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std::ranges::uninitialized_default_construct_n - cppreference.com

From cppreference.com

Defined in header <memory>

Call signature

 
template< /*nothrow-forward-iterator*/ I >
    requires std::default_initializable<std::iter_value_t<I>>
I uninitialized_default_construct_n( I first,
                                     std::iter_difference_t<I> count );
(1) (since C++20)
(constexpr since C++26)		 
template< /*execution-policy*/ Ep, /*nothrow-random-access-iterator*/ I >
    requires std::default_initializable<std::iter_value_t<I>>
I uninitialized_default_construct_n( Ep&& policy, I first,
                                     std::iter_difference_t<I> count );
(2) (since C++26)

For the definition of /*execution-policy*/, see this page; for the definition of other exposition-only concepts, see this page.

1) Constructs elements in the destination range first + [0, count) by default-initialization as if by

return ranges::uninitialized_default_construct(std::counted_iterator(first, count),
                                               std::default_sentinel).base();

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.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

Parameters

first - the beginning of the range of elements to initialize
count - the number of elements to construct
policy - the execution policy to use

Return value

As described above.

Exceptions

Any exception thrown on construction of the elements in the destination range.

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

An implementation may skip the objects construction (without changing the observable effect) if no non-trivial default constructor is called while default-initializing a std::iter_value_t<I> object, which can be detected by std::is_trivially_default_constructible.

Feature-test macro Value Std Feature
__cpp_lib_parallel_algorithm 202506L (C++26) Parallel range algorithms
__cpp_lib_raw_memory_algorithms 202411L (C++26) constexpr for specialized <memory> algorithms, (1)

Possible implementation

struct uninitialized_default_construct_n_fn
{
    template</*nothrow-forward-iterator*/ I>
        requires std::default_initializable<std::iter_value_t<I>>
    constexpr I operator()(I first, std::iter_difference_t<I> count) const
    {
        auto iter = std::counted_iterator(first, count);
        return ranges::uninitialized_default_construct(iter, std::default_sentinel).base();
    }
};

inline constexpr uninitialized_default_construct_n_fn uninitialized_default_construct_n{};

Example

#include <cstring>
#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 = std::ranges::uninitialized_default_construct_n(first, n);
        
        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";
    }
    
    // For scalar types, uninitialized_default_construct_n
    // generally does not zero-fill the given uninitialized memory area.
    constexpr int sample[]{1, 2, 3, 4, 5, 6};
    int v[]{1, 2, 3, 4, 5, 6};
    std::ranges::uninitialized_default_construct_n(std::begin(v), std::size(v));
    if (std::memcmp(v, sample, sizeof(v)) == 0)
    {
        // Maybe undefined behavior, pending CWG 1997:
        // for (const int i : v) { std::cout << i << ' '; }
        for (const int i : sample)
            std::cout << i << ' ';
    }
    else
        std::cout << "Unspecified!";
    std::cout << '\n';
}

Possible output: 

1 █▓▒░ █▓▒░
2 █▓▒░ █▓▒░
3 █▓▒░ █▓▒░
4 █▓▒░ █▓▒░
1 2 3 4 5 6

See also