array of pointers to functions

Question

i need to write a function that receives an array of pointers to functions. i wrote the following code, however i'm having trouble in testing it at the moment.

is this the correct way to define a pointers to function array?

typedef (*Function)(double);
void func(Function* arr);

and if i want to declare the size of the array with [20] do i write:

void func(Function arr[20]);

?

thanks for your help

Answer 1

First, my usual method for figuring out complex types. Start with the identifier, and then add on the rest one step at a time:

    f          -- f
    f[N]       -- is an N-element array
   *f[N]       -- of pointers
  (*f[N])()    -- to functions
T (*f[N])()    -- returning T

For an array of pointers to functions taking a double parameter and returning a double value, that would be

double (*f[N])(double);

However, remember that expressions of array type "decay" from type "N-element array of T" to "pointer to T" in most contexts. When you pass an array expression as an argument to a function, what the function actually receives is a pointer. So, instead of receiving an object of type "N-element array of pointer to function returning double", your function will receive an object of type "pointer to pointer to function returning double", or

double (**f)(double)

So your function definition would look something like

void func(double (**f)(double))
{
  int i;
  ...
  for (i = 0; f[i] != NULL; i++)
  {
    double x = (*f[i])((double) i);
  }
}

And the caller would look something like

double a(double x) {...}
double b(double x) {...}
double c(double x) {...}

void foo(void)
{
  double (*list[])(double) = {a, b, c, NULL};
  func(list);
}

If you want to use typedefs instead, you could use something like this:

typedef double Dblfunc(double);   // typedef for function type 
typedef Dblfunc *Dblfuncptr;      // typedef for function pointer type

void func(Dblfuncptr *f)
{
  int i;
  for (i = 0; f[i] != NULL; i++)
  {
    double x = (*f[i])((double) i);
    ...
  }
}
...
void foo(void)
{
  Dblfuncptr list[] = {a, b, c, NULL}; // EDIT: fixed type
  func(list);
}

Using the typedefs makes the array and function parameter look more like regular types. Personally, I prefer using the "raw" types, since it shows explicitly that I'm dealing with pointers to functions, and it shows what the return and parameter types are.

Answer 2

If you correct the typedef to include a return type typedef void (*Function)(double);, that array declaration will work fine. You'd call it by calling (arr[index])(3.14) for the array case.

BTW: http://www.newty.de/fpt/fpt.html is a handy reference for function pointers.

Answer 3

What does the function return? You're the return type in the function pointer typedef, like you should have something like

typedef double (*Function)(double);

Answer 4

it looks almost correct i think you forgot the return type when declaring Function:

typedef int (*Function)(double)

instead

the second declaration is invalid too i think because it is invalid to specify array size for function parameters

void func (Function*)

will do just fine