How to hide a field via define and provide only setter and getter?

Question

I wonder how to hide a real property field (not make it private or public but force to use setters and getters) and provide him with simple setter and getter. So I wonder how to create api like:

private:
    Property( int my_a);
public:
    Property( int my_b);
...
{
 set_my_a(1);
 cout << get_my_a() << endl;
 // my_a = 13; // will cause compiler error
...

How to create such thing via Boost preprocessor?

Answer 1

Do you really need to use boost preprocessor? you have a solution without boost below:

// property.h    
#include <stdio.h>

#define property(type) struct : public Property <type>  

template <typename T>
class Property 
{
protected:
  T value;
public:
  virtual T get() { 
    return value; 
  }
  virtual void set(T new_value) { 
    value = new_value; 
  }
};

usage example:

// test.cpp
#include "property.h"

class Test {
public:
  property(int) {} a;

  property(int)  {
    int get() { 
      return value * 10; 
    } 
  } b;

  property(int) {
    void set(int x) {
      value = x * 200;
    } 
  } c;

  property(int) {
    int get() { 
      return value * 3000; 
    } 
    void set(int x) {
      value = x * 443;
    } 
  } d;
};

main()
{
  Test t;

  printf("i\ta\tb\tc\td\t\n");
  for (int i=0; i<10; i++) { 
    t.a.set(i);
    t.b.set(i);
    t.c.set(i);
    t.d.set(i);
    printf("%i\t%i\t%i\t%i\t%i\n", i, t.a.get(), t.b.get(), t.c.get(), t.d.get());
  }
}

The wikipedia solution in http://en.wikipedia.org/wiki/Property_(programming)#C.2B.2B is good but needs a minimal modification to become useful, because without the protected statement you cant write your own getters and setters.

#include <iostream>

template <typename T> 
class property {
protected:
  T value;
public:
  T & operator = (const T &i) {
    ::std::cout << i << ::std::endl;
    return value = i;
  }
  operator T const & () const {
    return value;
  }
};

struct Bar {
  property <bool> alpha;
  struct :public property <int> {
    int & operator = (const int &i) {
      ::std::cout << "new setter " << i << ::std::endl;
      return value = i;
    }
  } bravo;
};

main() 
{
  Bar b;
  b.alpha = false;
  b.bravo = (unsigned int) 1;
}

You can change a little more if you want:

#include <iostream>
#define SETTER(type) public: type& operator=(const type new_value)
#define GETTER(type) public: operator type const & () const

template <typename T> 
class Property {
protected:
  T value;
public:
  T & operator = (const T &i) {
    ::std::cout << i << ::std::endl;
    return value = i;
  }
  template <typename T2> T2 & operator = (const T2 &i) {
    ::std::cout << "T2: " << i << ::std::endl;
    T2 &guard = value;
    throw guard; // Never reached.
  }
  operator T const & () const {
    return value;
  }
};

struct Bar {
  Property <bool> alpha;
  struct:Property <int> {
    SETTER(int) {
      value = new_value * 1000;
      ::std::cout << "new method " << new_value << ::std::endl;
      return value;
    }
    GETTER(int) {
      return value/1000;
    }
  } bravo;
};

main() 
{
  Bar b;
  b.alpha = false;
  b.bravo = (unsigned int) 1;
  ::std::cout << b.bravo << ::std::endl;
}

Answer 2

Rather than rewrite an example of the implementation, this is the link for one on Wikipedia: http://en.wikipedia.org/wiki/Property_(programming)#C.2B.2B

This basically forces the property to be accessed through getter/setter methods. The upgrade you would need to get your desired effect is the ability to pass functors to these properties. There are plenty of ideas on implementing these; the best approach I cannot advise and depends on your developmental needs. Personally, it feels like over engineering and prefer to just use Pimpl to hide my private details and just provide the getters/setters explicitly.