函数指针？

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 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?

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

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"
   

Without the typedef word, in C++ the declaration would declare a variable FunctionFunc of type pointer to function of no arguments, returning void.

With the typedef it instead defines FunctionFunc as a name for that type.

#include <stdio.h>
#include <math.h>

/*
To define a new type name with typedef, follow these steps:
1. Write the statement as if a variable of the desired type were being declared.
2. Where the name of the declared variable would normally appear, substitute the new type name.
3. In front of everything, place the keyword typedef.
*/

// typedef a primitive data type
typedef double distance;

// typedef struct 
typedef struct{
    int x;
    int y;
} point;

//typedef an array 
typedef point points[100]; 

points ps = {0}; // ps is an array of 100 point 

// typedef a function
typedef distance (*distanceFun_p)(point,point) ; // TYPE_DEF distanceFun_p TO BE int (*distanceFun_p)(point,point)

// prototype a function     
distance findDistance(point, point);

int main(int argc, char const *argv[])
{
    // delcare a function pointer 
    distanceFun_p func_p;

    // initialize the function pointer with a function address
    func_p = findDistance;

    // initialize two point variables 
    point p1 = {0,0} , p2 = {1,1};

    // call the function through the pointer
    distance d = func_p(p1,p2);

    printf("the distance is %f\n", d );

    return 0;
}

distance findDistance(point p1, point p2)
{
distance xdiff =  p1.x - p2.x;
distance ydiff =  p1.y - p2.y;

return sqrt( (xdiff * xdiff) + (ydiff * ydiff) );
}

If you can use C++11 you may want to use std::function and using keyword.

using FunctionFunc = std::function<void(int arg1, std::string arg2)>;