How do you pass a function as a parameter in C

Question

I want to create a function that performs a function passed by parameter on a set of data  How do you pass a function as a parameter in C

User · Answer

It s not really a function  but it is an localised piece of code  Of course it doesn t pass the code just the result   It won t work if passed to an event dispatcher to be run at a later time  as the result is calculated now and not when the event occurs   But it does localise your code into one place if that is all you are trying to do    include  lt stdio h gt   int IncMultInt int a  int b        a        return a   b     int main int argc  char  argv           int a   5      int b   7       printf   d    d    d n   a  b  IncMultInt a  b         b   9          Create some local code with it s own local variable     printf   d    d    d n   a  b       int  a   a 1   a   b              return 0

User · Answer

Declaration  A prototype for a function which takes a function parameter looks like the following   void func   void   f  int       This states that the parameter f will be a pointer to a function which has a void return type and which takes a single int parameter  The following function  print  is an example of a function which could be passed to func as a parameter because it is the proper type   void print   int x       printf   d n   x       Function Call  When calling a function with a function parameter  the value passed must be a pointer to a function  Use the function s name  without parentheses  for this   func print     would call func  passing the print function to it   Function Body  As with any parameter  func can now use the parameter s name in the function body to access the value of the parameter  Let s say that func will apply the function it is passed to the numbers 0-4  Consider  first  what the loop would look like to call print directly   for   int ctr   0   ctr  lt  5   ctr         print ctr       Since func s parameter declaration says that f is the name for a pointer to the desired function  we recall first that if f is a pointer then  f is the thing that f points to  i e  the function print in this case   As a result  just replace every occurrence of print in the loop above with  f   void func   void   f  int        for   int ctr   0   ctr  lt  5   ctr             f  ctr           Source

User · Answer

You need to pass a function pointer   The syntax is a little cumbersome  but it s really powerful once you get familiar with it

User · Answer

Functions can be  passed  as function pointers  as per ISO C11 6 7 6 3p8   A declaration of a parameter as       function returning type       shall be adjusted to       pointer to function returning type        as in 6 3 2 1      For example  this   void foo int bar int  int      is equivalent to this   void foo int   bar  int  int

User · Answer

I am gonna explain with a simple example code which takes a compare function as parameter to another sorting function  Lets say I have a bubble sort function that takes a custom compare function and uses it instead of a fixed if statement  Compare Function bool compare int a  int b        return a  gt  b     Now   the Bubble sort that takes another function as its parameter to perform comparison Bubble sort function void bubble sort int arr    int n  bool   amp cmp  int a  int b          for  int i   0 i  lt  n - 1 i              for  int j   0 j  lt   n - 1 - i  j                               if  cmp arr j   arr j   1                      swap arr j   arr j   1                                     Finally   the main which calls the Bubble sort function by passing the boolean compare function as argument  int main         int i  n   10  key   11      int arr 10      20  22  18  8  12  3  6  12  11  15         bubble sort arr  n  compare       cout lt  lt  quot Sorted Order quot  lt  lt endl      for  int i   0 i  lt  n i              cout  lt  lt  arr i   lt  lt   quot   quot            Output  Sorted Order 3 6 8 11 12 12 15 18 20 22

User · Answer

Since C  11 you can use the functional library to do this in a succinct and generic fashion  The syntax is  e g    std  function lt bool  int  gt    where bool is the return type here of a one-argument function whose first argument is of type int   I have included an example program below      g   test cpp --std c  11  include  lt functional gt   double Combiner double a  double b  std  function lt double  double double  gt  func     return func a b      double Add double a  double b     return a b     double Mult double a  double b     return a b     int main      Combiner 12 13 Add     Combiner 12 13 Mult       Sometimes  though  it is more convenient to use a template function      g   test cpp --std c  11  template lt class T gt  double Combiner double a  double b  T func     return func a b      double Add double a  double b     return a b     double Mult double a  double b     return a b     int main      Combiner 12 13 Add     Combiner 12 13 Mult

User · Answer

Pass address of a function as parameter to another function as shown below   include  lt stdio h gt   void print    void execute void      int main         execute print      sends address of print     return 0     void print         printf  Hello        void execute void f       receive address of print       f        Also we can pass function as parameter using function pointer   include  lt stdio h gt   void print    void execute void   f       int main         execute  amp print      sends address of print     return 0     void print         printf  Hello        void execute void   f        receive address of print       f

User · Answer

This question already has the answer for defining function pointers  however they can get very messy  especially if you are going to be passing them around your application  To avoid this unpleasantness I would recommend that you typedef the function pointer into something more readable  For example   typedef void   functiontype       Declares a function that returns void and takes no arguments  To create a function pointer to this type you can now do   void dosomething        functiontype func    amp dosomething  func      For a function that returns an int and takes a char you would do  typedef int   functiontype2  char     and to use it  int dosomethingwithchar char a    return 1     functiontype2 func2    amp dosomethingwithchar int result   func2  a      There are libraries that can help with turning function pointers into nice readable types  The boost function library is great and is well worth the effort   boost  function lt int  char a  gt  functiontype2    is so much nicer than the above

[c] How do you pass a function as a parameter in C?

Examples related to c

Examples related to function

Examples related to pointers

Examples related to syntax

Examples related to parameters