What does "Fred const& x" mean?, C++ FAQ

C++ FAQ / Section 18 / FAQ 18.6

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`?

[18.6] What does "Fred const& x" mean?

It means x aliases a Fred object, but x can't be used to change that Fred object.

Read it right-to-left: "x is a reference to a constant Fred."

For example, if class Fred has a const member function called inspect(), saying x.inspect() is OK. But if class Fred has a non-const member function called mutate(), saying x.mutate() is an error (the error is caught by the compiler; no run-time tests are done, which means const doesn't slow your program down).