Is "rebinding" references in C++ like this legal?

Question

Is the following legal in C++?

As far as I can tell, Reference has a trivial destructor, so it should be legal.
But I thought references can't be rebound legally... can they?

template<class T> struct Reference {     T &r;     Reference(T &r) : r(r) { } };  int main() {     int x = 5, y = 6;     Reference<int> r(x);     new (&r) Reference<int>(y); } 

Answer 1

You aren't rebinding a reference, you're creating a new object in the memory of another one with a placement new. Since the destructor of the old object was never run I think this would be undefined behavior.

Answer 2

There is no reference being rebound in your example. The first reference (constructed on line two with the name r.r) is bound to the int denoted by x for the entire of its lifetime. This reference's lifetime is ended when the storage for its containing object is re-used by the placement new expression on line three. The replacement object contains a reference which is bound y for its entire lifetime which lasts until the end of its scope - the end of main.