Call destructor and then constructor (resetting an object)

Question

I want to reset an object. Can I do it in the following way?

anObject->~AnObject();
anObject = new(anObject) AnObject();
// edit: this is not allowed: anO         


        

Answer 1

Yes, what you are doing is valid most of the time. [basic.life]p8 says:

If, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, a new object is created at the storage location which the original object occupied, a pointer that pointed to the original object, a reference that referred to the original object, or the name of the original object will automatically refer to the new object and, once the lifetime of the new object has started, can be used to manipulate the new object, if:

• the storage for the new object exactly overlays the storage location which the original object occupied, and

• the new object is of the same type as the original object (ignoring the top-level cv-qualifiers), and

• the type of the original object is not const-qualified, and, if a class type, does not contain any non-static data member whose type is const-qualified or a reference type, and

• neither the original object nor the new object is a potentially-overlapping subobject ([intro.object]).

So it is legal if you don't have a const or reference member.

If you don't have this guarantee, you need to use std::launder or use the pointer returned by placement new (like you're doing anyways) if you want to use the new object:

// no const/ref members
anObject->~AnObject(); // destroy object
new (anObject) AnObject(); // create new object in same storage, ok

anObject->f(); // ok

// const/ref members
anObject->~AnObject();
auto newObject = new (anObject) AnObject();

anObject->f(); // UB
newObject->f(); // ok
std::launder(anObject)->f(); // ok