std::ranges::construct_at(3)

🌸 NAME 🌸

std::ranges::construct_at - std::ranges::construct_at

okay bestie, welcome to the <memory> corner of the C++ Standard Libary manual — yes, that's the spelling up top, we're keeping it 💅. This page is literally std::ranges::construct_at(3), and it's giving "make an object at a pointer, but make it constexpr-friendly" energy ✨

✨ Synopsis ✨

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

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:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

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

🎀 Parameters 🎀

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

💖 Return value 💖

p

🦋 Possible implementation 🦋

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.

🍓 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 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();

💅 Defect reports 💅

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

• LWG 3870, applied to C++20 — as published: construct_at could create objects of a cv-qualified types; correct behavior: only cv-unqualified types are permitted.

👑 See also 👑

• ranges::destroy_at — destroys an object at a given address (C++20) (niebloid)
• construct_at — creates an object at a given address (C++20) (function template)

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💖 source/version: http://cppreference.com · date: 2024.06.10 · page: std::ranges::construct_at(3) — that's the cute sign-off, bestie, mwah 💋