Does "Fred& const x" make any sense?, C++ FAQ

C++ FAQ / Section 18 / FAQ 18.7

Section 18:

18.1	What is "`const` correctness"?
18.2	How is "`const` correctness" related to ordinary type safety?
18.3	Should I try to get things `const` correct "sooner" or "later"?
18.4	What does "`Fred const* p`" mean?
18.5	Difference between "`Fred const* p`", "`Fred* const p`" and "`Fred const* const p`"?
18.6	What does "`Fred const& x`" mean?
18.7	Does "`Fred& const x`" make any sense?
18.8	What does "`const X& x`" mean?
18.9	What does "`const X* x`" mean?
18.10	What is a "`const` member function"?
18.11	Using return-by-reference in a `const` member function?
18.12	What's the deal with "`const`-overloading"?
18.13	How a `const` member function can make an "invisible" change to a data member?
18.14	Does `const_cast` mean lost optimization opportunities?
18.15	Binding a pointer-to-`const` to a non-`const` object?
18.16	Does "`Fred const* p`" mean that `*p` can't change?
18.17	Errors trying to convert `Foo` → `Foo const`?

[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".