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std::ranges::construct_at - cppreference.com

Call signature

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

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:

• 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.

location

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

#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();

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

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