17 Language support library [support]

17.6 Dynamic memory management [support.dynamic]

17.6.5 Pointer optimization barrier [ptr.launder]

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template<class T> constexpr T* launder(T* p) noexcept;
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Mandates: !is_function_v<T> && !is_void_v<T> is true.
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Preconditions: p represents the address A of a byte in memory.
An object X that is within its lifetime and whose type is similar to T is located at the address A.
All bytes of storage that would be reachable through ([basic.compound]) the result are reachable through p.
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Returns: A value of type T* that points to X.
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Remarks: An invocation of this function may be used in a core constant expression if and only if the (converted) value of its argument may be used in place of the function invocation.
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[Note 1: 
If a new object is created in storage occupied by an existing object of the same type, a pointer to the original object can be used to refer to the new object unless its complete object is a const object or it is a base class subobject; in the latter cases, this function can be used to obtain a usable pointer to the new object.
— end note]
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[Example 1: struct X { int n; }; const X *p = new const X{3}; const int a = p->n; new (const_cast<X*>(p)) const X{5}; // p does not point to new object ([basic.life]) because its type is const const int b = p->n; // undefined behavior const int c = std::launder(p)->n; // OK — end example]