std::ranges::construct_at(3)

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

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.
      DR Applied to Behavior as published Correct behavior
   LWG 3870 C++20 construct_at could create objects only cv-unqualified types are
                       of a cv-qualified types 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)