std::ranges:: construct_at

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.

Parameters

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);
}

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.

See also