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& value );	(1)	(since C++20) (constexpr since C++26)
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& value );	(2)	(since C++20) (constexpr since C++26)

1) Copies value to an uninitialized memory area [first, last) as if by

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

If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.

2) Equivalent to return ranges::uninitialized_fill(ranges::begin(r), ranges::end(r), value);.

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

Explicit template argument lists cannot be specified when calling any of them.
None of them are visible to argument-dependent lookup.
When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

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

As described above.

Complexity

Linear in the size of the uninitialized memory area.

Exceptions

Any exception thrown on construction of the elements in the destination range.

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.

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

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&>
    constexpr I operator()(I first, S last, const T& value) const
    {
        I rollback{first};
        try
        {
            for (; !(first == last); ++first)
                ranges::construct_at(std::addressof(*first), value);
            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&>
    constexpr ranges::borrowed_iterator_t<R> operator()(R&& r, const T& value) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), value);
    }
};
 
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 ▄▀▄▀▄▀▄▀

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

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Standard library headers
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Feature test macros (C++20)
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Concepts library (C++20)
Diagnostics library
Memory management library
Metaprogramming library (C++11)
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Containers library
Iterators library
Ranges library (C++20)
Algorithms library
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Date and time library
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Concurrency support library (C++11)
Execution support library (C++26)
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 (algorithm function object)
uninitialized_fill	copies an object to an uninitialized area of memory, defined by a range (function template)