pointer to member function

[mchome]

I want to use in my class a pointer to function.
I could not find a way.
This is a simple example that explains the difficulty:

class C{
  public:
    int First (int i, int j){return i;}
    int Second(int i, int j){return j;}
    int Fun(int);
};
int C::Fun(int i){
  int (C::*pFun)(int, int);
  if(i==1)
    pFun=&C::First;
  else
    pFun=&C::Second;
  return pFun(1,2); //ERROR
}

class C{
  public:
    int First (int i, int j){return i;}
    int Second(int i, int j){return j;}
    int Fun(int);
};
int C::Fun(int i){
  int (C::*pFun)(int, int);
  if(i==1)
    pFun=&C::First;
  else
    pFun=&C::Second;
  return pFun(1,2); //ERROR
}

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this causes an error message to be displayed.
I cannot find a way to solve this. On the other hand the non-member version works correctly, as follows:

int First(int i, int j);
int Second(int i, int j);
int First(int i, int j){ return i;}
int Second(int i, int j){ return j;}
 
int main (int argc, const char * argv[])
{
  int i=1;
  float x;
  int (*fun)(int, int);
  if(i==1)
     fun=First;
  else
     fun=Second;
  x=fun(1,2); //OK
}

int First(int i, int j);
int Second(int i, int j);
int First(int i, int j){ return i;}
int Second(int i, int j){ return j;}

int main (int argc, const char * argv[])
{
  int i=1;
  float x;
  int (*fun)(int, int);
  if(i==1)
     fun=First;
  else
     fun=Second;
  x=fun(1,2); //OK
}

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Does anyone have any suggestion to give?

[ChrisW67]

I assume your requirement is more complicated than your example because you don't need function pointers to solve it as written.

You can have a simple function pointer to a free function or a static member function. Simple function pointers cannot reference a member function because they lack the "this" pointer to the instance of the object.

There are several approaches to this. One is in the TR1 extensions to the C++ library:

#include <tr1/functional>
 
int C::Fun(int i){
        std::tr1::function<int(int,int)> pFun;
        if(i==1)
                pFun = std::tr1::bind(&C::First,
                        this,
                        std::tr1::placeholders::_1,
                        std::tr1::placeholders::_2);
        else
                pFun = std::tr1::bind(&C::Second,
                        this,
                        std::tr1::placeholders::_1,
                        std::tr1::placeholders::_2);
        return pFun(1,2);
}

#include <tr1/functional>

int C::Fun(int i){
        std::tr1::function<int(int,int)> pFun;
        if(i==1)
                pFun = std::tr1::bind(&C::First,
                        this,
                        std::tr1::placeholders::_1,
                        std::tr1::placeholders::_2);
        else
                pFun = std::tr1::bind(&C::Second,
                        this,
                        std::tr1::placeholders::_1,
                        std::tr1::placeholders::_2);
        return pFun(1,2);
}

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[amleto]

to use pointers to member functions you also need to have an instance!

class C{
  public:
    int First (int i, int j){return i;}
    int Second(int i, int j){return j;}
    int Fun(int);
};
int C::Fun(int i){
  int (C::*pFun)(int, int);
  if(i==1)
    pFun=&C::First;
  else
    pFun=&C::Second;
  return this->*pFun(1,2); //<<<<<<<<<<<<<
}

class C{
  public:
    int First (int i, int j){return i;}
    int Second(int i, int j){return j;}
    int Fun(int);
};
int C::Fun(int i){
  int (C::*pFun)(int, int);
  if(i==1)
    pFun=&C::First;
  else
    pFun=&C::Second;
  return this->*pFun(1,2); //<<<<<<<<<<<<<
}

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untested...

[mchome]

ChrisW67, thank you very much for your complete explanation of the issue.
I can accept to convert the functions to static and everything works.

BTW, just in case you are curious, my mechanism of function pointer is inside a routine that calculates "round" numbers for the extrema of plots.
For instance a plot that ranges between 2.345 and 32.84 could not have on the axes those nasty float numbers. Better to choose, in this case, to plot between 0 and 40. To understand what extrema to choose in all possible situations is a demanding task and I decided to take advantage to a choice of functions to select from at run time.
To reduce the number of if's in my code I prefer to point to the right function and use later the pointer instead of the native function.

Amleto, your suggestion did not work. I think the reason its the one explained by ChrisW67.
Thanks again to both for helping me.

[amleto]

no its just a syntax mistake.

this works:
return (this->*pFun)(1,2);