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std::ranges::uninitialized_value_construct

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< cpp‎ | memory
 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
Dynamic memory management
Uninitialized memory algorithms
Constrained uninitialized memory algorithms
Allocators
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_value_construct( I first, S last );
 (1) (since C++20)
template< no-throw-forward-range R >

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

uninitialized_value_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 [first, last) by value-initialization, as if by 
for (; first != last; ++first)
    ::new (static_cast<void*>(std::addressof(*first)))
        std::remove_reference_t<std::iter_reference_t<I>>();
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.

Contents

[edit] Parameters

first, last - iterator-sentinel pair denoting the range of elements to value-initialize
r - the range of the elements to value-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 improve the efficiency of the ranges::uninitialized_value_construct, e.g. by using ranges::fill, if the value type of the range is TrivialType and CopyAssignable.

[edit] Possible implementation

struct uninitialized_value_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 T = std::remove_reference_t<std::iter_reference_t<I>>;
        if constexpr (std::is_trivial_v<T> && std::is_copy_assignable_v<T>)
            return ranges::fill(first, last, T());
        I rollback{first};
        try
        {
            for (; !(first == last); ++first)
                ::new (const_cast<void*>(static_cast<const volatile void*>
                    (std::addressof(*first)))) T();
            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>>
    ranges::borrowed_iterator_t<R>
    operator()(R&& r) const
    {
        return (*this)(ranges::begin(r), ranges::end(r));
    }
};
 
inline constexpr uninitialized_value_construct_fn uninitialized_value_construct{};

[edit] 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{first + n};
 
        std::ranges::uninitialized_value_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_value_construct
    // zero-fills the given uninitialized memory area.
    int v[]{0, 1, 2, 3};
    std::cout << ' ';
    for (const int i : v)
        std::cout << ' ' << static_cast<char>(i + 'A');
    std::cout << "\n ";
    std::ranges::uninitialized_value_construct(std::begin(v), std::end(v));
    for (const int i : v)
        std::cout << ' ' << static_cast<char>(i + 'A');
    std::cout << '\n';
}

Output: 

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

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