const int vs. int const as function parameter in C

Question

Quick question:

int testfunc1 (const int a)
{
  return a;
}

int testfunc2 (int const a)
{
  return a;
}

Are these two functions the same in every aspect or is there a difference? I'm interested in an answer for the C-language, but if there is something interesting in the C++ language, I'd like to know as well.

Answer 1

I think in this case they are the same, but here is an example where order matters:

const int* cantChangeTheData;
int* const cantChangeTheAddress;

Answer 2

Yes, they are same for just int

and different for int*

Answer 3

const T and T const are identical. With pointer types it becomes more complicated:

1. const char* is a pointer to a constant char
2. char const* is a pointer to a constant char
3. char* const is a constant pointer to a (mutable) char

In other words, (1) and (2) are identical. The only way of making the pointer (rather than the pointee) const is to use a suffix-const.

This is why many people prefer to always put const to the right side of the type (“East const” style): it makes its location relative to the type consistent and easy to remember (it also anecdotally seems to make it easier to teach to beginners).

Answer 4

There is no difference. They both declare "a" to be an integer that cannot be changed.

The place where differences start to appear is when you use pointers.

Both of these:

const int *a
int const *a

declare "a" to be a pointer to an integer that doesn't change. "a" can be assigned to, but "*a" cannot.

int * const a

declares "a" to be a constant pointer to an integer. "*a" can be assigned to, but "a" cannot.

const int * const a

declares "a" to be a constant pointer to a constant integer. Neither "a" nor "*a" can be assigned to.

static int one = 1;

int testfunc3 (const int *a)
{
  *a = 1; /* Error */
  a = &one;
  return *a;
}

int testfunc4 (int * const a)
{
  *a = 1;
  a = &one; /* Error */
  return *a;
}

int testfunc5 (const int * const a)
{
  *a = 1;   /* Error */
  a = &one; /* Error */
  return *a;
}

Answer 5

const int is identical to int const, as is true with all scalar types in C. In general, declaring a scalar function parameter as const is not needed, since C's call-by-value semantics mean that any changes to the variable are local to its enclosing function.

Answer 6

Prakash is correct that the declarations are the same, although a little more explanation of the pointer case might be in order.

"const int* p" is a pointer to an int that does not allow the int to be changed through that pointer. "int* const p" is a pointer to an int that cannot be changed to point to another int.

See http://www.parashift.com/c++-faq-lite/const-correctness.html#faq-18.5.