reference and const confusion in C++

Question

I knew that reference is just another name of a variable, they do not exist as a separate object in memory but what is happening here

double i = 24.7;
const int &ri = i;  //notice int here
std::cout << i << std::endl; //prints 24.7
i = 44.4;
std::cout << ri << std::endl; // prints 24
std::cout << i << std::endl; //prints 44.4

My question is ri is alias of what ? [value 24 in somewhere memory]

songyuanyao · Accepted Answer · 2017-12-06T01:30:32.797

3

You can't bind reference to object with different type directly.

For const int &ri = i;, i needs to be converted to int at first, then a temporary int is created and then bound to ri, it has nothing to do with the original object i.

BTW: The lifetime of the temporary is extended to match the lifetime of the reference here.

BTW2: Temporary can only be bound to lvalue-reference to const or rvalue-reference.

edited Dec 06 '17 at 01:30

answered Dec 06 '17 at 01:13

songyuanyao

169,198
16
310
405

I think this is correct, so +1. But is there a source for this? A requirement in the standard? – Fantastic Mr Fox Dec 06 '17 at 01:15
wow - i would expect that to not compile. I hope it warns – pm100 Dec 06 '17 at 01:15
@pm100 You would think so, but apparently no: [example](http://coliru.stacked-crooked.com/a/f8d5929cc8f5b347) – Fantastic Mr Fox Dec 06 '17 at 01:16
No warning on GNU. – Philip Brack Dec 06 '17 at 01:18
Correct answer, will accept in 5mins. – Kumar Roshan Mehta Dec 06 '17 at 01:20
@FantasticMrFox Their types don't match, so `double` needs to be converted to `int` before the binding. The converted `int` is a temporary, which is bound to the reference. Similar rule applies for pointer; we also can't take the address of `double` and assign it to a pointer to `int` without conversion. – songyuanyao Dec 06 '17 at 01:23
right but this is *very* fragile code. If I had loaded ri from a function returning a float which then later gets changed to one returning an int this code will compile cleanly but the semantics have significantly changed – pm100 Dec 06 '17 at 01:38
aha - fatals if you take out the const. So not so bad – pm100 Dec 06 '17 at 01:39

reference and const confusion in C++

1 Answers1