Is X const &x == X & const x?

Question

I understand that X & const x is redundant. It is not ok! But I want to know what is the difference between X const &x and X & const x? Is the first expression saying that x is a reference to a constant class X ?

@VJo Because the standard says so. Except in cases where it says it's OK. (If the `const` is inserted as the result of a `typedef` or a template expansion, for example.) — James Kanze, Oct 17 '11 at 16:30
How can something legal be equivalent to something illegal? :) — fredoverflow, Oct 17 '11 at 16:52

score 6 · Answer 1 · answered Oct 17 '11 at 16:28

6

X const &x is a reference to a const X, while X & const x is illegal. There is no such thing as a const reference since references aren't mutable to begin with.

answered Oct 17 '11 at 16:28

K-ballo

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Ironically, putting "mutable" in `backticks` may be read as reference to the very real C++ keyword of that name, which allows one to [change `const` members from inside `const` objects](http://stackoverflow.com/questions/105014/c-mutable-keyword). Not that any of this would make "constant references" any more meaningful or legal (as far as I know, that is). – Oct 17 '11 at 16:33
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@delnan: I added the backticks as a force of habit since `mutable` is indeed a keyword. Should I remove them to avoid confusion? – K-ballo Oct 17 '11 at 16:35
Well, it may save a reader or two a second of wonder, but I only added my comment because I found the irony amusing. – Oct 17 '11 at 16:36
@delnan: Let's go with irony then, its worth a second of wonder. – K-ballo Oct 17 '11 at 16:40

score 5 · Answer 2 · answered Oct 17 '11 at 16:28

5

References themselves are always constant. You cannot change the reference to refer to something else after initialization.

Yes first expression says that the referrence is referring a constant.

answered Oct 17 '11 at 16:28

Alok Save

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score 5 · Accepted Answer · answered Oct 17 '11 at 16:30

5

Read it from right to left : X& const x - x is a constant reference to object of type X which doesn't make sense, references are constant by definition. X const& x - x is a reference to constant of type X.

answered Oct 17 '11 at 16:30

a1ex07

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score 1 · Answer 4 · answered Oct 17 '11 at 16:47

X const& if a reference to an X which you are not allowed to modify through the reference. You can think of it as a read-only-view. Whether or not the X itself is const is not reflected in the reference type. Note that you can initialize X const& with both const and non-const objects:

X a;
X const b;
X const& r = a;   // read-only-view on non-const X
X const& s = b;   // read-only-view on const X

The important part is that you cannot change the X through the reference, but you can change a directly, and that change will be reflected via r.

X& const is forbidden by the standard since references themselves can never be modified, anyway.

Is X const &x == X & const x?

4 Answers4