Write the prototype for a function that takes an array of exactly 16 integers

Question

One of the interview questions asked me to "write the prototype for a C function that takes an array of exactly 16 integers" and I was wondering what it could be? Maybe a function declaration like this:

void foo(int a[], int len);

Or something else?

And what about if the language was C++ instead?

Answer 1

In C, this requires a pointer to an array of 16 integers:

void special_case(int (*array)[16]); 

It would be called with:

int array[16]; special_case(&array); 

In C++, you can use a reference to an array, too, as shown in Nawaz's answer. (The question asks for C in the title, and originally only mentioned C++ in the tags.)

---

Any version that uses some variant of:

void alternative(int array[16]); 

ends up being equivalent to:

void alternative(int *array); 

which will accept any size of array, in practice.

---

The question is asked - does special_case() really prevent a different size of array from being passed. The answer is 'Yes'.

void special_case(int (*array)[16]);  void anon(void) {      int array16[16];     int array18[18];     special_case(&array16);     special_case(&array18); } 

The compiler (GCC 4.5.2 on MacOS X 10.6.6, as it happens) complains (warns):

$ gcc -c xx.c xx.c: In function ‘anon’: xx.c:9:5: warning: passing argument 1 of ‘special_case’ from incompatible pointer type xx.c:1:6: note: expected ‘int (*)[16]’ but argument is of type ‘int (*)[18]’ $ 

Change to GCC 4.2.1 - as provided by Apple - and the warning is:

$ /usr/bin/gcc -c xx.c xx.c: In function ‘anon’: xx.c:9: warning: passing argument 1 of ‘special_case’ from incompatible pointer type $ 

The warning in 4.5.2 is better, but the substance is the same.

Answer 2

There are several ways to declare array-parameters of fixed size:

void foo(int values[16]);

accepts any pointer-to-int, but the array-size serves as documentation

void foo(int (*values)[16]);

accepts a pointer to an array with exactly 16 elements

void foo(int values[static 16]);

accepts a pointer to the first element of an array with at least 16 elements

struct bar { int values[16]; };
void foo(struct bar bar);

accepts a structure boxing an array with exactly 16 elements, passing them by value.

Answer 3

& is necessary in C++:

void foo(int (&a)[16]); // & is necessary. (in C++)

Note : & is necessary, otherwise you can pass array of any size!

---

For C:

void foo(int (*a)[16]) //one way
{
}

typedef int (*IntArr16)[16]; //other way
void bar(IntArr16 a)
{
}

int main(void) 
{
        int a[16];
        foo(&a); //call like this - otherwise you'll get warning!
        bar(&a); //call like this - otherwise you'll get warning!
        return 0;
}

Demo : http://www.ideone.com/fWva6