std::construct_at

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)

Parameters

Return value

location

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