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Difference between revisions of "cpp/memory/ranges/uninitialized default construct n"

From cppreference.com
< cpp‎ | memory
m (~ voidify: P0896R4 → n4861.)
m (- ranges::)
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template <no-throw-forward-iterator I>
 
template <no-throw-forward-iterator I>
 
requires std::default_initializable<std::iter_value_t<I>>
 
requires std::default_initializable<std::iter_value_t<I>>
−
I ranges::uninitialized_default_construct_n( I first, std::iter_difference_t<I> n );
+
I uninitialized_default_construct_n( I first, std::iter_difference_t<I> n );
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}

Revision as of 14:58, 8 July 2021

 
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>

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

I uninitialized_default_construct_n( I first, std::iter_difference_t<I> n );
 (1) (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 (/*VOIDIFY*/(*first))
      std::remove_reference_t<std::iter_reference_t<I>>;

where /*VOIDIFY*/(e) is: const_cast<void*>(static_cast<const volatile void*>(std::addressof(e))).

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.

Contents

Parameters

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

Return value

An iterator equal to ranges::next(first, n) that points to one past the last element of the range of constructed objects.

Complexity

Linear in n.

Notes

An implementation may skip the objects construction (without changing the observable effect) if the expression std::is_trivially_default_constructible_v<T> evaluates to true, where T is the value type of given range.

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};
        try {
            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)
                ranges::destroy_at(std::addressof(*rollback));
            throw;
        }
    }
};
 
inline constexpr uninitialized_default_construct_n_fn uninitialized_default_construct_n{};

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 = 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 etalon[] { 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, etalon, sizeof(v)) == 0) {
        // Maybe undefined behavior, pending CWG 1997:
        // for (const int i : v) { std::cout << i << ' '; }
        for (const int i : etalon) { std::cout << i << ' '; }
    } else {
        std::cout << "Unspecified!";
    }
    std::cout << '\n';
}

Possible output: 

1 █▓▒░ █▓▒░
2 █▓▒░ █▓▒░
3 █▓▒░ █▓▒░
4 █▓▒░ █▓▒░
1 2 3 4 5 6

See also

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