std::ranges::construct_at

 

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

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 of a cv-qualified types	only cv-unqualified types are permitted

See also

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