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 address 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 expressions.

When construct_at is called in the evaluation of some constant expression e, the
argument p must point to either storage obtained by std::allocator<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 retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3870 C++20 construct_at could create objects only cv-unqualified 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)