How can I pass a class member function as a callback

Question

I m using an API that requires me to pass a function pointer as a callback  I m trying to use this API from my class but I m getting compilation errors    Here is what I did from my constructor   m cRedundencyManager- gt Init this- gt RedundencyManagerCallBack     This doesn t compile - I get the following error       Error  8 error C3867   CLoggersInfra  RedundencyManagerCallBack   function call missing argument list  use   amp CLoggersInfra  RedundencyManagerCallBack  to create a pointer to member   I tried the suggestion to use  amp CLoggersInfra  RedundencyManagerCallBack - didn t work for me    Any suggestions explanation for this    I m using VS2008   Thanks

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What argument does Init take  What is the new error message   Method pointers in C   are a bit difficult to use  Besides the method pointer itself  you also need to provide an instance pointer  in your case this   Maybe Init expects it as a separate argument

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This is a simple question but the answer is surprisingly complex  The short answer is you can do what you re trying to do with std  bind1st or boost  bind  The longer answer is below  The compiler is correct to suggest you use  amp CLoggersInfra  RedundencyManagerCallBack  First  if RedundencyManagerCallBack is a member function  the function itself doesn t belong to any particular instance of the class CLoggersInfra  It belongs to the class itself  If you ve ever called a static class function before  you may have noticed you use the same SomeClass  SomeMemberFunction syntax  Since the function itself is  static  in the sense that it belongs to the class rather than a particular instance  you use the same syntax  The   amp   is necessary because technically speaking you don t pass functions directly -- functions are not real objects in C    Instead you re technically passing the memory address for the function  that is  a pointer to where the function s instructions begin in memory  The consequence is the same though  you re effectively  passing a function  as a parameter  But that s only half the problem in this instance  As I said  RedundencyManagerCallBack the function doesn t  belong  to any particular instance  But it sounds like you want to pass it as a callback with a particular instance in mind  To understand how to do this you need to understand what member functions really are  regular not-defined-in-any-class functions with an extra hidden parameter  For example  class A   public      A     data 0         void foo int addToData    this- gt data    addToData         int data           A an a object  an a object foo 5   A  foo  amp an a object  5      This is the same as the line above  std  cout  lt  lt  an a object data     Prints 10   How many parameters does A  foo take  Normally we would say 1  But under the hood  foo really takes 2  Looking at A  foo s definition  it needs a specific instance of A in order for the  this  pointer to be meaningful  the compiler needs to know what  this  is   The way you usually specify what you want  this  to be is through the syntax MyObject MyMemberFunction    But this is just syntactic sugar for passing the address of MyObject as the first parameter to MyMemberFunction  Similarly  when we declare member functions inside class definitions we don t put  this  in the parameter list  but this is just a gift from the language designers to save typing  Instead you have to specify that a member function is static to opt out of it automatically getting the extra  this  parameter  If the C   compiler translated the above example to C code  the original C   compiler actually worked that way   it would probably write something like this  struct A       int data      void a init A  to init        to init- gt data   0     void a foo A  this  int addToData         this- gt data    addToData          A an a object  a init 0      Before constructor call was implicit a foo  amp an a object  5      Used to be an a object foo 5    Returning to your example  there is now an obvious problem   Init  wants a pointer to a function that takes one parameter  But  amp CLoggersInfra  RedundencyManagerCallBack is a pointer to a function that takes two parameters  it s normal parameter and the secret  this  parameter  That s why you re still getting a compiler error  as a side note  If you ve ever used Python  this kind of confusion is why a  self  parameter is required for all member functions   The verbose way to handle this is to create a special object that holds a pointer to the instance you want and has a member function called something like  run  or  execute   or overloads the      operator  that takes the parameters for the member function  and simply calls the member function with those parameters on the stored instance  But this would require you to change  Init  to take your special object rather than a raw function pointer  and it sounds like Init is someone else s code  And making a special class for every time this problem comes up will lead to code bloat  So now  finally  the good solution  boost  bind and boost  function  the documentation for each you can find here  boost  bind docs  boost  function docs boost  bind will let you take a function  and a parameter to that function  and make a new function where that parameter is  locked  in place  So if I have a function that adds two integers  I can use boost  bind to make a new function where one of the parameters is locked to say 5  This new function will only take one integer parameter  and will always add 5 specifically to it  Using this technique  you can  lock in  the hidden  this  parameter to be a particular class instance  and generate a new function that only takes one parameter  just like you want  note that the hidden parameter is always the first parameter  and the normal parameters come in order after it   Look at the boost  bind docs for examples  they even specifically discuss using it for member functions  Technically there is a standard function called  std  bind1st  3  that you could use as well  but boost  bind is more general  Of course  there s just one more catch  boost  bind will make a nice boost  function for you  but this is still technically not a raw function pointer like Init probably wants  Thankfully  boost provides a way to convert boost  function s to raw pointers  as documented on StackOverflow here  How it implements this is beyond the scope of this answer  though it s interesting too  Don t worry if this seems ludicrously hard -- your question intersects several of C   s darker corners  and boost  bind is incredibly useful once you learn it  C  11 update  Instead of boost  bind you can now use a lambda function that captures  this   This is basically having the compiler generate the same thing for you

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What argument does Init take  What is the new error message   Method pointers in C   are a bit difficult to use  Besides the method pointer itself  you also need to provide an instance pointer  in your case this   Maybe Init expects it as a separate argument

