References don’t have top-level cv-qualifiers

Sometimes when reading about C++, for instance about template argument deduction, the term “top-level cv-qualifiers” comes up. I just spent an unreasonable amount of time being puzzled by something because I didn’t understand that references don’t have top-level cv-qualifiers. This post will hopefully help the next person (hi, next-year Anders) not make the same mistake.

Looking at

const int&

, I assumed without even thinking about it that the const here was the top-level one. It is not.

First, what’s a cv-qualifier? It’s const, volatile or both. Let’s just use const as an example for this article.

Then, what’s a top-level cv-qualifier? The best way to explain is with an example, and the best example is a pointer. There are two levels of constness to a pointer, the constness of the pointer itself, and the constness of what it’s pointing to.

Given const int *, a non-const pointer to const int, we can visualise it as

pointer (the * part)

to

const int (the const int part)

And given const int * const, a const pointer to const int, we can visualise it as

const pointer (the * const part)

to

const int (the const int part)

(And so on, you can imagine how it looks for pointers to non-const int.) The top-level cv-qualifier is the one on the top level, the cv-qualifier on the pointer itself.

Now, how does this look for references?

Given const int&, a reference to const int, we can visualise it as

reference (the & part)

to

const int (the const int part)

But there’s no such thing as a const reference! Constness applies to the object itself, and a reference is not an object, just an alternative name for an existing object. So there is no such thing as a const int& const, i.e. there’s no such thing as

const reference (the & const part)

to

const int (the const int part)

Which means, references don’t have top-level cv-qualifiers. The standard even has an example:

Example: The type corresponding to the type-id const int& has no top-level cv-qualifiers.

[basic.type.qualifier]

This is by the way a somewhat recent addition, until this Core Language Defect Report was resolved in 2014, the term “top-level cv-qualifier” was never actually defined in the standard.