`cpp.php`: The `fibonacci` example needs revision

Reference:

// bad_fibonacci.c

int fibonacci(int n) {
  if (n <= 1)
    return n;
  return fibonacci(n - 1) + fibonacci(n - 2);
}

$ clang -c bad_fibonacci.c
bad_fibonacci.c:4:12: error: implicit declaration of function 'fibonacci' is invalid in C99

bad_fibonacci did not declare the function fibonacci before using it which resulted in an error.

//  good_fibonacci.c

int fibonacci(int n);

int fibonacci(int n) {
  if (n <= 1)
    return n;
  return fibonacci(n - 1) + fibonacci(n - 2);
}

Adding the declaration allows the compiler to work in one pass.

$ clang -c good_fibonacci.c // It worked

The bad_fibonacci.c file compiles successfully, and there is no implicit declaration of the function fibonacci. Within the function body, the prototype of fibonacci is already known, so calls like fibonacci(n - 1) are valid. Therefore, the additional declaration in good_fibonacci.c is redundant.

Related: Comment on Hacker News:

I found the section on forward declarations at least partially off. I have never needed to use forward declarations for single recursion like the fibonacci example he gave. Mutual recursion does of course require forward declarations.

[fabiensanglard]

Indeed. I need a better exemple!