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Is m cRedundencyManager able to use member functions  Most callbacks are set up to use regular functions or static member functions  Take a look at this page at C   FAQ Lite for more information   Update  The function declaration you provided shows that m cRedundencyManager is expecting a function of the form  void yourCallbackFunction int  void     Member functions are therefore unacceptable as callbacks in this case  A static member function may work  but if that is unacceptable in your case  the following code would also work  Note that it uses an evil cast from void         in your CLoggersInfra constructor  m cRedundencyManager- Init myRedundencyManagerCallBackHandler  this         in your CLoggersInfra header  void myRedundencyManagerCallBackHandler int i  void   CLoggersInfraPtr         in your CLoggersInfra source file  void myRedundencyManagerCallBackHandler int i  void   CLoggersInfraPtr          CLoggersInfra   CLoggersInfraPtr - RedundencyManagerCallBack i

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A pointer to a class member function is not the same as a pointer to a function  A class member takes an implicit extra argument  the this pointer   and uses a different calling convention   If your API expects a nonmember callback function  that s what you have to pass to it

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The type of pointer to non-static member function is different from pointer to ordinary function  Type is void    int  if it   s an ordinary or static member function  Type is void CLoggersInfra     int  if it   s a non-static member function  So you cannot pass a pointer to a non-static member function if it is expecting an ordinary function pointer   Furthermore  a non-static member function has an implicit hidden parameter to the object  The this pointer is implicitly passed as an argument to the member function call  So the member functions can be invoked only by providing an object    If the API Init cannot be changed  a wrapper function  ordinary function or a class static member function  that invokes the member can be used  In the worst case  the object would be a global for the wrapper function to access   CLoggersInfra  pLoggerInfra   RedundencyManagerCallBackWrapper int val        pLoggerInfra- gt RedundencyManagerCallBack val       m cRedundencyManager- gt Init RedundencyManagerCallBackWrapper     If the API Init can be changed  there are many alternatives - Object non-static member function pointer  Function Object  std  function or Interface Function    See the post on callbacks for the different variations with C   working examples

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This question and answer from the C   FAQ Lite covers your question and the considerations involved in the answer quite nicely I think  Short snippet from the web page I linked      Don   t       Because a member function is meaningless without an object to invoke   it on  you can   t do this directly  if The X Window System was   rewritten in C    it would probably pass references to objects around    not just pointers to functions  naturally the objects would embody the   required function and probably a whole lot more

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This question and answer from the C   FAQ Lite covers your question and the considerations involved in the answer quite nicely I think  Short snippet from the web page I linked      Don   t       Because a member function is meaningless without an object to invoke   it on  you can   t do this directly  if The X Window System was   rewritten in C    it would probably pass references to objects around    not just pointers to functions  naturally the objects would embody the   required function and probably a whole lot more

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That doesn t work because a member function pointer cannot be handled like a normal function pointer  because it expects a  this  object argument    Instead you can pass a static member function as follows  which are like normal non-member functions in this regard   m cRedundencyManager- gt Init  amp CLoggersInfra  Callback  this     The function can be defined as follows   static void Callback int other arg  void   this pointer        CLoggersInfra   self   static cast lt CLoggersInfra  gt  this pointer       self- gt RedundencyManagerCallBack other arg

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I can see that the init has the following override    Init CALLBACK FUNC EX callback func  void   callback parm    where CALLBACK FUNC EX is  typedef void   CALLBACK FUNC EX  int  void

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I can see that the init has the following override    Init CALLBACK FUNC EX callback func  void   callback parm    where CALLBACK FUNC EX is  typedef void   CALLBACK FUNC EX  int  void

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This answer is a reply to a comment above and does not work with VisualStudio 2008 but should be preferred with more recent compilers     Meanwhile you don t have to use a void pointer anymore and there is also no need for boost since std  bind and std  function are available  One advantage  in comparison to void pointers  is type safety since the return type and the arguments are explicitly stated using std  function      std  function lt return type list of argument type s   gt  void Init std  function lt void void  gt  f     Then you can create the function pointer with std  bind and pass it to Init   auto cLoggersInfraInstance   CLoggersInfra    auto callback   std  bind  amp CLoggersInfra  RedundencyManagerCallBack  cLoggersInfraInstance   Init callback     Complete example for using std  bind with member  static members and non member functions    include  lt functional gt   include  lt iostream gt   include  lt string gt   class RedundencyManager    incl  Typo  -    public         std  function lt return type list of argument type s   gt      std  string Init std  function lt std  string void  gt  f                 return f              class CLoggersInfra   private      std  string member    Hello from non static member callback     public      static std  string RedundencyManagerCallBack                 return  Hello from static member callback               std  string NonStaticRedundencyManagerCallBack                 return member            std  string NonMemberCallBack         return  Hello from non member function       int main         auto instance   RedundencyManager         auto callback1   std  bind  amp NonMemberCallBack       std  cout  lt  lt  instance Init callback1   lt  lt    n           Similar to non member function      auto callback2   std  bind  amp CLoggersInfra  RedundencyManagerCallBack       std  cout  lt  lt  instance Init callback2   lt  lt    n           Class instance is passed to std  bind as second argument          heed that I call the constructor of CLoggersInfra      auto callback3   std  bind  amp CLoggersInfra  NonStaticRedundencyManagerCallBack                                 CLoggersInfra          std  cout  lt  lt  instance Init callback3   lt  lt    n       Possible output   Hello from non member function  Hello from static member callback  Hello from non static member callback    Furthermore using std  placeholders you can dynamically pass arguments to the callback  e g  this enables the usage of return f  MyString    in Init if f has a string parameter

