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std::ranges...e_construct(3) C++ Standard Libary std::ranges...e_construct(3) NAME std::ranges::uninitialized_value_construct - std::ranges::uninitial- ized_value_construct Synopsis Defined in header <memory> Call signature template< no-throw-forward-iterator I, no-throw-sentinel-for<I> S > (since requires std::default_initializable<std::iter_value_t<I>> (1) C++20) I uninitialized_value_construct( I first, S last ); template< no-throw-forward-range R > requires std::default_initializable<ranges::range_value_t<R>> (2) (since ranges::borrowed_iterator_t<R> C++20) uninitialized_value_construct( R&& r ); 1) Constructs objects of type std::iter_value_t<I> in the uninitial- ized storage designated by the range [first, last) by value-initialization, as if by for (; first != last; ++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. 2) Same as (1), but uses r as the range, as if using ranges::be- gin(r) as first, and ranges::end(r) as last. 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, last - iterator-sentinel pair denoting the range of elements to value-initialize r - the range of the elements to value-initialize Return value An iterator equal to last. Complexity Linear in the distance between first and last. 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, e.g. by using ranges::fill, if the value type of the range is TrivialType and CopyAssignable. Possible implementation struct uninitialized_value_construct_fn { template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S> requires std::default_initializable<std::iter_value_t<I>> I operator()(I first, S last) 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(first, last, T()); I rollback{first}; try { for (; !(first == last); ++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; } } template<no-throw-forward-range R> requires std::default_initializable<ranges::range_value_t<R>> ranges::borrowed_iterator_t<R> operator()(R&& r) const { return (*this)(ranges::begin(r), ranges::end(r)); } }; inline constexpr uninitialized_value_construct_fn uninitial- ized_value_construct{}; 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{first + n}; std::ranges::uninitialized_value_construct(first, last); 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 // zero-fills the given uninitialized memory area. int v[]{0, 1, 2, 3}; std::cout << ' '; for (const int i : v) std::cout << ' ' << static_cast<char>(i + 'A'); std::cout << "\n "; std::ranges::uninitialized_value_construct(std::begin(v), std::end(v)); for (const int i : v) std::cout << ' ' << static_cast<char>(i + 'A'); std::cout << '\n'; } Output: 1 2 3 4 A B C D A A A A 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_n in an uninitialized area of memory, (C++20) defined by a start and a count (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 in an uninitialized area of memory, (C++17) defined by a range (function template) http://cppreference.com 2024.06.10 std::ranges...e_construct(3)
NAME | Synopsis | Parameters | Return value | Complexity | Exceptions | Notes | Possible implementation | Example | Output: | See also
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