Re: [boost] properties in C++

From: Douglas Gregor (gregod_at_[hidden])
Date: 2002-02-13 14:38:44

On Wednesday 13 February 2002 12:55 pm, you wrote:
> IMHO, the ONLY advantage of a 'property' in comparison with the good old:
>
>     int const& foo() const { return m_foo ; }
>     int         & foo()          { return m_foo ; }
>     int m_foo ;
>
> is that the user don't have to write the extra '()'.
>
> I don't think that a property facility could add that much complexity just
> to allow syntactic sugar.
>
> Of course, I can be missing something and I'll be glad to know it.

There's no reason that properties be restricted to 'just' being syntactic 
sugar (note: in my world, syntax is _everything_, so that's enough of an 
argument for me already).

Since every property will know the address of the object containing it, 
properties can also be used for setting values at run-time and for 
reflection. Let's say we can do this (some abbreviations in the code):

class Window : public runtime_properties {
public:
  union {
    Window* self;
    property< read<&Window::get_top>, write<&Window::set_top> > top;
    property< read<&Window::get_left>, write<&Window::set_left> > left;
    property< read<&Window::get_width>, write<&Window::set_width> > width;
    property< read<&Window::get_height>, write<&Window::set_height> > height;
  }

  Window() : 
    self(this), top("Top", 0), left("Left", 0), 
    width("Width", 100), height("Height", 100)
  {
  }
};

The intention here is that the properties can register themselves with the 
"runtime_properties" base class of the Window class. They tell the 
runtime_properties base class:
  1) Their human-readable names
  2) Their default values (optional)
  3) How to read and/or write their values

Properties such as this have been made language features before (see Delphi 
or C++ Builder), and the degree to which one can interact with a component 
via its properties is wonderful. 

Did I mention that the syntax is nicer? :)

        Doug