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I can see that the init has the following override    Init CALLBACK FUNC EX callback func  void   callback parm    where CALLBACK FUNC EX is  typedef void   CALLBACK FUNC EX  int  void

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This is a simple question but the answer is surprisingly complex  The short answer is you can do what you re trying to do with std  bind1st or boost  bind  The longer answer is below  The compiler is correct to suggest you use  amp CLoggersInfra  RedundencyManagerCallBack  First  if RedundencyManagerCallBack is a member function  the function itself doesn t belong to any particular instance of the class CLoggersInfra  It belongs to the class itself  If you ve ever called a static class function before  you may have noticed you use the same SomeClass  SomeMemberFunction syntax  Since the function itself is  static  in the sense that it belongs to the class rather than a particular instance  you use the same syntax  The   amp   is necessary because technically speaking you don t pass functions directly -- functions are not real objects in C    Instead you re technically passing the memory address for the function  that is  a pointer to where the function s instructions begin in memory  The consequence is the same though  you re effectively  passing a function  as a parameter  But that s only half the problem in this instance  As I said  RedundencyManagerCallBack the function doesn t  belong  to any particular instance  But it sounds like you want to pass it as a callback with a particular instance in mind  To understand how to do this you need to understand what member functions really are  regular not-defined-in-any-class functions with an extra hidden parameter  For example  class A   public      A     data 0         void foo int addToData    this- gt data    addToData         int data           A an a object  an a object foo 5   A  foo  amp an a object  5      This is the same as the line above  std  cout  lt  lt  an a object data     Prints 10   How many parameters does A  foo take  Normally we would say 1  But under the hood  foo really takes 2  Looking at A  foo s definition  it needs a specific instance of A in order for the  this  pointer to be meaningful  the compiler needs to know what  this  is   The way you usually specify what you want  this  to be is through the syntax MyObject MyMemberFunction    But this is just syntactic sugar for passing the address of MyObject as the first parameter to MyMemberFunction  Similarly  when we declare member functions inside class definitions we don t put  this  in the parameter list  but this is just a gift from the language designers to save typing  Instead you have to specify that a member function is static to opt out of it automatically getting the extra  this  parameter  If the C   compiler translated the above example to C code  the original C   compiler actually worked that way   it would probably write something like this  struct A       int data      void a init A  to init        to init- gt data   0     void a foo A  this  int addToData         this- gt data    addToData          A an a object  a init 0      Before constructor call was implicit a foo  amp an a object  5      Used to be an a object foo 5    Returning to your example  there is now an obvious problem   Init  wants a pointer to a function that takes one parameter  But  amp CLoggersInfra  RedundencyManagerCallBack is a pointer to a function that takes two parameters  it s normal parameter and the secret  this  parameter  That s why you re still getting a compiler error  as a side note  If you ve ever used Python  this kind of confusion is why a  self  parameter is required for all member functions   The verbose way to handle this is to create a special object that holds a pointer to the instance you want and has a member function called something like  run  or  execute   or overloads the      operator  that takes the parameters for the member function  and simply calls the member function with those parameters on the stored instance  But this would require you to change  Init  to take your special object rather than a raw function pointer  and it sounds like Init is someone else s code  And making a special class for every time this problem comes up will lead to code bloat  So now  finally  the good solution  boost  bind and boost  function  the documentation for each you can find here  boost  bind docs  boost  function docs boost  bind will let you take a function  and a parameter to that function  and make a new function where that parameter is  locked  in place  So if I have a function that adds two integers  I can use boost  bind to make a new function where one of the parameters is locked to say 5  This new function will only take one integer parameter  and will always add 5 specifically to it  Using this technique  you can  lock in  the hidden  this  parameter to be a particular class instance  and generate a new function that only takes one parameter  just like you want  note that the hidden parameter is always the first parameter  and the normal parameters come in order after it   Look at the boost  bind docs for examples  they even specifically discuss using it for member functions  Technically there is a standard function called  std  bind1st  3  that you could use as well  but boost  bind is more general  Of course  there s just one more catch  boost  bind will make a nice boost  function for you  but this is still technically not a raw function pointer like Init probably wants  Thankfully  boost provides a way to convert boost  function s to raw pointers  as documented on StackOverflow here  How it implements this is beyond the scope of this answer  though it s interesting too  Don t worry if this seems ludicrously hard -- your question intersects several of C   s darker corners  and boost  bind is incredibly useful once you learn it  C  11 update  Instead of boost  bind you can now use a lambda function that captures  this   This is basically having the compiler generate the same thing for you

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Is m cRedundencyManager able to use member functions  Most callbacks are set up to use regular functions or static member functions  Take a look at this page at C   FAQ Lite for more information   Update  The function declaration you provided shows that m cRedundencyManager is expecting a function of the form  void yourCallbackFunction int  void     Member functions are therefore unacceptable as callbacks in this case  A static member function may work  but if that is unacceptable in your case  the following code would also work  Note that it uses an evil cast from void         in your CLoggersInfra constructor  m cRedundencyManager- Init myRedundencyManagerCallBackHandler  this         in your CLoggersInfra header  void myRedundencyManagerCallBackHandler int i  void   CLoggersInfraPtr         in your CLoggersInfra source file  void myRedundencyManagerCallBackHandler int i  void   CLoggersInfraPtr          CLoggersInfra   CLoggersInfraPtr - RedundencyManagerCallBack i

