std::construct_at
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 ifconstexpr(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.
Contents |
[edit]Parameters
| location | - | pointer to the uninitialized storage on which a T object will be constructed |
| args... | - | arguments used for initialization |
[edit]Return value
location
[edit]Example
#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!")]]constexprbool operator==(const S&)constnoexcept=default;}; consteval bool test(){ alignas(S)unsignedchar 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);constbool 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.
| 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 |
[edit]See also
| allocates uninitialized storage (public member function of std::allocator<T>) | |
[static] | constructs an object in the allocated storage (function template) |
(C++17) | destroys an object at a given address (function template) |
(C++20) | creates an object at a given address (algorithm function object) |
