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std::ranges...construct_n(3) C++ Standard Libary std::ranges...construct_n(3) NAME std::ranges::uninitialized_value_construct_n - std::ranges::uninitial- ized_value_construct_n Synopsis Defined in header <memory> Call signature template< no-throw-forward-iterator I > requires std::default_initializable<std::iter_value_t<I>> (since C++20) I uninitialized_value_construct_n( I first, std::iter_differ- ence_t<I> n ); Constructs n objects of type std::iter_value_t<I> in the uninitial- ized memory area starting at first by value-initialization, as if by for (; n-- > 0; ++first) ::new (static_cast<void*>(std::addressof(*first))) std::remove_reference_t<std::iter_reference_t<I>>(); If an exception is thrown during the initialization, the objects al- ready 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 call- ing 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 in- hibited. In practice, they may be implemented as function objects, or with special compiler extensions. Parameters first - the beginning of the range of elements to initialize n - the number of elements to construct Return value The end of the range of objects (i.e., ranges::next(first, n)). Complexity Linear in n. Exceptions The exception thrown on construction of the elements in the destina- tion range, if any. 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 TrivialType and CopyAssignable. Possible implementation struct uninitialized_value_construct_n_fn { template<no-throw-forward-iterator I> requires std::default_initializable<std::iter_value_t<I>> I operator()(I first, std::iter_difference_t<I> n) const { using T = std::remove_reference_t<std::iter_reference_t<I>>; if constexpr (std::is_trivial_v<T> && std::is_copy_assigna- ble_v<T>) return ranges::fill_n(first, n, T()); I rollback{first}; try { for (; n-- > 0; ++first) ::new (const_cast<void*>(static_cast<const volatile void*> (std::addressof(*first)))) T(); return first; } catch (...) // rollback: destroy constructed elements { for (; rollback != first; ++rollback) ranges::destroy_at(std::addressof(*rollback)); throw; } } }; inline constexpr uninitialized_value_construct_n_fn uninitial- ized_value_construct_n{}; Example // Run this code #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_value_con- struct_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"; } // Notice that for "trivial types" the uninitialized_value_con- struct_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 Defect reports The following behavior-changing defect reports were applied retroac- tively 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 constructs objects by value-initialization ranges::uninitialized_value_construct in an uninitialized area of memory, (C++20) defined by a range (niebloid) constructs objects by ranges::uninitialized_default_construct default-initialization in an uninitialized (C++20) area of memory, defined by a range (niebloid) constructs objects by ranges::uninitialized_default_construct_n default-initialization in an uninitialized (C++20) area of memory, defined by a start and count (niebloid) constructs objects by value-initialization uninitialized_value_construct_n in an uninitialized area of memory, (C++17) defined by a start and a count (function template) http://cppreference.com 2024.06.10 std::ranges...construct_n(3)
NAME | Synopsis | Parameters | Return value | Complexity | Exceptions | Notes | Possible implementation | Example | Output: | See also
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