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Necromancing   I think the answers to date are a little unclear    Let s make an example   Supposed you have an array of pixels  array of ARGB int8 t values      A RGB image int8 t  pixels   new int8 t 1024 768 4     Now you want to generate a PNG   To do so  you call the function toJpeg  bool ok   toJpeg writeByte  pixels  width  height     where writeByte is a callback-function  void writeByte unsigned char oneByte        fputc oneByte  output       The problem here  FILE  output has to be a global variable   Very bad if you re in a multithreaded environment  e g  a http-server      So you need some way to make output a non-global variable  while retaining the callback signature    The immediate solution that springs into mind is a closure  which we can emulate using a class with a member function    class BadIdea   private      FILE  m stream  public      BadIdea FILE  stream             this- gt m stream   stream             void writeByte unsigned char oneByte               fputc oneByte  this- gt m stream               And then do   FILE  fp   fopen filename   wb    BadIdea  foobar   new BadIdea fp    bool ok   TooJpeg  writeJpeg foobar- gt writeByte  image  width  height   delete foobar  fflush fp   fclose fp     However  contrary to expectations  this does not work   The reason is  C   member functions are kinda implemented like C  extension functions    So you have   class struct BadIdea       FILE  m stream      and   static class BadIdeaExtensions       public static writeByte this BadIdea instance  unsigned char oneByte                 fputc oneByte  instance- gt m stream              So when you want to call writeByte  you need pass not only the address of writeByte  but also the address of the BadIdea-instance    So when you have a typedef for the writeByte procedure  and it looks like this  typedef void   WRITE ONE BYTE  unsigned char     And you have a writeJpeg signature that looks like this  bool writeJpeg WRITE ONE BYTE output  uint8 t  pixels  uint32 t   width  uint32 t height                 it s fundamentally impossible to pass a two-address member function to a one-address function pointer  without modifying writeJpeg   and there s no way around it    The next best thing that you can do in C    is using a lambda-function   FILE  fp   fopen filename   wb    auto lambda    fp  unsigned char oneByte    fputc oneByte  fp       bool ok   TooJpeg  writeJpeg lambda  image  width  height     However  because lambda is doing nothing different  than passing an instance to a hidden class  such as the  BadIdea -class   you need to modify the signature of writeJpeg    The advantage of lambda over a manual class  is that you just need to change one typedef   typedef void   WRITE ONE BYTE  unsigned char     to  using WRITE ONE BYTE   std  function lt void unsigned char  gt      And then you can leave everything else untouched    You could also use std  bind  auto f   std  bind  amp BadIdea  writeByte   amp foobar     But this  behind the scene  just creates a lambda function  which then also needs the change in typedef    So no  there is no way to pass a member function to a method that requires a static function-pointer   But lambdas are the easy way around  provided that you have control over the source   Otherwise  you re out of luck   There s nothing you can do with C     Note   std  function requires  include  lt functional gt    However  since C   allows you to use C as well  you can do this with libffcall in plain C  if you don t mind linking a dependency   Download libffcall from GNU  at least on ubuntu  don t use the distro-provided package - it is broken   unzip      configure make make install  gcc main c -l libffcall a -o ma   main c    include  lt callback h gt      this is the closure function to be allocated  void function  void  data  va alist alist         int abc   va arg int alist         printf  data   08p n   data      hex 0x14   20      printf  abc   d n   abc            va start type alist   return type            arg   va arg type alist   arg type            va return type alist    return type   return value            va start int alist          int r   666         va return int alist  r        int main int argc  char  argv          int in1   10       void   data    void   20      void  incrementer1  int abc     void       alloc callback  amp function  data          void  incrementer1    can have unlimited arguments  e g  incrementer1 123 456          void  incrementer1  int abc  starts to throw errors        incrementer1 123          free callback callback       return EXIT SUCCESS      And if you use CMake  add the linker library after add executable  add library libffcall STATIC IMPORTED  set target properties libffcall PROPERTIES         IMPORTED LOCATION  usr local lib libffcall a  target link libraries BitmapLion libffcall    or you could just dynamically link libffcall  target link libraries BitmapLion ffcall    Note   You might want to include the libffcall headers and libraries  or create a cmake project with the contents of libffcall

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Looks like std  mem fn  C  11  does exactly what you need      Function template std  mem fn generates wrapper objects for pointers to members  which can store  copy  and invoke a pointer to member  Both references and pointers  including smart pointers  to an object can be used when invoking a std  mem fn

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A pointer to a class member function is not the same as a pointer to a function  A class member takes an implicit extra argument  the this pointer   and uses a different calling convention   If your API expects a nonmember callback function  that s what you have to pass to it

