std::ranges::uninitialized_value_construct_n

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< cpp‎ | memory
 
C++
 
Memory management library
(exposition only*)
Allocators
Uninitialized memory algorithms
Constrained uninitialized memory algorithms
Memory resources
Uninitialized storage (until C++20)
(until C++20*)
(until C++20*)
(until C++20*)

Garbage collector support (until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
 
Defined in header <memory>
Call signature
template< no-throw-forward-iterator I >

    requires std::default_initializable<std::iter_value_t<I>>
I uninitialized_value_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 value-initialization, as if by return ranges::uninitialized_value_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 improve the efficiency of the ranges::uninitialized_value_construct_n, e.g. by using ranges::fill_n, if the value type of the range is a CopyAssignable TrivialType.

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

Possible implementation

struct uninitialized_value_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_value_construct(iter, std::default_sentinel).base();
    }
};
 
inline constexpr uninitialized_value_construct_n_fn uninitialized_value_construct_n{};

Example

Run this code
#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_value_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_value_construct_n
    // zero-initializes the given uninitialized memory area.
    int v[]{1, 2, 3, 4, 5, 6, 7, 8};
    std::cout << ' ';
    for (const int i : v)
        std::cout << i << ' ';
    std::cout << "\n ";
    std::ranges::uninitialized_value_construct_n(std::begin(v), std::size(v));
    for (const int i : v)
        std::cout << i << ' ';
    std::cout << '\n';
}

Output: 

1 █▓▒░ █▓▒░ █▓▒░ 
2 █▓▒░ █▓▒░ █▓▒░ 
3 █▓▒░ █▓▒░ █▓▒░ 
4 █▓▒░ █▓▒░ █▓▒░
1 2 3 4 5 6 7 8
0 0 0 0 0 0 0 0

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