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# std::launder

From cppreference.com
< cpp‎ | utility

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

### [edit] Exceptions

`noexcept` specification:
noexcept

### [edit] Notes

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

### [edit] 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
    }

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