C++: const reference, before vs after type-specifier

Question

What is the difference between the arguments in:

int foo1(const Fred &arg) {
...
}

and

int foo2(Fred const &arg) {
...
}

? I don't see this case covered in the parashift FAQ.

Answer 1

No difference, both are syntactically and semantically same.

Answer 2

No difference http://c-faq.com/ansi/constptrconst.html

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

References doesn't work the same way as pointers: for pointers you can have 'const pointers' (type * const p) and 'pointer to const' (const type * p or type const * p).

But you don't have this for references: a reference will always refer to the same object; in that sense you can consider that 'references' are 'const references' (the same way you can have 'const pointers').

Therefore something like 'type & const ref' is not legal. You can only have 'reference to type' (type &ref) and 'reference to constant type' (const type &ref or type const &ref; both are exactly equivalent).

One last thing: even if const type sounds more correct in English, writing type const allows a more systematic understanding of declarations "right to left" : int const & ref can be read has 'ref is a reference to a constant int'. Or more complicated example: int const * const & ref, ref is a reference to a constant pointer to a constant int.

Conclusion: in your question, both are exactly equivalent.

Answer 7

I'll dissent from a lot of the other answers and prefer const T& (and const T*):

• It's the style used in Stroustrup's The C++ Programming Language book.
• It's the style used in the C++ standard itself.
• const T* seems way more common to me than T const* in all of the C code I've seen. (It's the style used in K&R's The C Programming Language and the style used in the C standard.)
• Due to the above factors, I think const T& has way more inertia than T const&, and I think following common practices is more readable than dogmatically adhering to right-to-left parsing rules. (I don't agree that const T& breaks that rule anyway.)
• With T const*, it seems easier to misenter it as T* const (especially if people aren't as accustomed to it).