std::ranges::construct_at

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< cpp‎ | memory
 
 
Memory management library
(exposition only*)
Uninitialized memory algorithms
(C++17)
(C++17)
(C++17)
Constrained uninitialized
memory algorithms
ranges::construct_at
(C++20)
C Library

Allocators
Memory resources
Garbage collection support
(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)
Uninitialized storage
(until C++20*)
(until C++20*)
Explicit lifetime management
 
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 if constexpr (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:

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:

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

#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 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

destroys an object at a given address
(algorithm function object)
creates an object at a given address
(function template)