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Function pointer syntax tutorial

Function pointer syntax is one of those weird, weird things that just doesn’t seem right.

double (*fun)(int,int,double*)

Can you read that?

In less than 2 paragraphs, you will!

First things first. What is a function pointer anyway? Why, its simply a variable that, instead of pointing to an object (like an int* will point to an integer, and a double* will point to a double), this weird, weird “function pointer” will actually point to __A FUNCTION__!!

WHAT?? You say. POINT TO A FUNCTION! Why, yes! It is no different than me pointing to a recipe. Its one thing to have a copy of the recipe for chocolate mousse in hand (an actual function body), and its another thing to have a hyperlink that merely __points__ to that chocolate mousse recipe (a pointer to the function). When you have a pointer to a function (hyperlink to a recipe) you can still execute it (make the recipe) but you need to remember to dereference the pointer first (follow the hyperlink to pull up the recipe).

Does that make a little bit of sense? Me hopes so.

Moving on to function pointer syntax then, it is actually not that bad to read. Say you had a function:

void SayIt()
  puts( "It. There.  Happy now?" ) ;

And you wanted to declare a function pointer to the SayIt() function in your main function:

int main()
  // SayIt() function by the name of ptrSayIt
  void (*ptrSayIt)() ;
  // The above is probably the weirdest, non-C++ish
  // syntax you've come across.  It looks weird because
  // it is.  No, really, function pointer declaration
  // syntax is kind of mixed up.
  // 1.  First, the word VOID specifies the
  // return type of the function that you
  // will point to...
  // 2.  Second is (*ptrSayIt).  The bracket
  // and star combination (* there
  // TO VOID* (which is what "void *ptrSayIt();" would mean!)
  // 3.  Third, the trailing brackets at the end ()
  // say that the function we are pointing to
  // won't accept any arguments when it is called.

  // If it helps, you can look at it as:
  // void(*)().  That is the type of the
  // ptrSayIt variable.  The "mixed up" part
  // is because this breaks the usual C declaration
  // syntax of TYPE    InstanceName,
  // and so its really weird to look at at first.
  // Then you kind of get used to it.  Keep reading!

  // Now the ptrSayIt variable points to the
  // SayIt function..
  ptrSayIt = &SayIt ; // you don't really need the & in front
  // but I put it to be clear

  // Now we can call the SayIt function __through__
  // the ptrSayIt pointer we just declared and set up!
  (*ptrSayIt)() ; // (*DEREFERENCE) the pointer first!
  // OUTPUTS:  "It.  There.  Happy now?"

So this all seems very weird. But the program above works, I guarantee!

Lets do some more examples.

Declare a function pointer named Foo that points to a function marked by prototype:

int add( int a, int b ) ;


int (*Foo)(int,int) ;

Reasoning: the return type of the function you are about to point to goes first.

int        // function pointer will point
// to a function returning int..

Then, you do this mixed up thing where you write the name of the pointer variable NOW, in brackets, with a * in front:

int (*Foo) // function pointer VARIABLE
// identifier/handle is Foo.  This is actually
// the WEIRDEST part of the syntax.

Then you follow up with the types of arguments that the function you are trying to point to will contain:

int (*Foo)( int, int )   // will point 
// to a function accepting 2 ints

Further reading

So what’s the use of function pointers? Why, many, many uses!

Now you can write functions that accept pointers to functions.. and so you can have a function have variable behavior based on what function you passed it…-

But you probably already think function pointers are cool. That’s why you stopped in to read about them!

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