std::ranges::uninitialized_default_construct_n

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ranges::uninitialized_default_construct_n
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C Library

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Defined in header <memory>
Call signature
template< no-throw-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 );
 (since C++20)
(constexpr since C++26)

Constructs objects of type std::iter_value_t<I> in the uninitialized memory area 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.

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

Return value

As described above.

Complexity

Linear in count.

Exceptions

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

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.

Notes

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

Possible implementation

struct uninitialized_default_construct_n_fn
{
    template<no-throw-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

Run this code
#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

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

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