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std::construct_at(3) C++ Standard Libary std::construct_at(3) NAME std::construct_at - std::construct_at Synopsis Defined in header <memory> template< class T, class... Args > (since C++20) constexpr T* construct_at( T* p, Args&&... args ); Creates a T object initialized with arguments args... at given ad- dress p. Specialization of this function template participates in overload resolution only if ::new(std::declval<void*>()) T(std::declval<Args>()...) is well- formed in an unevaluated context. Equivalent to return ::new (static_cast<void*>(p)) T(std::forward<Args>(args)...); except that construct_at may be used in evaluation of constant ex- pressions. When construct_at is called in the evaluation of some constant ex- pression e, the argument p must point to either storage obtained by std::alloca- tor<T>::allocate or an object whose lifetime began within the evaluation of e. Parameters p - pointer to the uninitialized storage on which a T object will be constructed args... - arguments used for initialization Return value p Example // Run this code #include <bit> #include <memory> class S { int x_; float y_; double z_; public: constexpr S(int x, float y, double z) : x_{x}, y_{y}, z_{z} {} [[nodiscard("no side-effects!")]] constexpr bool operator==(const S&) const noexcept = default; }; consteval bool test() { alignas(S) unsigned char storage[sizeof(S)]{}; S uninitialized = std::bit_cast<S>(storage); std::destroy_at(&uninitialized); S* ptr = std::construct_at(std::addressof(uninitialized), 42, 2.71f, 3.14); const bool res{*ptr == S{42, 2.71f, 3.14}}; std::destroy_at(ptr); return res; } static_assert(test()); int main() {} 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 construct_at could create objects only cv-un- qualified types are of a cv-qualified types permitted See also allocate allocates uninitialized storage (public member function of std::allocator<T>) construct constructs an object in the allocated storage [static] (function template) destroy_at destroys an object at a given address (C++17) (function template) ranges::construct_at creates an object at a given address (C++20) (niebloid) http://cppreference.com 2024.06.10 std::construct_at(3)
NAME | Synopsis | Parameters | Return value | Example | See also
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