std::ranges::uninitialized_fill_n

Copies value to an uninitialized memory area first + [​0​, count) as if by return ranges::uninitialized_fill (std::counted_iterator (first, count),
                                  std::default_sentinel, value).base();

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 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.

[edit] Parameters

[edit] Return value

As described above.

[edit] Complexity

Linear in count.

[edit] Exceptions

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

[edit] 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.

[edit] 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{};

[edit] Example

#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";
 }
}

cppreference
cppreference
cppreference

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also

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