C++: const reference, before vs after type-specifier
Both work, and here is the explanation from the man who wrote it.
To quote him:
Why? When I invented "const" (initially named "readonly" and had a corresponding "writeonly"), I allowed it to go before or after the type because I could do so without ambiguity.
No difference as const is read right-to-left with respect to the &, so both represent a reference to an immutable Fred instance.
Fred& const
would mean the reference itself is immutable, which is redundant; when dealing with const pointers both Fred const*
and Fred* const
are valid but different.
It's a matter of style, but I prefer using const
as a suffix since it can be applied consistently including const member functions.
Though they are one and the same, to retain consistency with the RIGHT-LEFT rule about parsing C and C++ declarations, it is better to write Fred const &arg
Also refer this for developing more understanding about declarations, qualifiers and declarators.
Behavior
There is no semantic difference between const T&
and T const&
; the language treats them as the same type. (The same thing applies to const T*
and T const*
.)
As a matter of style
Regarding which you should prefer stylistically, however, I'll dissent from a lot of the other answers and prefer const T&
(and const T*
):
const T&
is the style used in Stroustrup's The C++ Programming Language book.const T&
is the style used in the C++ standard itself.const T*
is the style used in K&R's The C Programming Language book.const T*
is the style used in the C standard.- Due to the above factors, I think
const T&
/const T*
have way more inertia thanT const&
/T const*
.const T&
/const T*
empirically seem way more common to me thanT const&
/T const*
in all of the C++ and C code that I've seen. I think following common practices is more readable than dogmatically adhering to right-to-left parsing rules. - With
T const*
, it seems easier to misplace the*
asT* const
(especially if people aren't as accustomed to it). In contrast,const* T
is not legal syntax.
What about the right-to-left parsing rule?
Regarding the whole right-to-left parsing argument that people seem to love to use: as I mentioned in a comment to another answer, const T&
reads fine right-to-left too. It's a reference to a T constant. "T" and "constant" each can work as an adjective or a noun. (Additionally, reading T const*
right-to-left can be ambiguous since it could be incorrectly interpreted as "pointer constant to T" instead of as "pointer to constant T".)