std::ranges::uninitialized_default_construct_n

Constructs objects of type std::iter_value_t <I> in the uninitialized memory area first + [​0​, count) by default-initialization, as if by return ranges::uninitialized_default_construct (std::counted_iterator (first, count),
                                               std::default_sentinel ).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 skip the objects construction (without changing the observable effect) if no non-trivial default constructor is called while default-initializing a std::iter_value_t <I> object, which can be detected by std::is_trivially_default_constructible .

[edit] Notes

[edit] Possible implementation

struct uninitialized_default_construct_n_fn
{
 template<no-throw-forward-iterator I>
 requires std::default_initializable <std::iter_value_t <I>>
 constexpr I operator()(I first, std::iter_difference_t <I> count) const
 {
 auto iter = std::counted_iterator (first, count);
 return ranges::uninitialized_default_construct (iter, std::default_sentinel ).base();
 }
};
 
inline constexpr uninitialized_default_construct_n_fn uninitialized_default_construct_n{};

[edit] Example

#include <cstring>
#include <iostream>
#include <memory>
#include <string>
 
int main()
{
 struct S { std::string m{"█▓▒░ █▓▒░ "}; };
 
 constexpr int n{4};
 alignas(alignof(S)) char out[n * sizeof(S)];
 
 try
 {
 auto first{reinterpret_cast<S*>(out)};
 auto last = std::ranges::uninitialized_default_construct_n(first, n);
 
 auto count{1};
 for (auto it{first}; it != last; ++it)
 std::cout << count++ << ' ' << it->m << '\n';
 
 std::ranges::destroy (first, last);
 }
 catch (...)
 {
 std::cout << "Exception!\n";
 }
 
 // For scalar types, uninitialized_default_construct_n
 // generally does not zero-fill the given uninitialized memory area.
 constexpr int sample[]{1, 2, 3, 4, 5, 6};
 int v[]{1, 2, 3, 4, 5, 6};
 std::ranges::uninitialized_default_construct_n(std::begin (v), std::size (v));
 if (std::memcmp (v, sample, sizeof(v)) == 0)
 {
 // Maybe undefined behavior, pending CWG 1997:
 // for (const int i : v) { std::cout << i << ' '; }
 for (const int i : sample)
 std::cout << i << ' ';
 }
 else
 std::cout << "Unspecified!";
 std::cout << '\n';
}

1 █▓▒░ █▓▒░
2 █▓▒░ █▓▒░
3 █▓▒░ █▓▒░
4 █▓▒░ █▓▒░
1 2 3 4 5 6

[edit] Defect reports

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

[edit] See also

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