# std::construct_at

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< cpp‎ | memory

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. 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 <iostream>
    #include <memory>

    struct S
    {
        int x;
        float y;
        double z;

        S(int x, float y, double z) : x{x}, y{y}, z{z} { std::cout << "S::S();\n"; }

        ~S() { std::cout << "S::~S();\n"; }

        void print() const
        {
            std::cout << "S { x=" << x << "; y=" << y << "; z=" << z << "; };\n";
        }
    };

    int main()
    {
        alignas(S) unsigned char storage[sizeof(S)];

        S* ptr = std::construct_at(reinterpret_cast<S*>(storage), 42, 2.71828f, 3.1415);
        ptr->print();

        std::destroy_at(ptr);
    }

Output:

    S::S();
    S { x=42; y=2.71828; z=3.1415; };
    S::~S();

### 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

### 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 (niebloid)

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