std::launder

 

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Defined in header <new>
template <class T> constexpr T* launder(T* p)		(since C++17)

Obtains a pointer to an object created in storage occupied by an existing object of the same type, even if it has const or reference members.

Formally, given

• the pointer p represents the address A of a byte in memory
• an object X is located at the address A
• X is within its lifetime
• the type of X is the same as T, ignoring cv-qualifiers at every level
• Every byte that would be reachable through the result is reachable through p (bytes are reachable through a pointer that points to an object if those bytes are within that object's storage, or within the immediately enclosing array of which the object is an element)

Then std::launder(p) returns a value of type T* that points to the object X.

The program is ill-formed if T is a function type or (possibly cv-qualified) void.

std::launder may be used in a core constant expression if the value of its argument may be used in a core constant expression

Exceptions

noexcept specification:  

noexcept

  

Notes

For objects that do not have const or reference members, std::launder is not necessary; pointers and references can be reused

Example

Run this code

#include <new>
 
struct X {
  const int n; // note: X has a const member
  int m;
};
int main()
{
  X *p = new X{3};
  const int a = p->n;
  new (p) X{5};       // p does not point to new object because X::n is const
  const int b = p->n; // undefined behavior
  const int x = p->m; // undefined behavior (even though m is non-const, p can't be used)
  const int c = std::launder(p)->n; // OK, std::launder(p) points to new object
}