< cpp‎ | memory
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Dynamic memory management
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(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)

Defined in header <memory>
Call signature
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. construct_at participates in overload resolution only if ::new(std::declval<void*>()) T(std::declval<Args>()...) is well-formed in 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.

The function-like entities described on this page are niebloids, that is:

In practice, they may be implemented as function objects, or with special compiler extensions.


[edit] Parameters

p - pointer to the uninitialized storage on which a T object will be constructed
args... - arguments used for initialization

[edit] Return value


[edit] Possible implementation

struct construct_at_fn
    template<class T, class...Args>
            requires (void* vp, Args&&... args)
                { ::new (vp) T(static_cast<Args&&>(args)...); }
    constexpr T* operator()(T* p, Args&&... args) const
        return std::construct_at(p, static_cast<Args&&>(args)...);
inline constexpr construct_at_fn construct_at{};

[edit] Notes

std::ranges::construct_at behaves exactly same as std::construct_at, except that it is invisible to argument-dependent lookup.

[edit] Example

#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 buf[sizeof(S)];
    S* ptr = std::ranges::construct_at(reinterpret_cast<S*>(buf), 42, 2.71828f, 3.1415);


S { x=42; y=2.71828; z=3.1415; };

[edit] 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

[edit] See also

destroys an object at a given address
creates an object at a given address
(function template) [edit]