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This is a simple question but the answer is surprisingly complex  The short answer is you can do what you re trying to do with std  bind1st or boost  bind  The longer answer is below  The compiler is correct to suggest you use  amp CLoggersInfra  RedundencyManagerCallBack  First  if RedundencyManagerCallBack is a member function  the function itself doesn t belong to any particular instance of the class CLoggersInfra  It belongs to the class itself  If you ve ever called a static class function before  you may have noticed you use the same SomeClass  SomeMemberFunction syntax  Since the function itself is  static  in the sense that it belongs to the class rather than a particular instance  you use the same syntax  The   amp   is necessary because technically speaking you don t pass functions directly -- functions are not real objects in C    Instead you re technically passing the memory address for the function  that is  a pointer to where the function s instructions begin in memory  The consequence is the same though  you re effectively  passing a function  as a parameter  But that s only half the problem in this instance  As I said  RedundencyManagerCallBack the function doesn t  belong  to any particular instance  But it sounds like you want to pass it as a callback with a particular instance in mind  To understand how to do this you need to understand what member functions really are  regular not-defined-in-any-class functions with an extra hidden parameter  For example  class A   public      A     data 0         void foo int addToData    this- gt data    addToData         int data           A an a object  an a object foo 5   A  foo  amp an a object  5      This is the same as the line above  std  cout  lt  lt  an a object data     Prints 10   How many parameters does A  foo take  Normally we would say 1  But under the hood  foo really takes 2  Looking at A  foo s definition  it needs a specific instance of A in order for the  this  pointer to be meaningful  the compiler needs to know what  this  is   The way you usually specify what you want  this  to be is through the syntax MyObject MyMemberFunction    But this is just syntactic sugar for passing the address of MyObject as the first parameter to MyMemberFunction  Similarly  when we declare member functions inside class definitions we don t put  this  in the parameter list  but this is just a gift from the language designers to save typing  Instead you have to specify that a member function is static to opt out of it automatically getting the extra  this  parameter  If the C   compiler translated the above example to C code  the original C   compiler actually worked that way   it would probably write something like this  struct A       int data      void a init A  to init        to init- gt data   0     void a foo A  this  int addToData         this- gt data    addToData          A an a object  a init 0      Before constructor call was implicit a foo  amp an a object  5      Used to be an a object foo 5    Returning to your example  there is now an obvious problem   Init  wants a pointer to a function that takes one parameter  But  amp CLoggersInfra  RedundencyManagerCallBack is a pointer to a function that takes two parameters  it s normal parameter and the secret  this  parameter  That s why you re still getting a compiler error  as a side note  If you ve ever used Python  this kind of confusion is why a  self  parameter is required for all member functions   The verbose way to handle this is to create a special object that holds a pointer to the instance you want and has a member function called something like  run  or  execute   or overloads the      operator  that takes the parameters for the member function  and simply calls the member function with those parameters on the stored instance  But this would require you to change  Init  to take your special object rather than a raw function pointer  and it sounds like Init is someone else s code  And making a special class for every time this problem comes up will lead to code bloat  So now  finally  the good solution  boost  bind and boost  function  the documentation for each you can find here  boost  bind docs  boost  function docs boost  bind will let you take a function  and a parameter to that function  and make a new function where that parameter is  locked  in place  So if I have a function that adds two integers  I can use boost  bind to make a new function where one of the parameters is locked to say 5  This new function will only take one integer parameter  and will always add 5 specifically to it  Using this technique  you can  lock in  the hidden  this  parameter to be a particular class instance  and generate a new function that only takes one parameter  just like you want  note that the hidden parameter is always the first parameter  and the normal parameters come in order after it   Look at the boost  bind docs for examples  they even specifically discuss using it for member functions  Technically there is a standard function called  std  bind1st  3  that you could use as well  but boost  bind is more general  Of course  there s just one more catch  boost  bind will make a nice boost  function for you  but this is still technically not a raw function pointer like Init probably wants  Thankfully  boost provides a way to convert boost  function s to raw pointers  as documented on StackOverflow here  How it implements this is beyond the scope of this answer  though it s interesting too  Don t worry if this seems ludicrously hard -- your question intersects several of C   s darker corners  and boost  bind is incredibly useful once you learn it  C  11 update  Instead of boost  bind you can now use a lambda function that captures  this   This is basically having the compiler generate the same thing for you

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What argument does Init take  What is the new error message   Method pointers in C   are a bit difficult to use  Besides the method pointer itself  you also need to provide an instance pointer  in your case this   Maybe Init expects it as a separate argument

User · Answer

Looks like std  mem fn  C  11  does exactly what you need      Function template std  mem fn generates wrapper objects for pointers to members  which can store  copy  and invoke a pointer to member  Both references and pointers  including smart pointers  to an object can be used when invoking a std  mem fn

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A simple solution  workaround  still is to create a class of virtual functions  interface  and inherit it in the caller class  Then pass it as a parameter  could be in the constructor  of the other class that you want to call your caller class back   DEFINE Interface   class CallBack       virtual callMeBack             This is the class that you want to call you back   class AnotherClass           public void RegisterMe CallBack  callback                  m callback   callback               public void DoSomething                      DO STUFF                             then call         if  m callback  m callback- gt callMeBack                private CallBack  m callback   NULL       And this is the class that will be called back   class Caller   public CallBack       void DoSomthing                     void callMeBack                std  cout  lt  lt   I got your message   lt  lt  std  endl

