std::construct_at - cppreference.com (original) (raw)

| | | | | ------------------------------------------------------------------------------------------------ | | ------------- | | 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_](voidify.html "cpp/memory/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.

[edit] Parameters

[edit] Return value

location

[edit] Example

#include #include   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(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() {}

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also

| | allocates uninitialized storage (public member function of std::allocator) [edit] | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | constructs an object in the allocated storage (function template) [edit] | | | destroys an object at a given address (function template) [edit] | | | creates an object at a given address(algorithm function object)[edit] |