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

This isn't a direct answer but a related tip. To keep things straight, I always use the convection "put const on the outside", where by "outside" I mean the far left or far right. That way there is no confusion -- the const applies to the closest thing (either the type or the *). E.g.,

int * const foo = ...; // Pointer cannot change, pointed to value can change
const int * bar = ...; // Pointer can change, pointed to value cannot change
int * baz = ...; // Pointer can change, pointed to value can change
const int * const qux = ...; // Pointer cannot change, pointed to value cannot change

Answer 2

The trick is to read the declaration backwards (right-to-left):

const int a = 1; // read as "a is an integer which is constant"
int const a = 1; // read as "a is a constant integer"

Both are the same thing. Therefore:

a = 2; // Can't do because a is constant

The reading backwards trick especially comes in handy when you're dealing with more complex declarations such as:

const char *s;      // read as "s is a pointer to a char that is constant"
char c;
char *const t = &c; // read as "t is a constant pointer to a char"

*s = 'A'; // Can't do because the char is constant
s++;      // Can do because the pointer isn't constant
*t = 'A'; // Can do because the char isn't constant
t++;      // Can't do because the pointer is constant

Answer 3

They are the same, but in C++ there's a good reason to always use const on the right. You'll be consistent everywhere because const member functions must be declared this way:

int getInt() const;

It changes the this pointer in the function from Foo * const to Foo const * const. See here.