function pointer in c++ class throws compiler error

Question

I am new to function pointer and tried to pass the function pointer as parameter from one class to other and getting compiler error.

'fncptr1': is not a class or namespace name"

what am I doing wrong?

fncptr1.h

#ifndef FNCPTR1
#define FNCPTR1

#include "fncptr2.h"
class fncptr1
{
  public:
    int addition(int a,int b);
    void testfncptr();  
};

#endif // !FNCPTR1

fncptr1.cpp

 #include "fncptr1.h"
 #include <iostream>

 using namespace std;

 int fncptr1::addition(int a,int b)
 {
   return a + b;
 }

 void fncptr1::testfncptr()
 {
    fncptr2 f;
    f.implfncptr(&addition);
 }

fncptr2.h

 #ifndef FNCPTR2
 #define FNCPTR2

 #include "fncptr1.h"
 class fncptr2
 {
   public:
      int implfncptr(int (fncptr1::*add)(int,int));
 };

 #endif // !FNCPTR2

fncptr2.cpp

  #include "stdafx.h"
  #include "fncptr2.h"
  #include <iostream>

  using namespace std;
  fncptr1 ff;

  int fncptr2::implfncptr(int (fncptr1::*add)(int, int))
  {
    return (ff.*add)(1,2);
  }

main.cpp

fncptr1 f;
f.testfncptr();

fncptr1.h includes fncptr2.h and fncptr2.h includes fncptr1.h, this is seriously wrong. — Jabberwocky
– Jabberwocky, Commented Aug 25, 2021 at 11:55
And f.implfncptr(&addition); should be f.implfncptr(&fncptr1::addition); — Ted Lyngmo
– Ted Lyngmo, Commented Aug 25, 2021 at 11:59
Also show main.cpp, the one you show is incomplete, and show the full error log. — Jabberwocky
– Jabberwocky, Commented Aug 25, 2021 at 12:00
It's also unclear what you're hoping for when calling f.testfncptr(); in main. The result of the addition is just thrown away. — Ted Lyngmo
– Ted Lyngmo, Commented Aug 25, 2021 at 12:04

Jabberwocky · Accepted Answer · 2021-08-25 12:20:19Z

Remove the includes of fncptr2.h and fncptr1.h from all .h files. Instead add them to fncptr1.c and fncptr2.c.
you need f.implfncptr(&fncptr1::addition); instead of f.implfncptr(&addition); in fncptr1.cpp

fncptr1.h

#ifndef FNCPTR1
#define FNCPTR1

class fncptr1
{
public:
  int addition(int a, int b);
  void testfncptr();
};

#endif // !FNCPTR1

fncptr2.h

#ifndef FNCPTR2
#define FNCPTR2

class fncptr1;

class fncptr2
{
public:
  int implfncptr(int (fncptr1::* add)(int, int));
};

#endif // !FNCPTR2

fncptr1.cpp

#include "fncptr1.h"
#include "fncptr2.h"
#include <iostream>

using namespace std;

int fncptr1::addition(int a, int b)
{
  return a + b;
}

void fncptr1::testfncptr()
{
  fncptr2 f;
  f.implfncptr(&fncptr1::addition);
}

fncptr2.cpp

#include "fncptr2.h"
#include "fncptr1.h"
#include <iostream>

using namespace std;
fncptr1 ff;

int fncptr2::implfncptr(int (fncptr1::* add)(int, int))
{
  return (ff.*add)(1, 2);
}

main.cpp

#include "fncptr1.h"

int main()
{
  fncptr1 f;
  f.testfncptr();
}

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