TITLE: returning self reference from mutators

(Newsgroups: comp.lang.c++.moderated, 28 Mar 97)


CAMBELL: Mike Campbell <mcampbel@tvmaster.turner.com>

|>  I've recently seen, and have become rather accustomed to, mutators
|>  returning a reference to this.
|>  
|>  class foo  // has members f, i, and s (float, int, and string)
|>  {
|>  	public:
|>  		foo& F(float in) { _f = in; return *this; }
|>  		...
|>  
|>  	private:
|>  		float _f;
|>  		...
|>  };
|>  
|>  
|>  This allows one to write:
|>  
|>  foo obj;  obj.F(3.14).I(2).S("bar");
|>  
|>  to set multiple members in one line.
|>  
|>  Am I being seduced by something evil, or is this a Common Way To Do
|>  It(tm)?


KANZE: James Kanze <james-albert.kanze@vx.cit.alcatel.fr>

Or both:-).

Seriously, I find that in general, this probably doesn't do much for the
maintainability, but in specific cases, it is a good solution.  I can
think of at least one case where it is certainly appropriate: for
handling argument lists.  A typical example would be a window
descripter.  The constructor creates the object (with default
parameters) and you add the custom parameters before passing it, e.g.:

	createWindow( WindowParameters( defaultWindowStyle )
					  .backgroundColor( "chartreuse" )
				      .foregroundColor( "magenta" )
					  .name( "sickening" ) ) ;

I use this in my GB_Format class, for example:

	cout << GB_Format( "The value is %6.2f" ).with( theValue ) << endl ;

For that matter, although the spelling is different, this is exactly
what is happening in:

	cout << "Some text" << aNumber << endl ;

The above is actually:

	cout.operator<<( "Some text" ).operator<<( aNumber ).operator<<( endl ) ;

Despite all this, I think that such chaining should be the exception,
not the rule.

(Pete Becker mentions that he hasn't seen this idiom much in production
code.  I would guess that he wasn't considering the use of operator<<.)