Go4Expert - functors in C++

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hi i was going through Functor tutorial on net which had the following example

Code:

class TFunctor

   {

   public:

      virtual void operator()(const char* string)=0;

      virtual void Call(const char* string)=0;

 };



   template <class TClass> class TSpecificFunctor : public TFunctor

   {

   private:

      void (TClass::*fpt)(const char*);

      TClass* pt2Object;



   public:

      TSpecificFunctor(TClass* _pt2Object, void(TClass::*_fpt)(const char*))

         { pt2Object = _pt2Object;  fpt=_fpt; };



      virtual void operator()(const char* string)

       { (*pt2Object.*fpt)(string);};              // execute member function



      virtual void Call(const char* string)

        { (*pt2Object.*fpt)(string);};             // execute member function

   };

Code:

class TClassA{

   public:



      TClassA(){};

      void Display(const char* text) { cout << text << endl; };



      /* more of TClassA */

   };



   // dummy class B

   class TClassB{

   public:



      TClassB(){};

      void Display(const char* text) { cout << text << endl; };



      /* more of TClassB */

   };





   // main program

   int main(int /*argc*/, char* /*argv[]*/)

   {

      // 1. instantiate objects of TClassA and TClassB

      TClassA objA;

      TClassB objB;



  TSpecificFunctor<TClassA> specFuncA(&objA, &TClassA::Display);

  TSpecificFunctor<TClassB> specFuncB(&objB, &TClassB::Display);

  TFunctor* vTable[] = { &specFuncA, &specFuncB };

  vTable[0]->Call("TClassA::Display called!");        // via function "Call"

  (*vTable[1])   ("TClassB::Display called!");        // via operator "()"

   cout << endl << "Hit Enter to terminate!" << endl;

   cin.get();

   return 0;

   }

Now i tried to replace main() with a class

Code:

class test

{

public:

TClassA objA;

This is not possible now

Code:

TSpecificFunctor<TClassA> specFuncA(&objA, &TClassA::Display);

};

How to achieve this when main() is removed.

Re: functors in C++

You always have to have a main function in C++; it's the starting point for your program.

Re: functors in C++

Quote:

Originally Posted by xpi0t0s (Post 54687)

You always have to have a main function in C++; it's the starting point for your program.

But i need this to be a library and the main is present somewhere else
I tried to compile this piece of code with -c option

Re: functors in C++

OK. What does "This is not possible" mean? Do you get an error?
If so, what is that error?
If not, please explain why you think it is not possible.

Re: functors in C++

Sorry for the confusion. I will add it properly this time .

This is my hdr file (hdr.h) file.

Code:

class TFunctor

   {

   public:



      // two possible functions to call member function. virtual cause derived

      // classes will use a pointer to an object and a pointer to a member function

      // to make the function call

      virtual void operator()(const char* string)=0;  // call using operator

      virtual void Call(const char* string)=0;        // call using function

   };





   // derived template class

   template <class TClass> class TSpecificFunctor : public TFunctor

   {

   private:

      void (TClass::*fpt)(const char*);   // pointer to member function

      TClass* pt2Object;                  // pointer to object



   public:



      // constructor - takes pointer to an object and pointer to a member and stores

      // them in two private variables

      TSpecificFunctor(TClass* _pt2Object, void(TClass::*_fpt)(const char*))

         { pt2Object = _pt2Object;  fpt=_fpt; };



      // override operator "()"

      virtual void operator()(const char* string)

       { (*pt2Object.*fpt)(string);};              // execute member function



      // override function "Call"

      virtual void Call(const char* string)

        { (*pt2Object.*fpt)(string);};             // execute member function

   };

This is my CPP file
[code]

Code:

#include "hdr.h"

#include <iostream>

using namespace std;



class TClassA{

   public:



      TClassA(){};

      void Display(const char* text) { cout << text << endl; };



      /* more of TClassA */

   };



   // dummy class B

   class TClassB{

   public:



      TClassB(){};

      void Display(const char* text) { cout << text << endl; };



      /* more of TClassB */

   };



class test

     27    {

     28       public:

     29 

     30       // 1. instantiate objects of TClassA and TClassB

     31       TClassA objA;

     32       TClassB objB;

     33 

     34 

     35       TSpecificFunctor<TClassA> specFuncA(&objA, &TClassA::Display);

};

I get this error while compiling

Code:

[root@localhost example]# g++ -c hdr.cpp 

hdr.cpp:35: error: expected identifier before ‘&’ token

hdr.cpp:35: error: expected identifier before ‘&’ token

hdr.cpp:35: error: expected ‘,’ or ‘...’

[root@localhost example]#

Re: functors in C++

OK, so it looks like you've taken the code from main() and are trying to use it as a class definition.
This won't work; you need to write function code in functions, not in class definitions.
One way if the code must go in a class definition is to have a member function that contains the code from main; that should work OK.

Code:

class test {

public:

  void func() {

    TClassA objA;

    TClassB objB;

    TSpecificFunctor<TClassA> specFuncA(&objA, &TClassA::Display);

  }

Re: functors in C++

hi now it compiles. I was under the impression that

Code:

TSpecificFunctor<TClassA> specFuncA(&objA, &TClassA::Display);

is actually creating a member function specFuncA(..) under the class test.

Re: functors in C++

thanks for replying xpi0t0s now it works for me