< 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, no-throw-sentinel-for<I> S >

requires std::default_initializable<std::iter_value_t<I>>

I uninitialized_default_construct( I first, S last );
(1) (since C++20)
template< no-throw-forward-range R >

requires std::default_initializable<ranges::range_value_t<R>>

uninitialized_default_construct( R&& r );
(2) (since C++20)
1) Constructs objects of type std::iter_value_t<I> in the uninitialized storage designated by the range [firstlast) by default-initialization, as if by
for (; first != last; ++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.
2) Same as (1), but uses r as the range, as if using ranges::begin(r) as first, and ranges::end(r) as last.

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, last - iterator-sentinel pair denoting the range of the elements to initialize
r - the range of the elements to initialize

[edit] Return value

An iterator equal to last.

[edit] Complexity

Linear in the distance between first and last.

[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_fn
    template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S>
    requires std::default_initializable<std::iter_value_t<I>>
    I operator()(I first, S last) 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, last); // skip initialization
        I rollback{first};
            for (; !(first == last); ++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)
    template<no-throw-forward-range R>
    requires std::default_initializable<ranges::range_value_t<R>>
    operator()(R&& r) const
        return (*this)(ranges::begin(r), ranges::end(r));
inline constexpr uninitialized_default_construct_fn uninitialized_default_construct{};

[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{first + n};
        std::ranges::uninitialized_default_construct(first, last);
        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
    // generally does not zero-fill the given uninitialized memory area.
    constexpr char sample[]{'A', 'B', 'C', 'D', '\n'};
    char v[]{'A', 'B', 'C', 'D', '\n'};
    std::ranges::uninitialized_default_construct(std::begin(v), std::end(v));
    if (std::memcmp(v, sample, sizeof(v)) == 0)
        std::cout << "  ";
        // Maybe undefined behavior, pending CWG 1997:
        // for (const char c : v) { std::cout << c << ' '; }
        for (const char c : sample)
            std::cout << c << ' ';
        std::cout << "Unspecified\n";

Possible output:

1 ▄▀▄▀▄▀▄▀
2 ▄▀▄▀▄▀▄▀
3 ▄▀▄▀▄▀▄▀
4 ▄▀▄▀▄▀▄▀
  A B C D

[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 start and count
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 range
(function template) [edit]