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std::ranges::uninitialized_default_construct

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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, 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)
(constexpr since C++26)
template< no-throw-forward-range R >

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

    uninitialized_default_construct( R&& r );
 (2) (since C++20)
(constexpr since C++26)
1) Constructs objects of type std::iter_value_t<I> in the uninitialized memory area [first, last) by default-initialization, as if by

for (; first != last; ++first)
    ::new (voidify(*first))
        std::remove_reference_t<std::iter_reference_t<I>>;
return first;

If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.
2) Equivalent to ranges::uninitialized_default_construct(ranges::begin(r), ranges::end(r)).

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

Contents

[edit] Parameters

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

[edit] Return value

As described above.

[edit] Complexity

Linear in the distance between first and last.

[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.

Feature-test macro Value Std Feature
__cpp_lib_raw_memory_algorithms 202411L (C++26) constexpr for specialized memory algorithms, (1,2)

[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>>
    constexpr 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};
        try
        {
            for (; !(first == last); ++first)
                ::new (static_cast<void*>(std::addressof(*first))) ValueType;
            return first;
        }
        catch (...) // rollback: destroy constructed elements
        {
            for (; rollback != first; ++rollback)
                ranges::destroy_at(std::addressof(*rollback));
            throw;
        }
    }
 
    template<no-throw-forward-range R>
        requires std::default_initializable<ranges::range_value_t<R>>
    constexpr ranges::borrowed_iterator_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

Run this code
#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{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 to be resolved:
        // for (const char c : v) { std::cout << c << ' '; }
        for (const char c : sample)
            std::cout << c << ' ';
    }
    else
        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

(C++20)
 constructs objects by default-initialization in an uninitialized area of memory, defined by a start and count
(algorithm function object)[edit]
(C++20)
 constructs objects by value-initialization in an uninitialized area of memory, defined by a range
(algorithm function object)[edit]
(C++20)
 constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count
(algorithm function object)[edit]
(C++17)
 constructs objects by default-initialization in an uninitialized area of memory, defined by a range
(function template) [edit]
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