NAME¶

std::ranges::uninitialized_fill_n - std::ranges::uninitialized_fill_n

Synopsis¶

Defined in header <memory>
Call signature
template< no-throw-forward-range I, class T >

requires std::constructible_from<std::iter_value_t<I>, const T&> (since C++20)

I uninitialized_fill_n( I first, std::iter_difference_t<I> n, const
T& x );

Constructs n copies of the given value x in an uninitialized memory area, designated
by the range [first, first + n), as if by

for (; n--; ++first) {
::new (static_cast<void*>(std::addressof(*first)))
std::remove_reference_t<std::iter_reference_t<I>>(x);
}

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:

* 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 - the beginning of the range of the elements to initialize
n - number of elements to construct
x - the value to construct the elements with

Return value¶

An iterator equal to first + n.

Complexity¶

Linear in 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_n,
e.g. by using ranges::fill_n, if the value type of the output range is TrivialType.

Possible implementation¶

struct uninitialized_fill_n_fn
{
template<no-throw-forward-range I, class T>
requires std::constructible_from<std::iter_value_t<I>, const T&>
I operator()(I first, std::iter_difference_t<I> n, const T& x) const
{
I rollback{first};
try
{
for (; n-- > 0; ++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;
}
}
};

inline constexpr uninitialized_fill_n_fn uninitialized_fill_n{};

Example¶

// Run this code

#include <iostream>
#include <memory>
#include <string>

int main()
{
constexpr int n{3};
alignas(alignof(std::string)) char out[n * sizeof(std::string)];

try
{
auto first{reinterpret_cast<std::string*>(out)};
auto last = std::ranges::uninitialized_fill_n(first, n, "cppreference");

for (auto it{first}; it != last; ++it)
std::cout << *it << '\n';

std::ranges::destroy(first, last);
}
catch (...)
{
std::cout << "Exception!\n";
}
}

Output:¶

cppreference
cppreference
cppreference

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