How can I pass a member function where a free function is expected

Question

The question is the following  consider this piece of code   include  lt iostream gt    class aClass   public      void aTest int a  int b                printf  quot  d    d    d quot   a  b  a   b             void function1 void   function  int  int         function 1  1      void test int a int b        printf  quot  d -  d    d quot   a   b   a - b      int main         aClass a       function1  amp test       function1  amp aClass  aTest       lt -- How should I point to a s aClass  test function     How can I use the a s aClass  test as an argument to function1  I would like to access a member of the class

User · Answer

I made the member function as static and all works    include  lt iostream gt   class aClass   public      static void aTest int a  int b                printf   d    d    d n   a  b  a   b             void function1 int a int b void function int  int         function a  b      void test int a int b        printf   d -  d    d n   a   b   a - b      int main  int argc  const char  argv          aClass a       function1 10 12 test       function1 10 12 a aTest       lt -- How should I point to a s aClass  test function       getchar   return 0

User · Answer

Pete Becker s answer is fine but you can also do it without passing the class instance as an explicit parameter to function1 in C   11    include  lt functional gt  using namespace std  placeholders   void function1 std  function lt void int  int  gt  fun        fun 1  1      int main  int argc  const char   argv                 aClass a     auto fp   std  bind  amp aClass  test  a   1   2      function1 fp       return 0

User · Answer

There isn t anything wrong with using function pointers  However  pointers to non-static member functions are not like normal function pointers  member functions need to be called on an object which is passed as an implicit argument to the function  The signature of your member function above is  thus void  aClass     int  int   rather than the type you try to use void     int  int   One approach could consist in making the member function static in which case it doesn t require any object to be called on and you can use it with the type void     int  int   If you need to access any non-static member of your class  and you need to stick with function pointers  e g   because the function is part of a C interface  your best option is to always pass a void  to your function taking function pointers and call your member through a forwarding function which obtains an object from the void  and then calls the member function  In a proper C   interface you might want to have a look at having your function take templated argument for function objects to use arbitrary class types  If using a templated interface is undesirable you should use something like std  function lt void int  int  gt   you can create a suitably callable function object for these  e g   using std  bind    The type-safe approaches using a template argument for the class type or a suitable std  function lt     gt  are preferable than using a void  interface as they remove the potential for errors due to a cast to the wrong type  To clarify how to use a function pointer to call a member function  here is an example     the function using the function pointers  void somefunction void   fptr  void   int  int   void  context        fptr context  17  42      void non member void   int i0  int i1        std  cout  lt  lt   quot I don t need any context  i0  quot   lt  lt  i0  lt  lt   quot  i1  quot   lt  lt  i1  lt  lt   quot  n quot      struct foo       void member int i0  int i1            std  cout  lt  lt   quot member function  this  quot   lt  lt  this  lt  lt   quot  i0  quot   lt  lt  i0  lt  lt   quot  i1  quot   lt  lt  i1  lt  lt   quot  n quot             void forwarder void  context  int i0  int i1        static cast lt foo  gt  context - gt member i0  i1      int main         somefunction  amp non member  nullptr       foo object      somefunction  amp forwarder   amp object

User · Answer

A pointer to member function is different from a pointer to function  In order to use a member function through a pointer you need a pointer to it  obviously   and an object to apply it to  So the appropriate version of function1 would be  void function1 void  aClass   function  int  int   aClass amp  a         a  function  1  1       and to call it   aClass a     note  no parentheses  with parentheses it s a function declaration function1  amp aClass  test  a

User · Answer

Since 2011  if you can change function1  do so  like this    include  lt functional gt   include  lt cstdio gt   using namespace std   class aClass   public      void aTest int a  int b                printf   d    d    d   a  b  a   b             template  lt typename Callable gt  void function1 Callable f        f 1  1      void test int a int b        printf   d -  d    d   a   b   a - b      int main         aClass obj          Free function     function1  amp test           Bound member function     using namespace std  placeholders      function1 std  bind  amp aClass  aTest  obj   1   2            Lambda     function1   amp   int a  int b            obj aTest a  b                live demo   Notice also that I fixed your broken object definition  aClass a    declares a function

User · Answer

Not sure why this incredibly simple solution has been passed up    include  lt stdio h gt   class aClass   public      void aTest int a  int b                printf   d    d    d n   a  b  a   b             template lt class C gt  void function1 void  C   function  int  int   C amp  c         c  function  1  1     void function1 void   function  int  int       function 1  1      void test int a int b        printf   d -  d    d n   a   b   a - b      int main  int argc  const char  argv          aClass a       function1  amp test       function1 lt aClass gt   amp aClass  aTest  a       return 0      Output   1 - 1   0 1   1   2

User · Answer

I asked a similar question  C   openframeworks passing void from other classes  but the answer I found was clearer so here the explanation for future records   it   s easier to use std  function as in    void draw int grid  std  function lt void   gt  element    and then call as    grid draw 12  std  bind  amp BarrettaClass  draw  a  std  placeholders   1      or even easier     grid draw 12    amp   a draw        where you create a lambda that calls the object capturing it by reference

User · Answer

If you actually don t need to use the instance a  i e  you can make it static like  mathengineer  s answer  you can simply pass in a non-capture lambda   which decay to function pointer    include  lt iostream gt   class aClass   public     void aTest int a  int b             printf  quot  d    d    d quot   a  b  a   b            void function1 void   function  int  int         function 1  1      int main          note  you don t need the        function1     int a int b  return aClass   aTest a b          Wandbox  note  if aClass is costly to construct or has side effect  this may not be a good way

User · Answer

You can stop banging your heads now  Here is the wrapper for the member function to support existing functions taking in plain C functions as arguments  thread local directive is the key here   http   cpp sh 9jhk3     Example program  include  lt iostream gt   include  lt string gt   using namespace std   typedef int FooCooker   int       Existing function extern  C  void cook 10 foo  FooCooker  FooCooker        cout  lt  lt   Cooking 10 Foo       lt  lt  endl      cout  lt  lt   FooCooker    lt  lt  endl      FooCooker  10      struct Bar        Bar   int Foo   0    Foo  Foo          int cook  int Foo            cout  lt  lt   This Bar got    lt  lt  this- gt Foo  lt  lt  endl          if  this- gt Foo  gt   Foo                this- gt Foo -  Foo              cout  lt  lt  Foo  lt  lt    cooked   lt  lt  endl              return Foo            else               cout  lt  lt   Can t cook    lt  lt   Foo  lt  lt  endl              return 0                      int Foo   0         Each Bar  object and a member function need to define    their own wrapper with a global thread local object ptr    to be called as a plain C function  thread local static Bar   BarPtr   NULL  static int cook in Bar  int Foo        return BarPtr- gt cook  Foo      thread local static Bar   Bar2Ptr   NULL  static int cook in Bar2  int Foo        return Bar2Ptr- gt cook  Foo      int main        BarPtr   new Bar   20     cook 10 foo  cook in Bar      Bar2Ptr   new Bar   40     cook 10 foo  cook in Bar2      delete BarPtr    delete Bar2Ptr    return 0      Please comment on any issues with this approach   Other answers fail to call existing plain C functions  http   cpp sh 8exun

[c++] How can I pass a member function where a free function is expected?

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