In C++ is “const” after type ID acceptable?

Question

My co-worker is 0 for 2 on questions he has inspired (1, 2), so I thought I'd give him a chance to catch up.

Our latest disagreement is over the style issue of where to put "const" on declarations.

He is of the opinion that it should go either in front of the type, or after the pointer. The reasoning is that this is what is typically done by everyone else, and other styles are liable to be confusing. Thus a pointer to a constant int, and a constant pointer to int would be respectively:

const int *i;
      int * const i;

However, I'm confused anyway. I need rules that are consistent and easy to understand, and the only way I can make sense of "const" is that it goes after the thing it is modifying. There's an exception that allows it to go in front of the final type, but that's an exception, so it's easier on me if I don't use it.

Thus a pointer to a constant int, and a constant pointer to int would be respectively:

int const * i;
int * const i;

As an added benefit, doing things this way makes deeper levels of indirection easier to understand. For example, a pointer to a constant pointer to int would clearly be:

int * const * i;

My contention is that if someone just learns it his way, they'll have little trouble figuring out what the above works out to.

The ultimate issue here is that he thinks that putting const after int is so unspeakably ugly, and so harmful to readability that it should be banned in the style guide. Of course, I think if anything the guide should suggest doing it my way, but either way we shouldn't be banning one approach.

Edit: I've gotten a lot of good answers, but none really directly address my last paragraph ("The ultimate issue"). A lot of people argue for consistency, but is that so desirable in this case that it is a good idea to ban the other way of doing it, rather that just discouraging it?

Answer 1

Let me put my 2¢ into the discussion.

In modern C++ I'd recommend to stick to int const instead of const int to highlight the difference between const and constexpr. It would help programmer to remember that const int cannot be always thought as compile-time constant, because:

• compiler may (will?) allocate memory for it,
• this memory may even be not write-protected (stack segment),
• such a constant cannot serve all the duties constexpr can.

Answer 2

The most important thing is consistency. If there aren't any coding guidelines for this, then pick one and stick with it. But, if your team already has a de facto standard, don't change it!

That said, I think by far the more common is

const int* i;
int* const j;

because most people write

const int n;

instead of

int const n;

A side note -- an easy way to read pointer constness is to read the declaration starting at the right.

const int* i; // pointer to an int that is const
int* const j; // constant pointer to a (non-const) int
int const* aLessPopularWay; // pointer to a const int

Answer 3

The ultimate issue here is that he thinks that putting const after int is so unspeakably ugly, and so harmful to readability that it should be banned in the style guide

Really?

Show me a programmer who gets bogged down when he sees:

int foo() {
}

vs

int foo()
{
}

...and I'll show you a programmer who doesn't pay close enough attention to detail.

No professional programmer worth his salt will have a problem with superficial differences in style.

EDIT: It is true that const int* and int* const don't mean exactly the same thing, but that wasn't the point. The point made by OP's coworker was that differences in style make code difficult to understand & maintain. It is this claim I disagree with.

Answer 4

Rules are good to follow. Simpler rules are better. Const goes to the right of what's const.

Take this declaration:

int main ( int const argc , char const * const * const argv ) ...

Answer 5

In modern C++11, you could also use template typedefs to add clarity to complicated declarations:

template<typename T>
using Const = const T;

template<typename T>
using Ptr = T*;

The three declarations you mentioned in your question:

int const * a;
int * const b;
int * const * c;

would then be written as:

Ptr<Const<int>> a;
Const<Ptr<int>> b;
Ptr<Const<Ptr<int>>> c;

Answer 6

I hope this explanation from B. Stroustrup's FAQ on Style & Techniques will give you a definite answer.

Bjarne Stroustrup's C++ Style and Technique FAQ

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I personaly prefer:

int const* pi;
int* const pi;

Because const identifies the left token which is intended to be const.

And you definitely keep the same consistency when using smth like that:

int const* const pi;

Instead of writing inconsistently:

const int* const pi;

And what happens if you have a pointer to pointer and so on:

int const* const* const pi;

Instead of:

const int* const* const pi;