std::ranges::construct_at

 

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

Creates a T object initialized with the arguments in args at given address location.

Equivalent to ifconstexpr(std::is_array_v<T>)
    return::new(voidify (*location)) T[1]();
else
    return::new(voidify (*location)) T(std::forward<Args>(args)...);, except that construct_at may be used in evaluation of constant expressions(until C++26).

When construct_at is called in the evaluation of some constant expression expr, location must point to either a storage obtained by std::allocator<T>::allocate or an object whose lifetime began within the evaluation of expr.

This overload participates in overload resolution only if all following conditions are satisfied:

• std::is_unbounded_array_v<T> is false.
• ::new(std::declval<void*>()) T(std::declval<Args>()...) is well-formed when treated as an unevaluated operand.

If std::is_array_v<T> is true and sizeof...(Args) is nonzero, the program is ill-formed.

The function-like entities described on this page are algorithm function objects (informally known as 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.

Parameters

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

Return value

location

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)unsignedchar 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 3436	C++20	construct_at could not create objects of array types	can value-initialize bounded arrays
LWG 3870	C++20	construct_at could create objects of cv-qualified types	only cv-unqualified types are permitted

See also

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