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std::ranges...t_construct(3) C++ Standard Libary std::ranges...t_construct(3) NAME std::ranges::uninitialized_default_construct - std::ranges::uninitial- ized_default_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_default_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_default_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 default-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 the ele- ments to initialize r - the range of the elements to 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 skip the objects construction (without chang- ing the observable effect) if no non-trivial default constructor is called while de- fault-initializing a std::iter_value_t<I> object, which can be detected by std::is_trivially_default_constructible_v. Possible implementation struct uninitialized_default_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 ValueType = std::remove_reference_t<std::iter_refer- ence_t<I>>; if constexpr (std::is_trivially_default_constructible_v<Val- ueType>) return ranges::next(first, last); // skip initialization I rollback{first}; try { for (; !(first == last); ++first) ::new (const_cast<void*>(static_cast<const volatile void*> (std::addressof(*first)))) ValueType; 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_default_construct_fn uninitial- ized_default_construct{}; Example // Run this code #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{first + n}; std::ranges::uninitialized_default_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_default_con- struct // generally does not zero-fill the given uninitialized memory area. constexpr char sample[]{'A', 'B', 'C', 'D', '\n'}; char v[]{'A', 'B', 'C', 'D', '\n'}; std::ranges::uninitialized_default_construct(std::begin(v), std::end(v)); if (std::memcmp(v, sample, sizeof(v)) == 0) { std::cout << " "; // Maybe undefined behavior, pending CWG 1997: // for (const char c : v) { std::cout << c << ' '; } for (const char c : sample) std::cout << c << ' '; } else std::cout << "Unspecified\n"; } Possible output: 1 2 3 4 A B C D 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 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 ranges::uninitialized_value_construct in an uninitialized area of memory, (C++20) defined by a range (niebloid) 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 uninitialized_default_construct default-initialization in an uninitialized (C++17) area of memory, defined by a range (function template) http://cppreference.com 2024.06.10 std::ranges...t_construct(3)
NAME | Synopsis | Parameters | Return value | Complexity | Exceptions | Notes | Possible implementation | Example | Possible output: | See also
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