[boost] Object Hierarchies using Interface Types with the BIL

From: christopher diggins (cdiggins_at_[hidden])
Date: 2004-12-30 16:05:18

On the subject of GUI object hierarchies, I wanted to bring to everyone's 
attention the Boost Interfaces Library (BIL) currently being developed by 
Jonathan Turkanis which could be an excellent tool for building a flexible 
and efficient GUI library. There is a brief introduction to the library on 
the Boost wiki ( 
http://www.crystalclearsoftware.com/cgi-bin/boost_wiki/wiki.pl?Boost.Interfaces )

The BIL library, provides a mechanism for non-intrusive structural 
subtyping. In other words any object can be used polymorphically if it 
provides function signatures which match those of the interface type. The 
advantages are that it eliminates the overhead of virtual tables in the 
objects. Also dynamic function dispatch only occurs when it is needed (i.e. 
through an interface reference). Most importantly the BIL removes the 
requirement of inheritance for polymorphism.

The code for declaring the interfaces would look like this:

BOOST_IDL_BEGIN(ILabel)
  BOOST_IDL_CONST_FN0(GetText, string)
  BOOST_IDL_CONST_FN0(GetAlignment, align_T)
BOOST_IDL_END(ILabel)

BOOST_IDL_BEGIN(IStandardButton)
  BOOST_IDL_FN0(GetLabel, ILabel)
  BOOST_IDL_FN0(GetDims, dim_struct)
BOOST_IDL_END(IStandardButton)

Using the interfaces would be done by simply declaring classes which 
implement the interfaces implicitly as below:

class CenteredLabel {
  public:
    CenteredLabel(string s) : text(s) { };
    string GetText() { return text; }
    string GetAlignment() { retun enumAlignHCenterVCenter; }
  private:
    string text;
}

class StandardOkButton  {
  public:
    OkButton() : label("OK") { }
    dim_struct GetDims() { return dim_struct(100,50); }
    ILabel GetLabel() { return label; }
  private:
    CenteredLabel label;
}

Is the value of such as approach apparent or are more detailed examples 
warrranted?

Christopher Diggins
http://www.cdiggins.com