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    Why is typedef used  The syntax looks odd  after void should there not be a function name or something  It looks like an anonymous function  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

User · Answer

For general case of syntax you can look at annex A of the ANSI C standard.

In the Backus-Naur form from there, you can see that typedef has the type storage-class-specifier.

In the type declaration-specifiers you can see that you can mix many specifier types, the order of which does not matter.

For example, it is correct to say,

long typedef long a;

to define the type a as an alias for long long. So , to understand the typedef on the exhaustive use you need to consult some backus-naur form that defines the syntax (there are many correct grammars for ANSI C, not only that of ISO).

When you use typedef to define an alias for a function type you need to put the alias in the same place where you put the identifier of the function. In your case you define the type FunctionFunc as an alias for a pointer to function whose type checking is disabled at call and returning nothing.

User · Answer

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

User · Answer

If you can use C  11 you may want to use std  function and using keyword   using FunctionFunc   std  function lt void int arg1  std  string arg2  gt

User · Answer

typedef is used to alias types  in this case you re aliasing FunctionFunc to void       Indeed the syntax does look odd  have a look at this   typedef   void        FunctionFunc                                                       return type      type name  arguments  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    amp doSomething   x      prints  Hello there

User · Answer

include  lt stdio h gt   include  lt math h gt      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

User · Answer

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    amp product      int x2     afunc  123  456      call product   to calculate 123 456

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