Named Parameter Idiom using a pointer to a class private method

Question

I got stuck with a C++ compilation error while doing something that is probably not really "conventional". To make things easier I just re-wrote the mechanism I am trying to use in a easier-to-read way and I checked that I got the same issue.

First of all here is the code:

test.h // -- C++ --

template <typename MODULE> class item;

template <typename MODULE>
class init {
public:
  typedef int (MODULE::*funcPtr)(int);
private:
  funcPtr m_fp;
public:
  init& has_funcPtr(funcPtr fp) { m_fp = fp;}
  init() {}
  virtual ~init() {}
private:
  friend class item<MODULE>;
};

template <typename MODULE>
class item {
public:
  typedef int (MODULE::*funcPtr)(int);
private:
  funcPtr m_fp;
public:
  item(init<MODULE> params) : m_fp(params.m_fp) {}
  virtual ~item() {}
};

class user {
public:
  typedef init<user>::funcPtr funcPtr;
private:
  // Method CB
  int func1(int i);
  // Item member
  item<user> m_item;
public:
  user();
  virtual ~user();
};

test.cpp // -- C++ --

#include "test.h"

user::user() : m_item(init<user>().has_funcPtr(this->func1) ) {}

int user::func1(int i) {return 1;}

and here is the error:

/test.cpp:5:59: error: invalid use of non-static member function
 user::user() : m_item(init<user>().has_funcPtr(this->func1) ) {
                                                       ^

So, I am not sure this is the best way to achieve what I want (probably not, anyway if you have other suggestions they are very welcome) but my goal now is to make it work or to understand exactly why it can't work so that I learn something from it!

The basic idea is that:

• the class "item" can be initialized with the named parameter idiom using the method "has_funcPtr" of the class "init" concatenated to its constructor like: "init().has_funcPtr(&function_name)".
• the class "user" can store a pointer to its private method "func1" as a private member of its private member of type "item".

In this way, when a specific method of an object "item" is called (for simplicity I don't include this long part here since it is not relevant to the error but it is just to describe the goal of this snippet of code) that method can do stuff and call the private method of its father object "user" through that pointer to function (I hope this is clear enough...).

Now, I think there is an issue with the order of initialization of the objects but I am not sure where and how to fix it. In particular I thought that since the "func1" method doesn't operate on any member of the class "user", then its reference could be used directly in the initialization list to initialize an "init" object and feed it to an "item" object.

Thank you all in advance

Answer 1

this->func1 doesn't form a member function pointer. It should look like &user::func1 if you are in the user class.