Python style properties in C++

properties.cpp

/*
** Python style properties in C++
** (C) 2010 Arkanosis
** jroquet@arkanosis.net
**
** Example code released under the MIT license
** http://www.opensource.org/licenses/mit-license.php
**
** This is an original portable version of
**  http://www.codeproject.com/KB/cpp/properties.aspx
** which only worked under MSVC
*/

/*
** This is the Python-like version, for the C#-like version, see:
** http://gist.github.com/632294
*/


#include <iostream>
 
/************************************************************
** This is where the magic goes
*************************************************************/
 
template <typename ClassT, typename TypeT, TypeT (ClassT::*Getter)() const>
class _Get
{
public:
  explicit _Get(ClassT const* self)
    : _this(self) {
  }
  inline operator TypeT() const
  {
    return (_this->*Getter)();
  }
private:
  TypeT operator=(TypeT value) const;
private:
  ClassT const* _this;
};
template <typename ClassT, typename TypeT, void (ClassT::*Setter)(TypeT)>
class _Set
{
public:
  explicit _Set(ClassT* self)
    : _this(self) {
  }
  inline TypeT operator=(TypeT value) const
  {
    (_this->*Setter)(value);
    return value;
  }
private:
  operator TypeT() const;
private:
  ClassT* _this;
};
template <typename ClassT, typename TypeT, TypeT (ClassT::*Getter)() const, void (ClassT::*Setter)(TypeT)>
class _GetSet
{
public:
  explicit _GetSet(ClassT* self)
    : _this(self) {
  }
  inline operator TypeT() const
  {
    return (_this->*Getter)();
  }
  inline TypeT operator=(TypeT value) const
  {
    (_this->*Setter)(value);
    return value;
  }
private:
  ClassT* _this;
};
 
#define RO_PROPERTY(CLASS, TYPE, GETTER) _Get<CLASS, TYPE, &CLASS::GETTER>
#define WO_PROPERTY(CLASS, TYPE, SETTER) _Set<CLASS, TYPE, &CLASS::SETTER>
#define RW_PROPERTY(CLASS, TYPE, GETTER, SETTER) _GetSet<CLASS, TYPE, &CLASS::GETTER, &CLASS::SETTER>
 
/************************************************************
** End of magic, now demo!
*************************************************************/
 
class MyClass
{
private:
  int _attribute;
 
public:
  inline int getAttribute() const {
    std::clog << "Getting attribute equals to " << _attribute << std::endl;
    return _attribute;
  }
  inline void setAttribute(int value) {
    std::clog << "Setting attribute to " << value << std::endl;
    _attribute = value;
  }
 
  RO_PROPERTY(MyClass, int, getAttribute) readOnlyProperty;
  WO_PROPERTY(MyClass, int, setAttribute) writeOnlyProperty;
  RW_PROPERTY(MyClass, int, getAttribute, setAttribute) readWriteProperty;
 
  MyClass()
    : _attribute(0),
      readOnlyProperty(this),
      writeOnlyProperty(this),
      readWriteProperty(this) {
  }
};
 
int main()
{
  MyClass myVar;
 
  // Read only
  std::cout << myVar.readOnlyProperty << std::endl; // OK
  //myVar.readOnlyProperty = 42; // Will fail
 
  // Write only
  //std::cout << myVar.writeOnlyProperty << std::endl; // Will fail
  myVar.writeOnlyProperty = 42; // OK
 
  // Read write
  std::cout << myVar.readWriteProperty << std::endl; // OK
  myVar.readWriteProperty = 42; // OK
}