Using generic std function objects with member functions in one class

Question

For one class I want to store some function pointers to member functions of the same class in one map storing std  function objects  But I fail right at the beginning with this code   include  lt functional gt   class Foo       public          void doSomething              void bindFunction                    ERROR             std  function lt void void  gt  f    amp Foo  doSomething                I receive error C2064  term does not evaluate to a function taking 0 arguments in xxcallobj combined with some weird template instantiation errors  Currently I am working on Windows 8 with Visual Studio 2010 2011 and on Win 7 with VS10 it fails too  The error must be based on some weird C   rules i do not follow

User · Accepted Answer

A non-static member function must be called with an object. That is, it always implicitly passes "this" pointer as its argument.

Because your std::function signature specifies that your function doesn't take any arguments (<void(void)>), you must bind the first (and the only) argument.

std::function<void(void)> f = std::bind(&Foo::doSomething, this);

If you want to bind a function with parameters, you need to specify placeholders:

using namespace std::placeholders;
std::function<void(int,int)> f = std::bind(&Foo::doSomethingArgs, this, std::placeholders::_1, std::placeholders::_2);

Or, if your compiler supports C++11 lambdas:

std::function<void(int,int)> f = [=](int a, int b) {
    this->doSomethingArgs(a, b);
}

(I don't have a C++11 capable compiler at hand right now, so I can't check this one.)

User · Answer

If you need to store a member function without the class instance  you can do something like this   class MyClass   public      void MemberFunc int value                do something              Store member function binding auto callable   std  mem fn  amp MyClass  MemberFunc       Call with late supplied  this  MyClass myInst  callable  amp myInst  123     What would the storage type look like without auto  Something like this   std   Mem fn wrap lt void void    cdecl TestA     int  TestA int gt  callable   You can also pass this function storage to a standard function binding  std  function lt void int  gt  binding   std  bind callable   amp testA  std  placeholders   1   binding 123      Call   Past and future notes  An older interface std  mem func existed  but has since been deprecated  A proposal exists  post C  17  to make pointer to member functions callable  This would be most welcome

User · Answer

You can avoid std  bind doing this    std  function lt void void  gt  f    this - gt   Foo  doSomething

User · Answer

Unfortunately  C   does not allow you to directly get a callable object referring to an object and one of its member functions   amp Foo  doSomething gives you a  pointer to member function  which refers to the member function but not the associated object   There are two ways around this  one is to use std  bind to bind the  pointer to member function  to the this pointer  The other is to use a lambda that captures the this pointer and calls the member function   std  function lt void void  gt  f   std  bind  amp Foo  doSomething  this   std  function lt void void  gt  g    this    doSomething        I would prefer the latter   With g   at least binding a member function to this will result in an object three-pointers in size  assigning this to an std  function will result in dynamic memory allocation   On the other hand  a lambda that captures this is only one pointer in size  assigning it to an std  function will not result in dynamic memory allocation with g     While I have not verified this with other compilers  I suspect similar results will be found there

User · Answer

Either you need  std  function lt void Foo   gt  f    amp Foo  doSomething    so that you can call it on any instance  or you need to bind a specific instance  for example this  std  function lt void void  gt  f   std  bind  amp Foo  doSomething  this

User · Answer

You can use functors if you want a less generic and more precise control under the hood  Example with my win32 api to forward api message from a class to another class   IListener h   include  lt windows h gt  class IListener        public      virtual  IListener          virtual LRESULT operator   HWND hWnd  UINT uMsg  WPARAM wParam  LPARAM lParam    0       Listener h   include  IListener h  template  lt typename D gt  class Listener   public IListener       public      typedef LRESULT  D   WMFuncPtr  HWND hWnd  UINT uMsg  WPARAM wParam  LPARAM lParam         private      D   instance      WMFuncPtr  wmFuncPtr        public      virtual  Listener          virtual LRESULT operator   HWND hWnd  UINT uMsg  WPARAM wParam  LPARAM lParam  override           return   instance- gt   wmFuncPtr  hWnd  uMsg  wParam  lParam              Listener D  instance  WMFuncPtr wmFuncPtr             instance   instance           wmFuncPtr   wmFuncPtr             Dispatcher h   include  lt map gt   include  Listener h   class Dispatcher       private            Storage map for message pointers         std  map lt UINT   WM MESSAGE    IListener  gt   listeners        public          virtual  Dispatcher       clear the map              Return a previously registered callable funtion pointer for uMsg          IListener  get UINT uMsg                typename std  map lt UINT  IListener  gt   iterator itEvt              if  itEvt    listeners find uMsg       listeners end                      return NULL                            return itEvt- gt second                       Set a member function to receive message             Example Button- gt add lt MyClass gt  WM COMMAND  this   amp MyClass  myfunc           template  lt typename D gt  void add UINT uMsg  D  instance  typename Listener lt D gt   WMFuncPtr wmFuncPtr                 listeners uMsg    new Listener lt D gt  instance  wmFuncPtr                   Usage principles  class Button       public      Dispatcher  dispatcher        button window forward all received message to a listener     LRESULT onMessage HWND hWnd  UINT uMsg  WPARAM w  LPARAM l              to return a precise message like WM CREATE  you have just           search it in the map          return  dispatcher uMsg  hWnd  uMsg  w  l             class Myclass       Button  button        the listener for Button messages     LRESULT button listener HWND hWnd  UINT uMsg  WPARAM w  LPARAM l            return 0               Register the listener for Button messages     void initialize               now all message received from button are forwarded to button listener function          button  dispatcher add WM CREATE  this   amp Myclass  button listener              Good luck and thank to all for sharing knowledge

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