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Necromancing   I think the answers to date are a little unclear    Let s make an example   Supposed you have an array of pixels  array of ARGB int8 t values      A RGB image int8 t  pixels   new int8 t 1024 768 4     Now you want to generate a PNG   To do so  you call the function toJpeg  bool ok   toJpeg writeByte  pixels  width  height     where writeByte is a callback-function  void writeByte unsigned char oneByte        fputc oneByte  output       The problem here  FILE  output has to be a global variable   Very bad if you re in a multithreaded environment  e g  a http-server      So you need some way to make output a non-global variable  while retaining the callback signature    The immediate solution that springs into mind is a closure  which we can emulate using a class with a member function    class BadIdea   private      FILE  m stream  public      BadIdea FILE  stream             this- gt m stream   stream             void writeByte unsigned char oneByte               fputc oneByte  this- gt m stream               And then do   FILE  fp   fopen filename   wb    BadIdea  foobar   new BadIdea fp    bool ok   TooJpeg  writeJpeg foobar- gt writeByte  image  width  height   delete foobar  fflush fp   fclose fp     However  contrary to expectations  this does not work   The reason is  C   member functions are kinda implemented like C  extension functions    So you have   class struct BadIdea       FILE  m stream      and   static class BadIdeaExtensions       public static writeByte this BadIdea instance  unsigned char oneByte                 fputc oneByte  instance- gt m stream              So when you want to call writeByte  you need pass not only the address of writeByte  but also the address of the BadIdea-instance    So when you have a typedef for the writeByte procedure  and it looks like this  typedef void   WRITE ONE BYTE  unsigned char     And you have a writeJpeg signature that looks like this  bool writeJpeg WRITE ONE BYTE output  uint8 t  pixels  uint32 t   width  uint32 t height                 it s fundamentally impossible to pass a two-address member function to a one-address function pointer  without modifying writeJpeg   and there s no way around it    The next best thing that you can do in C    is using a lambda-function   FILE  fp   fopen filename   wb    auto lambda    fp  unsigned char oneByte    fputc oneByte  fp       bool ok   TooJpeg  writeJpeg lambda  image  width  height     However  because lambda is doing nothing different  than passing an instance to a hidden class  such as the  BadIdea -class   you need to modify the signature of writeJpeg    The advantage of lambda over a manual class  is that you just need to change one typedef   typedef void   WRITE ONE BYTE  unsigned char     to  using WRITE ONE BYTE   std  function lt void unsigned char  gt      And then you can leave everything else untouched    You could also use std  bind  auto f   std  bind  amp BadIdea  writeByte   amp foobar     But this  behind the scene  just creates a lambda function  which then also needs the change in typedef    So no  there is no way to pass a member function to a method that requires a static function-pointer   But lambdas are the easy way around  provided that you have control over the source   Otherwise  you re out of luck   There s nothing you can do with C     Note   std  function requires  include  lt functional gt    However  since C   allows you to use C as well  you can do this with libffcall in plain C  if you don t mind linking a dependency   Download libffcall from GNU  at least on ubuntu  don t use the distro-provided package - it is broken   unzip      configure make make install  gcc main c -l libffcall a -o ma   main c    include  lt callback h gt      this is the closure function to be allocated  void function  void  data  va alist alist         int abc   va arg int alist         printf  data   08p n   data      hex 0x14   20      printf  abc   d n   abc            va start type alist   return type            arg   va arg type alist   arg type            va return type alist    return type   return value            va start int alist          int r   666         va return int alist  r        int main int argc  char  argv          int in1   10       void   data    void   20      void  incrementer1  int abc     void       alloc callback  amp function  data          void  incrementer1    can have unlimited arguments  e g  incrementer1 123 456          void  incrementer1  int abc  starts to throw errors        incrementer1 123          free callback callback       return EXIT SUCCESS      And if you use CMake  add the linker library after add executable  add library libffcall STATIC IMPORTED  set target properties libffcall PROPERTIES         IMPORTED LOCATION  usr local lib libffcall a  target link libraries BitmapLion libffcall    or you could just dynamically link libffcall  target link libraries BitmapLion ffcall    Note   You might want to include the libffcall headers and libraries  or create a cmake project with the contents of libffcall

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That doesn t work because a member function pointer cannot be handled like a normal function pointer  because it expects a  this  object argument    Instead you can pass a static member function as follows  which are like normal non-member functions in this regard   m cRedundencyManager- gt Init  amp CLoggersInfra  Callback  this     The function can be defined as follows   static void Callback int other arg  void   this pointer        CLoggersInfra   self   static cast lt CLoggersInfra  gt  this pointer       self- gt RedundencyManagerCallBack other arg

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What argument does Init take  What is the new error message   Method pointers in C   are a bit difficult to use  Besides the method pointer itself  you also need to provide an instance pointer  in your case this   Maybe Init expects it as a separate argument

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Is m cRedundencyManager able to use member functions  Most callbacks are set up to use regular functions or static member functions  Take a look at this page at C   FAQ Lite for more information   Update  The function declaration you provided shows that m cRedundencyManager is expecting a function of the form  void yourCallbackFunction int  void     Member functions are therefore unacceptable as callbacks in this case  A static member function may work  but if that is unacceptable in your case  the following code would also work  Note that it uses an evil cast from void         in your CLoggersInfra constructor  m cRedundencyManager- Init myRedundencyManagerCallBackHandler  this         in your CLoggersInfra header  void myRedundencyManagerCallBackHandler int i  void   CLoggersInfraPtr         in your CLoggersInfra source file  void myRedundencyManagerCallBackHandler int i  void   CLoggersInfraPtr          CLoggersInfra   CLoggersInfraPtr - RedundencyManagerCallBack i

