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)