std::construct_at
<memory>
constexpr T* construct_at( T* location, Args&&... args );
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.
[edit] Parameters
T object will be constructed
[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!")]] 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() {}
[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 |