std::construct_at() algorithm

od C++20

// (1)
template< class T, class... Args >
constexpr T* construct_at( T* p, Args&&... args );

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

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.

Overload Resolution

Participates in overload resolution only if ::new(std::declval<void*>()) T(std::declval<Args>()...) is well-formed in unevaluated context.

Parameters

p	Pointer to the uninitialized storage on which a T object will be constructed.
args..	Args to initialize the object with.

Return value

p

(none)

Complexity

O(1)

Exceptions

(none)

Possible implementation

ranges::construct_at(1)

struct construct_at_fn
{
    template<class T, class...Args>
        requires
            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{};

Notes

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

Examples

Main.cpp

#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);
    ptr->print();

    std::ranges::destroy_at(ptr);
}

Output

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