How can I call a function using a function pointer

Question

Suppose I have these three functions   bool A    bool B    bool C      How do I call one of these functions conditionally using a function pointer  and how do I declare the function pointer

User · Answer

I usually use typedef to do it  but it may be overkill  if you do not have to use the function pointer too often      assuming bool is available  where I come from it is an enum   typedef bool   pmyfun t      pmyfun t pMyFun   pMyFun A    pMyFun  amp A is actually same  pMyFun

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Note that when you say   bool   a       you are declaring a of type  pointer to function returning bool and taking an unspecified number of parameters    Assuming bool is defined  maybe you re using C99 and have included stdbool h  or it may be a typedef   this may or may not be what you want   The problem here is that there is no way for the compiler to now check if a is assigned to a correct value   The same problem exists with your function declarations  A    B    and C   are all declared as functions  returning bool and taking an unspecified number of parameters    To see the kind of problems that may have  let s write a program    include  lt stdio h gt   int test zero void        return 42     static int test one char  data        return printf   s n   data      int main void           a is of type  pointer to function returning int        and taking unspecified number of parameters        int   a             b is of type  pointer to function returning int        and taking no parameters        int   b  void           This is OK        a   test zero      printf  a   d n   a          a   test one     OK  since compiler doesn t check the parameters        printf  a   d n   a        oops  wrong number of args            This is OK too        b   test zero      printf  b   d n   b             The compiler now does type checking  and sees that the        assignment is wrong  so it can warn us        b   test one      printf  b   d n   b        Wrong again         return 0      When I compile the above with gcc  it says      warning  assignment from incompatible pointer type   for the line b   test one   which is good   There is no warning for the corresponding assignment to a   So  you should declare your functions as   bool A void   bool B void   bool C void     And then the variable to hold the function should be declared as   bool   choice  void

User · Answer

Declare your function pointer like this   bool   f     f   A  f

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Calling a function through a function pointer  float add int  float   result   int main         float   fp  int  float       float result      fp   add      result   add 5  10 9         Normal calling     printf   f n n   result        result     fp  5  10 9       Calling via a function pointer     printf   f n n   result        result    fp  5  10 9        Calling via function pointer  The                                  indirection operator can be omitted      printf   f   result       getch       float add int a  float b        return a b         Output  15 90000 15 90000 15 90000

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I think your question has already been answered more than adequately  but it might be useful to point out explicitly that given a function pointer  void   pf  int foo  int bar     the two calls  pf 1  0     pf  1  0     are exactly equivalent in every way by definition   The choice of which to use is up to you  although it s a good idea to be consistent   For a long time  I preferred   pf  1  0  because it seemed to me that it better reflected the type of pf  however in the last few years I ve switched to pf 1  0

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You can do the following  Suppose you have your A B  amp  C function as the following   bool A                 bool B                 bool C                 Now at some other function  say at main   int main       bool   choice            now if there is if-else statement for making  choice  to       point at a particular function then proceed as following    if   x    1      choice   A     else if   x    2      choice   B      else    choice   C   if choice     printf  Success n     else  printf  Failure n                                 Remember this is one example for function pointer  there are several other method and for which you have to learn function pointer clearly

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Declare the pointer and asign it to the function bool   pFunc      A    Call the function A pFunc      Call function B pFunc   B  pFunc      Call function C pFunc   C  pFunc

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bool   FuncPtr     FuncPtr   A  FuncPtr      If you want to call one of those functions conditionally  you should consider using an array of function pointers  In this case you d have 3 elements pointing to A  B  and C and you call one depending on the index to the array  such as funcArray0 for A

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Slightly different approach   bool A         bool B         bool C          int main void               Declare an array of pointers to functions returning bool      and initialize with A  B  and C         bool   farr         A  B  C                    Call A  B  or C based on the value of i       assumes i is in range of array          if  farr i        or   farr i

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The best way to read that is the clockwise spiral rule by David Anderson

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If you need help with complex definitions  like  double     pf     3  4     take a look at my right-left rule here

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You can declare the function pointer as follows   bool  funptr        Which says we are declaring a function pointer to a function which does not take anything and return a bool   Next assignment   funptr   A    To call the function using the function pointer   funptr

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bool   fptr      int main void                        printf  Enter your choice        scanf   d   amp a       switch a                case 0                 fptr   A                 break          case 1                 fptr   B                 break          case 2                 fptr   C                 break          case 3                  break              fptr         return 0      Your choice is stored in a  Then accordingly  functions are assigned in the function pointer  Finally  depending on your choice  the very same function is called to return the desired result

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This has been more than adequately answered  but you may find this useful  The Function Pointer Tutorials   It is a truly comprehensive treatment of the subject in five chapters

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Initially define a function pointer array which takes a void and returns a void   Assuming that your function is taking a void and returning a void   typedef void   func ptr  void     Now you can use this to create function pointer variables of such functions   Like below   func ptr array of fun ptr 3     Now store the address of your functions in the three variables   array of fun ptr 0    amp A  array of fun ptr 1    amp B  array of fun ptr 2    amp C    Now you can call these functions using function pointers as below   some a    array of fun ptr 0       some b    array of fun ptr 1       some c    array of fun ptr 2

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You declare a function pointer variable for the given signature of your functions like this   bool    fnptr       you can assign it one of your functions   fnptr   A    and you can call it   bool result   fnptr      You might consider using typedefs to define a type for every distinct function signature you need  This will make the code easier to read and to maintain  i e  for the signature of functions returning bool with no arguments this could be   typdef bool    BoolFn       and then you can use like this to declare the function pointer variable for this type   BoolFn fnptr

[c] How can I call a function using a function pointer?

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