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std::ranges::construct_at

From cppreference.com
< cpp‎ | memory
 
 
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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 (const_cast<void*>(static_cast<const volatile 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:

In practice, they may be implemented as function objects, or with special compiler extensions.

Contents

[edit] Parameters

p - pointer to the uninitialized storage on which a T object will be constructed
args... - arguments used for initialization

[edit] Return value

p

[edit] 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{};

[edit] Notes

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

[edit] 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();

[edit] See also

destroys an object at a given address
(niebloid) [edit]
creates an object at a given address
(function template) [edit]