std:: construct_at

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

Return value

p

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() {}

Defect reports

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

See also