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This is a simple question but the answer is surprisingly complex  The short answer is you can do what you re trying to do with std  bind1st or boost  bind  The longer answer is below  The compiler is correct to suggest you use  amp CLoggersInfra  RedundencyManagerCallBack  First  if RedundencyManagerCallBack is a member function  the function itself doesn t belong to any particular instance of the class CLoggersInfra  It belongs to the class itself  If you ve ever called a static class function before  you may have noticed you use the same SomeClass  SomeMemberFunction syntax  Since the function itself is  static  in the sense that it belongs to the class rather than a particular instance  you use the same syntax  The   amp   is necessary because technically speaking you don t pass functions directly -- functions are not real objects in C    Instead you re technically passing the memory address for the function  that is  a pointer to where the function s instructions begin in memory  The consequence is the same though  you re effectively  passing a function  as a parameter  But that s only half the problem in this instance  As I said  RedundencyManagerCallBack the function doesn t  belong  to any particular instance  But it sounds like you want to pass it as a callback with a particular instance in mind  To understand how to do this you need to understand what member functions really are  regular not-defined-in-any-class functions with an extra hidden parameter  For example  class A   public      A     data 0         void foo int addToData    this- gt data    addToData         int data           A an a object  an a object foo 5   A  foo  amp an a object  5      This is the same as the line above  std  cout  lt  lt  an a object data     Prints 10   How many parameters does A  foo take  Normally we would say 1  But under the hood  foo really takes 2  Looking at A  foo s definition  it needs a specific instance of A in order for the  this  pointer to be meaningful  the compiler needs to know what  this  is   The way you usually specify what you want  this  to be is through the syntax MyObject MyMemberFunction    But this is just syntactic sugar for passing the address of MyObject as the first parameter to MyMemberFunction  Similarly  when we declare member functions inside class definitions we don t put  this  in the parameter list  but this is just a gift from the language designers to save typing  Instead you have to specify that a member function is static to opt out of it automatically getting the extra  this  parameter  If the C   compiler translated the above example to C code  the original C   compiler actually worked that way   it would probably write something like this  struct A       int data      void a init A  to init        to init- gt data   0     void a foo A  this  int addToData         this- gt data    addToData          A an a object  a init 0      Before constructor call was implicit a foo  amp an a object  5      Used to be an a object foo 5    Returning to your example  there is now an obvious problem   Init  wants a pointer to a function that takes one parameter  But  amp CLoggersInfra  RedundencyManagerCallBack is a pointer to a function that takes two parameters  it s normal parameter and the secret  this  parameter  That s why you re still getting a compiler error  as a side note  If you ve ever used Python  this kind of confusion is why a  self  parameter is required for all member functions   The verbose way to handle this is to create a special object that holds a pointer to the instance you want and has a member function called something like  run  or  execute   or overloads the      operator  that takes the parameters for the member function  and simply calls the member function with those parameters on the stored instance  But this would require you to change  Init  to take your special object rather than a raw function pointer  and it sounds like Init is someone else s code  And making a special class for every time this problem comes up will lead to code bloat  So now  finally  the good solution  boost  bind and boost  function  the documentation for each you can find here  boost  bind docs  boost  function docs boost  bind will let you take a function  and a parameter to that function  and make a new function where that parameter is  locked  in place  So if I have a function that adds two integers  I can use boost  bind to make a new function where one of the parameters is locked to say 5  This new function will only take one integer parameter  and will always add 5 specifically to it  Using this technique  you can  lock in  the hidden  this  parameter to be a particular class instance  and generate a new function that only takes one parameter  just like you want  note that the hidden parameter is always the first parameter  and the normal parameters come in order after it   Look at the boost  bind docs for examples  they even specifically discuss using it for member functions  Technically there is a standard function called  std  bind1st  3  that you could use as well  but boost  bind is more general  Of course  there s just one more catch  boost  bind will make a nice boost  function for you  but this is still technically not a raw function pointer like Init probably wants  Thankfully  boost provides a way to convert boost  function s to raw pointers  as documented on StackOverflow here  How it implements this is beyond the scope of this answer  though it s interesting too  Don t worry if this seems ludicrously hard -- your question intersects several of C   s darker corners  and boost  bind is incredibly useful once you learn it  C  11 update  Instead of boost  bind you can now use a lambda function that captures  this   This is basically having the compiler generate the same thing for you

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This answer is a reply to a comment above and does not work with VisualStudio 2008 but should be preferred with more recent compilers     Meanwhile you don t have to use a void pointer anymore and there is also no need for boost since std  bind and std  function are available  One advantage  in comparison to void pointers  is type safety since the return type and the arguments are explicitly stated using std  function      std  function lt return type list of argument type s   gt  void Init std  function lt void void  gt  f     Then you can create the function pointer with std  bind and pass it to Init   auto cLoggersInfraInstance   CLoggersInfra    auto callback   std  bind  amp CLoggersInfra  RedundencyManagerCallBack  cLoggersInfraInstance   Init callback     Complete example for using std  bind with member  static members and non member functions    include  lt functional gt   include  lt iostream gt   include  lt string gt   class RedundencyManager    incl  Typo  -    public         std  function lt return type list of argument type s   gt      std  string Init std  function lt std  string void  gt  f                 return f              class CLoggersInfra   private      std  string member    Hello from non static member callback     public      static std  string RedundencyManagerCallBack                 return  Hello from static member callback               std  string NonStaticRedundencyManagerCallBack                 return member            std  string NonMemberCallBack         return  Hello from non member function       int main         auto instance   RedundencyManager         auto callback1   std  bind  amp NonMemberCallBack       std  cout  lt  lt  instance Init callback1   lt  lt    n           Similar to non member function      auto callback2   std  bind  amp CLoggersInfra  RedundencyManagerCallBack       std  cout  lt  lt  instance Init callback2   lt  lt    n           Class instance is passed to std  bind as second argument          heed that I call the constructor of CLoggersInfra      auto callback3   std  bind  amp CLoggersInfra  NonStaticRedundencyManagerCallBack                                 CLoggersInfra          std  cout  lt  lt  instance Init callback3   lt  lt    n       Possible output   Hello from non member function  Hello from static member callback  Hello from non static member callback    Furthermore using std  placeholders you can dynamically pass arguments to the callback  e g  this enables the usage of return f  MyString    in Init if f has a string parameter

