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

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 than T const& / T const* . const T& / const T* empirically seem way more common to me than T 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 * as T* 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".)

Answer 2

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 3

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 4

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 5

没有区别， 两者在语法和语义上都是相同的。

Answer 6

没有区别http://c-faq.com/ansi/constptrconst.html

Answer 7

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.