Question

Short question, is the following ok:

struct X
{
    A& x;
    A  y;
    X() : x(y) {}
};

Reversing the order of the two members in the struct is definitely ok, since it guarantees y is initialized first, but does this work or invoke UB?

Moreso, would the following be ok:

struct X
{
   X& x;
   X() : x(*this) {}
};

?

Answer 1

I don’t think that would invoke undefined behavior.

I don’t see that case is any different from this:

 int *p = new int;

The expression new int is a pointer to an uninitialized int. Here we are initializing the pointer p with new int. The content is not read.

Similarly,

 int & r = *p; //or *new int

Here we are initializing the reference r with *p. The content is not read

In both cases, the content is not read. Reading an uninitialized variable invokes undefined behavior. In both cases, it is the content which is uninitialized, not the address, and we are not reading the content.