Variable declaration with const and reference

Question

I am confused about variable declarations with references. I read some pages and that made me more confused so preferred to ask here.

I was asked 4 questions in a C++ interview; which one of them is wrong and why?

(1) int & n;
(2) const int & n=15;
(3) float x; const int & n =x;

Could some one please explain which in incorrect and why?

Answer 1

tl;dr References must be initialised on declaration (either directly or in the class initialisation).

int& n;

This leaves the reference n uninitialised. References must be initialised on declaration.

const int& n = 15;

Nothing wrong here. The reference to a const int is initialised to 15.

float x; const int& n = x;

The float is ok, but a bad idea to be left uninitialised. The reference n being initialised to x (float/integral conversions apply), it's not a great idea either.

Answer 2

A reference must always be initialized.