NAME¶

std::ranges::uninitialized_fill - std::ranges::uninitialized_fill

Synopsis¶

Defined in header <memory>
Call signature
template< no-throw-forward-iterator I, no-throw-sentinel-for<I> S, class
T >
(1) (since
requires std::constructible_from<std::iter_value_t<I>, const T&> C++20)

I uninitialized_fill( I first, S last, const T& x );
template< no-throw-forward-range R, class T >

requires std::constructible_from<ranges::range_value_t<R>, const T&> (2) (since
ranges::borrowed_iterator_t<R> C++20)

uninitialized_fill( R&& r, const T& x );

1) Constructs \(\scriptsize N\)N copies of the given value x in an uninitialized
memory area, designated by the range [first, last), where \(\scriptsize N\)N is
ranges::distance(first, last).
The function has the effect equivalent to:

for (; first != last; ++first)
::new (static_cast<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 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.

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 ▄▀▄▀▄▀▄▀

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 kept disallowed
storage

See also¶

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