std::uninitialized_default_construct_n - cppreference.com

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

1) Constructs elements in the destination range first + [0, count) by default-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 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	-	the beginning of the range of elements to initialize
count	-	the number of elements to construct
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.

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, class Size>
constexpr ForwardIt uninitialized_default_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

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

struct S
{
    std::string m{"default value"};
};

int main()
{
    constexpr int n{3};
    alignas(alignof(S)) unsigned char mem[n * sizeof(S)];
    
    try
    {
        auto first{reinterpret_cast<S*>(mem)};
        auto last = std::uninitialized_default_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_default_construct_n
    // generally does not zero-initialize the given uninitialized memory area.
    int v[]{1, 2, 3, 4};
    const int original[]{1, 2, 3, 4};
    std::uninitialized_default_construct_n(std::begin(v), std::size(v));
    
    // An attempt to access v might be an undefined behavior,
    // pending CWG 1997 to be resolved:
    // for (const int i : v)
    //     std::cout << i << ' ';
    
    // The result is unspecified:
    std::cout << (std::memcmp(v, original, sizeof(v)) == 0 ? "un" : "") << "modified\n";
}

Possible output:

default value
default value
default value
unmodified