std::ranges::uninitialized_value_construct_n

 

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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_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:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

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

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