 
    template< class NoThrowForwardIt >
void uninitialized_value_construct( NoThrowForwardIt first,
                                    NoThrowForwardIt last );
    		(1) (since C++17)
    (constexpr since C++26)    		 
    template< class ExecutionPolicy, class NoThrowForwardIt >
void uninitialized_value_construct( ExecutionPolicy&& policy,
                                    NoThrowForwardIt first,
                                    NoThrowForwardIt last );
    		(2) (since C++17)
    1) Constructs elements in the destination range [first, last) by value-initialization as if by

    for (; first != last; ++first)
        ::new (voidify(*first))
            typename std::iterator_traits<NoThrowForwardIt>::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 participates in overload resolution only if the value of the following expression is true:

    std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>

    		(until C++20)

    std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>

    		(since C++20)

    Parameters

    first, last - the pair of iterators defining the range 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.

    Exceptions

    2) During the execution process:
    • If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
    • If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).

    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>
constexpr void uninitialized_value_construct(NoThrowForwardIt first,
                                             NoThrowForwardIt last)
{
    using Value = typename std::iterator_traits<NoThrowForwardIt>::value_type;
    NoThrowForwardIt current = first;
    try
    {
        for (; current != last; ++current)
        {
            ::new (static_cast<void*>(std::addressof(*current))) Value();
        }
    }
    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{first + n};
        
        std::uninitialized_value_construct(first, last);
        
        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
    // zero-fills 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(std::begin(v), std::end(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

    See also

    (C++17)
    		 constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count
    (function template & algorithm function object)[edit]
    (C++20)
    (C++17)
    		 constructs objects by default-initialization in an uninitialized area of memory, defined by a range
    (function template & algorithm function object)[edit]
    (C++20)
    (C++20)
    		 constructs objects by value-initialization in an uninitialized area of memory, defined by a range
    (algorithm function object)[edit]