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std::ranges::uninitialized_default_construct_n

From cppreference.com
< cpp‎ | memory
 
 
Memory management library
(exposition only*)
Uninitialized memory algorithms
(C++17)
(C++17)
(C++17)
Constrained uninitialized
memory algorithms
ranges::uninitialized_default_construct_n
(C++20) 
C Library

Allocators
Memory resources
Garbage collection support
(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)
Uninitialized storage
(until C++20*)
(until C++20*)
Explicit lifetime management
 
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 + [0count) 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:

Contents

[edit] Parameters

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

[edit] Return value

As described above.

[edit] Complexity

Linear in count.

[edit] Exceptions

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

[edit] 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.

[edit] Notes

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

[edit] 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{};

[edit] 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

[edit] 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

[edit] See also

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