std::ranges::uninitialized_fill

Defined in header `<memory>`
Call signature
template< no-throw-forward-iterator I, no-throw-sentinel-for<I> S, class T > requires std::constructible_from<std::iter_value_t<I>, const T&> I uninitialized_fill( I first, S last, const T& x );	(1)	(since C++20)
template< no-throw-forward-range R, class T > requires std::constructible_from<ranges::range_value_t<R>, const T&> ranges::borrowed_iterator_t<R> uninitialized_fill( R&& r, const T& x );	(2)	(since C++20)

1) Constructs

N

copies of the given value x in an uninitialized memory area, designated by the range [first, last), where

N

is ranges::distance(first, last).

The function has the effect equivalent to:

for (; first != last; ++first) {
    ::new (
        const_cast<void*>(static_cast<const volatile void*>(std::addressof(*first)))
        ) std::remove_reference_t<std::iter_reference_t<I>>(x);
}
return 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:

Explicit template argument lists may not be specified when calling any of them.
None of them is visible to argument-dependent lookup.
When one of them is found by normal unqualified lookup for the name to the left of the function-call operator, it inhibits argument-dependent lookup.

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

Parameters

first, last	-	iterator-sentinel pair denoting the range of elements to initialize
r	-	the range of the elements to initialize
value	-	the value to construct the elements with

Return value

An iterator equal to last.

Complexity

$\scriptsize\mathcal{O}(N)$ $𝓞(N)$ .

Exceptions

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

Notes

An implementation may improve the efficiency of the ranges::uninitialized_fill, e.g. by using ranges::fill, if the value type of the output range is TrivialType.

Possible implementation

struct uninitialized_fill_fn {
    template <no-throw-forward-iterator I, no-throw-sentinel-for<I> S, class T>
    requires std::constructible_from<std::iter_value_t<I>, const T&>
    I operator()( I first, S last, const T& x ) const {
        I rollback {first};
        try {
            for (; !(first == last); ++first)
                ranges::construct_at(std::addressof(*first), x);
            return first;
        } catch (...) { // rollback: destroy constructed elements
            for (; rollback != first; ++rollback)
                ranges::destroy_at(std::addressof(*rollback));
            throw;
        }
    }
 
    template <no-throw-forward-range R, class T>
    requires std::constructible_from<ranges::range_value_t<R>, const T&>
    ranges::borrowed_iterator_t<R>
    operator()( R&& r, const T& x ) const {
        return (*this)(ranges::begin(r), ranges::end(r), x);
    }
};
 
inline constexpr uninitialized_fill_fn uninitialized_fill{};

Example

Run this code

#include <iostream>
#include <memory>
#include <string>
 
int main()
{
    constexpr int n {4};
    alignas(alignof(std::string)) char out[n * sizeof(std::string)];
 
    try
    {
        auto first {reinterpret_cast<std::string*>(out)};
        auto last {first + n};
        std::ranges::uninitialized_fill(first, last, "▄▀▄▀▄▀▄▀");
 
        int count {1};
        for (auto it {first}; it != last; ++it) {
            std::cout << count++ << ' ' << *it << '\n';
        }
 
        std::ranges::destroy(first, last);
    }
    catch(...)
    {
        std::cout << "Exception!\n";
    }
}

Output:

1 ▄▀▄▀▄▀▄▀
2 ▄▀▄▀▄▀▄▀
3 ▄▀▄▀▄▀▄▀
4 ▄▀▄▀▄▀▄▀

Compiler support
Freestanding and hosted
Language
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Technical specifications
Symbols index
External libraries

ranges::uninitialized_fill_n (C++20)	copies an object to an uninitialized area of memory, defined by a start and a count (niebloid)
uninitialized_fill	copies an object to an uninitialized area of memory, defined by a range (function template)