std::ranges::uninitialized_value_construct_n - cppreference.com (original) (raw)

| Defined in header | | | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | ------------------------------------- | | Call signature | | | | template< no-throw-forward-iterator I > requires std::default_initializable<std::iter_value_t<I>>I uninitialized_value_construct_n( I first, std::iter_difference_t<I> count ); | | (since C++20) (constexpr since C++26) |

Constructs objects of type std::iter_value_t<I> in the uninitialized memory area first+ [​0​, count) by value-initialization, as if byreturn ranges::uninitialized_value_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.

Contents

• 1 Parameters
• 2 Return value
• 3 Complexity
• 4 Exceptions
• 5 Notes
• 6 Possible implementation
• 7 Example
• 8 Defect reports
• 9 See also

[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_value_construct_n, e.g. by using ranges::fill_n, if the value type of the range is a CopyAssignable TrivialType.

[edit] Possible implementation

[edit] Example

#include #include #include   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_value_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_value_construct_n // zero-initializes the given uninitialized memory area. int v[]{1, 2, 3, 4, 5, 6, 7, 8}; std::cout << ' '; for (const int i : v) std::cout << i << ' '; std::cout << "\n "; std::ranges::uninitialized_value_construct_n(std::begin(v), std::size(v)); for (const int i : v) std::cout << i << ' '; std::cout << '\n'; }

Output:

1 █▓▒░ █▓▒░ █▓▒░ 2 █▓▒░ █▓▒░ █▓▒░ 3 █▓▒░ █▓▒░ █▓▒░ 4 █▓▒░ █▓▒░ █▓▒░ 1 2 3 4 5 6 7 8 0 0 0 0 0 0 0 0

[edit] Defect reports

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

[edit] See also