C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
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Section 18:
[18.7] Does "Fred& const x" make any sense?

No, it is nonsense.

To find out what the above declaration means, you have to read it right-to-left. Thus "Fred& const x" means "x is a const reference to a Fred". But that is redundant, since references are always const. You can't reseat a reference. Never. With or without the const.

In other words, "Fred& const x" is functionally equivalent to "Fred& x". Since you're gaining nothing by adding the const after the &, you shouldn't add it since it will confuse people — the const will make some people think that the Fred is const, as if you had said "Fred const& x".