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    std::construct_at

    From cppreference.com
     
    C++
     
    Memory management library
    (exposition only*)
    Allocators
    Uninitialized memory algorithms
    Constrained uninitialized memory algorithms
    Memory resources
    Uninitialized storage (until C++20)
    (until C++20*)
    (until C++20*)
    (until C++20*)

    Garbage collector support (until C++23)
    (C++11)(until C++23)
    (C++11)(until C++23)
    (C++11)(until C++23)
    (C++11)(until C++23)
    (C++11)(until C++23)
    (C++11)(until C++23)
     
    Defined in header <memory>
    template< class T, class... Args > constexpr T* construct_at( T* location, Args&&... args );
    		 (since C++20)

    Creates a T object initialized with the arguments in args at given address location.

    Equivalent to if constexpr (std::is_array_v<T>)
        return ::new (voidify (*location)) T[1]();
    else
        return ::new (voidify (*location)) T(std::forward<Args>(args)...);     , except that construct_at may be used in evaluation of constant expressions(until C++26).

    When construct_at is called in the evaluation of some constant expression expr, location must point to either a storage obtained by std::allocator<T>::allocate or an object whose lifetime began within the evaluation of expr.

    This overload participates in overload resolution only if all following conditions are satisfied:

    • std::is_unbounded_array_v<T> is false.
    • ::new(std::declval<void*>()) T(std::declval<Args>()...) is well-formed when treated as an unevaluated operand.

    If std::is_array_v<T> is true and sizeof...(Args) is nonzero, the program is ill-formed.

    Parameters

    location - pointer to the uninitialized storage on which a T object will be constructed
    args... - arguments used for initialization

    Return value

    location

    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 retroactively to previously published C++ standards.

    DR Applied to Behavior as published Correct behavior
    LWG 3436 C++20 construct_at could not create objects of array types can value-initialize bounded arrays
    LWG 3870 C++20 construct_at could create objects of cv-qualified types only cv-unqualified types are permitted

    See also

    		 allocates uninitialized storage
    (public member function of std::allocator<T>) [edit]
    [static]
    		 constructs an object in the allocated storage
    (function template) [edit]
    (C++17)
    		 destroys an object at a given address
    (function template) [edit]
    (C++20)
    		 creates an object at a given address
    (algorithm function object)[edit]