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A pointer to a class member function is not the same as a pointer to a function  A class member takes an implicit extra argument  the this pointer   and uses a different calling convention   If your API expects a nonmember callback function  that s what you have to pass to it

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I can see that the init has the following override    Init CALLBACK FUNC EX callback func  void   callback parm    where CALLBACK FUNC EX is  typedef void   CALLBACK FUNC EX  int  void

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That doesn t work because a member function pointer cannot be handled like a normal function pointer  because it expects a  this  object argument    Instead you can pass a static member function as follows  which are like normal non-member functions in this regard   m cRedundencyManager- gt Init  amp CLoggersInfra  Callback  this     The function can be defined as follows   static void Callback int other arg  void   this pointer        CLoggersInfra   self   static cast lt CLoggersInfra  gt  this pointer       self- gt RedundencyManagerCallBack other arg

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Is m cRedundencyManager able to use member functions  Most callbacks are set up to use regular functions or static member functions  Take a look at this page at C   FAQ Lite for more information   Update  The function declaration you provided shows that m cRedundencyManager is expecting a function of the form  void yourCallbackFunction int  void     Member functions are therefore unacceptable as callbacks in this case  A static member function may work  but if that is unacceptable in your case  the following code would also work  Note that it uses an evil cast from void         in your CLoggersInfra constructor  m cRedundencyManager- Init myRedundencyManagerCallBackHandler  this         in your CLoggersInfra header  void myRedundencyManagerCallBackHandler int i  void   CLoggersInfraPtr         in your CLoggersInfra source file  void myRedundencyManagerCallBackHandler int i  void   CLoggersInfraPtr          CLoggersInfra   CLoggersInfraPtr - RedundencyManagerCallBack i

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A simple solution  workaround  still is to create a class of virtual functions  interface  and inherit it in the caller class  Then pass it as a parameter  could be in the constructor  of the other class that you want to call your caller class back   DEFINE Interface   class CallBack       virtual callMeBack             This is the class that you want to call you back   class AnotherClass           public void RegisterMe CallBack  callback                  m callback   callback               public void DoSomething                      DO STUFF                             then call         if  m callback  m callback- gt callMeBack                private CallBack  m callback   NULL       And this is the class that will be called back   class Caller   public CallBack       void DoSomthing                     void callMeBack                std  cout  lt  lt   I got your message   lt  lt  std  endl

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This question and answer from the C   FAQ Lite covers your question and the considerations involved in the answer quite nicely I think  Short snippet from the web page I linked      Don   t       Because a member function is meaningless without an object to invoke   it on  you can   t do this directly  if The X Window System was   rewritten in C    it would probably pass references to objects around    not just pointers to functions  naturally the objects would embody the   required function and probably a whole lot more

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That doesn t work because a member function pointer cannot be handled like a normal function pointer  because it expects a  this  object argument    Instead you can pass a static member function as follows  which are like normal non-member functions in this regard   m cRedundencyManager- gt Init  amp CLoggersInfra  Callback  this     The function can be defined as follows   static void Callback int other arg  void   this pointer        CLoggersInfra   self   static cast lt CLoggersInfra  gt  this pointer       self- gt RedundencyManagerCallBack other arg

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The type of pointer to non-static member function is different from pointer to ordinary function  Type is void    int  if it   s an ordinary or static member function  Type is void CLoggersInfra     int  if it   s a non-static member function  So you cannot pass a pointer to a non-static member function if it is expecting an ordinary function pointer   Furthermore  a non-static member function has an implicit hidden parameter to the object  The this pointer is implicitly passed as an argument to the member function call  So the member functions can be invoked only by providing an object    If the API Init cannot be changed  a wrapper function  ordinary function or a class static member function  that invokes the member can be used  In the worst case  the object would be a global for the wrapper function to access   CLoggersInfra  pLoggerInfra   RedundencyManagerCallBackWrapper int val        pLoggerInfra- gt RedundencyManagerCallBack val       m cRedundencyManager- gt Init RedundencyManagerCallBackWrapper     If the API Init can be changed  there are many alternatives - Object non-static member function pointer  Function Object  std  function or Interface Function    See the post on callbacks for the different variations with C   working examples

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This question and answer from the C   FAQ Lite covers your question and the considerations involved in the answer quite nicely I think  Short snippet from the web page I linked      Don   t       Because a member function is meaningless without an object to invoke   it on  you can   t do this directly  if The X Window System was   rewritten in C    it would probably pass references to objects around    not just pointers to functions  naturally the objects would embody the   required function and probably a whole lot more

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A pointer to a class member function is not the same as a pointer to a function  A class member takes an implicit extra argument  the this pointer   and uses a different calling convention   If your API expects a nonmember callback function  that s what you have to pass to it

[c++] How can I pass a class member function as a callback?

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