Function pointer to member function

Question

I d like to set up a function pointer as a member of a class that is a pointer to another function in the same class  The reasons why I m doing this are complicated   In this example  I would like the output to be  1   class A   public   int f     int   x        int A  f      return 1      int main      A a   a x   a f   printf   d n  a x        But this fails at compiling  Why

User · Answer

While you unfortunately cannot convert an existing member function pointer to a plain function pointer, you can create an adapter function template in a fairly straightforward way that wraps a member function pointer known at compile-time in a normal function like this:

template <class Type>
struct member_function;

template <class Type, class Ret, class... Args>
struct member_function<Ret(Type::*)(Args...)>
{
    template <Ret(Type::*Func)(Args...)>
    static Ret adapter(Type &obj, Args&&... args)
    {
        return (obj.*Func)(std::forward<Args>(args)...);
    }
};

template <class Type, class Ret, class... Args>
struct member_function<Ret(Type::*)(Args...) const>
{
    template <Ret(Type::*Func)(Args...) const>
    static Ret adapter(const Type &obj, Args&&... args)
    {
        return (obj.*Func)(std::forward<Args>(args)...);
    }
};

int (*func)(A&) = &member_function<decltype(&A::f)>::adapter<&A::f>;

Note that in order to call the member function, an instance of A must be provided.

User · Answer

Building on  IllidanS4  s answer  I have created a template class that allows virtually any member function with predefined arguments and class instance to be passed by reference for later calling     template lt class RET  class    RArgs gt  class Callback t   public      virtual RET call RArgs amp  amp     rargs    0        virtual RET call     0      template lt class T  class RET  class    RArgs gt  class CallbackCalltimeArgs   public Callback t lt RET  RArgs    gt    public      T   owner      RET T   x  RArgs          RET call RArgs amp  amp     rargs            return   owner   x   std  forward lt RArgs gt  rargs                  CallbackCalltimeArgs T  t  RET T   x  RArgs        owner t   x x         template lt class T  class RET  class    Args gt  class CallbackCreattimeArgs   public Callback t lt RET gt    public      T  owner      RET T   x  Args          RET call             return   owner   x   std  get lt Args amp  amp  gt  args                  std  tuple lt Args amp  amp     gt  args      CallbackCreattimeArgs T  t  RET T   x  Args      Args amp  amp     args    owner t   x x           args std  tuple lt Args amp  amp     gt  std  forward lt Args gt  args               Test   example   class container   public      static void printFrom container  c    c- gt print           container int data    data data           container           void print     printf   d n   data          void printTo FILE  f    fprintf f    d n   data          void printWith int arg    printf   d  d n   data  arg      private      int data      int main         container c1 1   c2 20       CallbackCreattimeArgs lt container  void gt  f1  amp c1   amp container  print       Callback t lt void gt   fp1    amp f1      fp1- gt call     1     CallbackCreattimeArgs lt container  void  FILE  gt  f2  amp c2   amp container  printTo  stdout       Callback t lt void gt   fp2    amp f2      fp2- gt call     20     CallbackCalltimeArgs lt container  void  int gt  f3  amp c2   amp container  printWith       Callback t lt void  int gt   fp3    amp f3      fp3- gt call 15    20 15     Obviously  this will only work if the given arguments and owner class are still valid  As far as readability    please forgive me   Edit  removed unnecessary malloc by making the tuple normal storage  Added inherited type for the reference  Added option to provide all arguments at calltime instead  Now working on having both      Edit 2  As promised  both  Only restriction  that I see  is that the predefined arguments must come before the runtime supplied arguments in the callback function  Thanks to  Chipster for some help with gcc compliance  This works on gcc on ubuntu and visual studio on windows    ifdef  WIN32  define wintypename typename  else  define wintypename  endif  template lt class RET  class    RArgs gt  class Callback t   public      virtual RET call RArgs    rargs    0      virtual  Callback t     default      template lt class RET  class    RArgs gt  class CallbackFactory   private      template lt class T  class    CArgs gt  class Callback   public Callback t lt RET  RArgs    gt        private          T   owner          RET T   x  CArgs     RArgs              std  tuple lt CArgs    gt  cargs          RET call RArgs    rargs                return   owner   x   std  get lt CArgs gt  cargs      rargs                     public          Callback T  t  RET T   x  CArgs     RArgs      CArgs    pda            Callback              public      template lt class U  class    CArgs gt  static Callback t lt RET  RArgs    gt   make U  owner  CArgs    cargs  RET U   func  CArgs     RArgs          template lt class RET2  class    RArgs2 gt  template lt class T2  class    CArgs2 gt  CallbackFactory lt RET2  RArgs2    gt   Callback lt T2  CArgs2    gt   Callback T2  t  RET2 T2   x  CArgs2     RArgs2      CArgs2    pda    x x   owner t   cargs std  forward lt CArgs2 gt  pda         template lt class RET  class    RArgs gt  template lt class U  class    CArgs gt  Callback t lt RET  RArgs    gt   CallbackFactory lt RET  RArgs    gt   make U  owner  CArgs    cargs  RET U   func  CArgs     RArgs            return new wintypename CallbackFactory lt RET  RArgs    gt   Callback lt U  CArgs    gt  owner  func  std  forward lt CArgs gt  cargs

User · Answer

While this is based on the sterling answers elsewhere on this page  I had a use case which wasn t completely solved by them  for a vector of pointers to functions do the following    include  lt iostream gt   include  lt vector gt   include  lt stdio h gt   include  lt stdlib h gt   class A  public    typedef vector lt int gt   A   AFunc  int I1 int I2     vector lt AFunc gt  FuncList    inline int Subtract int I1 int I2  return I1-I2      inline int Add int I1 int I2  return I1 I2            void Populate      void ExecuteAll        void A  Populate        FuncList push back  amp A  Subtract       FuncList push back  amp A  Add              void A  ExecuteAll      int In1 1 In2 2 Out 0    for size t FuncId 0 FuncId lt FuncList size   FuncId         Out  this- gt  FuncList FuncId   In1 In2       printf  Function  ld output  d n  FuncId Out          int main      A Demo    Demo Populate      Demo ExecuteAll      return 0      Something like this is useful if you are writing a command interpreter with indexed functions that need to be married up with parameter syntax and help tips etc  Possibly also useful in menus

User · Answer

Call member function on string command   include  lt iostream gt   include  lt string gt    class A    public       void call    private      void printH        void command std  string a  std  string b  void  A   func          void A  printH         std  cout lt  lt   H n      void A  call         command  a   a    amp A  printH      void A  command std  string a  std  string b  void  A   func           if a    b                 this- gt  func              int main         A a      a call        return 0      Pay attention to  this- gt  func     and the way to declare the function pointer with class name void  A   func

User · Answer

The syntax is wrong  A member pointer is a different type category from a ordinary pointer  The member pointer will have to be used together with an object of its class   class A   public   int f     int  A   x         lt - declare by saying what class it is a pointer to     int A  f      return 1      int main      A a   a x    amp A  f     use the    syntax  printf   d n   a   a x          use together with an object of its class     a x does not yet say on what object the function is to be called on  It just says that you want to use the pointer stored in the object a  Prepending a another time as the left operand to the    operator will tell the compiler on what object to call the function on

User · Answer

int   x    is not a pointer to member function  A pointer to member function is written like this  int  A   x  void     amp A  f

User · Answer

You need to use a pointer to a member function  not just a pointer to a function   class A        int f     return 1    public      int  A   x          A     x  amp A  f         int main         A a     std  cout  lt  lt   a  a x        return 0

[c++] Function pointer to member function

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