< cpp‎ | memory
Utilities library
Language support
Type support (basic types, RTTI)
Library feature-test macros (C++20)
Dynamic memory management
Program utilities
Coroutine support (C++20)
Variadic functions
Debugging support
Three-way comparison
General utilities
Date and time
Function objects
Formatting library (C++20)
Relational operators (deprecated in C++20)
Integer comparison functions
Swap and type operations
Common vocabulary types
Elementary string conversions

Dynamic memory management
Uninitialized memory algorithms
Constrained uninitialized memory algorithms
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)

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> n );
(since C++20)

Constructs n objects of type std::iter_value_t<I> in the uninitialized memory area starting at first by default-initialization, as if by

for (; n-- > 0; ++first)
    ::new (static_cast<void*>(std::addressof(*first)))

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 niebloids, that is:

In practice, they may be implemented as function objects, or with special compiler extensions.


[edit] Parameters

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

[edit] Return value

The end of the range of objects (i.e., ranges::next(first, n)).

[edit] Complexity

Linear in n.

[edit] Exceptions

The exception thrown on construction of the elements in the destination range, if any.

[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_v.

[edit] Possible implementation

struct uninitialized_default_construct_n_fn
    template<no-throw-forward-iterator I>
    requires std::default_initializable<std::iter_value_t<I>>
    I operator()(I first, std::iter_difference_t<I> n) const
        using ValueType = std::remove_reference_t<std::iter_reference_t<I>>;
        if constexpr (std::is_trivially_default_constructible_v<ValueType>)
            return ranges::next(first, n); // skip initialization
        I rollback{first};
            for (; n-- > 0; ++first)
                ::new (const_cast<void*>(static_cast<const volatile void*>
                    (std::addressof(*first)))) ValueType;
            return first;
        catch (...) // rollback: destroy constructed elements
            for (; rollback != first; ++rollback)
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)];
        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";
    // Notice that for "trivial types" the 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 << ' ';
        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
constructs objects by value-initialization in an uninitialized area of memory, defined by a range
constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count
constructs objects by default-initialization in an uninitialized area of memory, defined by a start and a count
(function template) [edit]