std::uninitialized_value_construct - cppreference.com

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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

#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: