std::construct_at

 

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Defined in header <memory>
template< class T, class... Args > constexpr T* construct_at( T* p, Args&&... args );		(since C++20)

Creates a T object initialized with arguments args... at given address p.

Equivalent to:

if constexpr (std::is_array_v<T>)
    return ::new (static_cast<void*>(p)) T[1]();
else
    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.

Specialization of this function template participates in overload resolution only if:

• std::is_unbounded_array_v<T> is false, and
• ::new(std::declval<void*>()) T(std::declval<Args>()...) is well-formed in an unevaluated context.

The program is ill-formed if sizeof...(Args) is nonzero when std::is_array_v<T> is true.

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 of a cv-qualified types	only cv-unqualified types are permitted
LWG 3436	C++20	construct_at on pointers to bounded array was probihited	allowed only in value-initialization

See also

allocate	allocates uninitialized storage (public member function of std::allocator<T>)
construct [static]	constructs an object in the allocated storage (function template)
destroy_at (C++17)	destroys an object at a given address (function template)
ranges::construct_at (C++20)	creates an object at a given address (algorithm function object)