Typedef function pointer?

Question

I'm learning how to dynamically load DLL's but what I don't understand is this line

typedef void (*FunctionFunc)(); 

I have a few questions. If someone is able to answer them I would be grateful.

1. Why is typedef used?
2. The syntax looks odd; after void should there not be a function name or something? It looks like an anonymous function.
3. Is a function pointer created to store the memory address of a function?

So I'm confused at the moment; can you clarify things for me?

Answer 1

1. typedef is used to alias types; in this case you're aliasing FunctionFunc to void(*)().

2. Indeed the syntax does look odd, have a look at this:

   typedef   void      (*FunctionFunc)  ( ); //         ^                ^         ^ //     return type      type name  arguments 

3. No, this simply tells the compiler that the FunctionFunc type will be a function pointer, it doesn't define one, like this:

   FunctionFunc x; void doSomething() { printf("Hello there\n"); } x = &doSomething;  x(); //prints "Hello there" 

Answer 2

typedef is a language construct that associates a name to a type.
You use it the same way you would use the original type, for instance

typedef int myinteger; typedef char *mystring; typedef void (*myfunc)(); 

using them like

myinteger i;   // is equivalent to    int i; mystring s;    // is the same as      char *s; myfunc f;      // compile equally as  void (*f)(); 

As you can see, you could just replace the typedefed name with its definition given above.

The difficulty lies in the pointer to functions syntax and readability in C and C++, and the typedef can improve the readability of such declarations. However, the syntax is appropriate, since functions - unlike other simpler types - may have a return value and parameters, thus the sometimes lengthy and complex declaration of a pointer to function.

The readability may start to be really tricky with pointers to functions arrays, and some other even more indirect flavors.

To answer your three questions

• Why is typedef used? To ease the reading of the code - especially for pointers to functions, or structure names.

• The syntax looks odd (in the pointer to function declaration) That syntax is not obvious to read, at least when beginning. Using a typedef declaration instead eases the reading

• Is a function pointer created to store the memory address of a function? Yes, a function pointer stores the address of a function. This has nothing to do with the typedef construct which only ease the writing/reading of a program ; the compiler just expands the typedef definition before compiling the actual code.

Example:

typedef int (*t_somefunc)(int,int);  int product(int u, int v) {   return u*v; }  t_somefunc afunc = &product; ... int x2 = (*afunc)(123, 456); // call product() to calculate 123*456