[boost] Re: Pretty Good Initialization Library

From: Vladimir Prus (ghost_at_[hidden])
Date: 2004-07-07 01:21:43

Andrei Alexandrescu (See Website for Email) wrote:

> Just sent the message below to clc++m. Any comments are welcome.
> 
> There's a library that I understand is being proposed for Boost:
> 
> http://www.codeproject.com/vcpp/stl/PGIL.asp

Actually, it's even already accepted: see the first entry of "Past Review
Results" in 

    http://boost-consulting.com/boost/more/formal_review_schedule.html

> It does useful things, but it runs straight against the commonly given
> advice (with which I happen to agree) that one should not override
> operator, unless using expression templates that ensure left-to-right
> evaluation.
> 
> So now, in the referenced article, the author makes the comment: "
> Overloading operator comma is sometimes viewed as a bad practice [3].
> However, it has been done with success in eg. the Generative Matrix
> Computation Library and Blitz to initialize matrices (see [4]) and [5]).
> The Initialization Library overloads the comma operator in a safe manner
> by letting set_cont and set_map return an object that is responsible for
> the initialization. Therefore it takes explicit action from the programmer
> to begin using the overloaded operator,(). "
> 
> I think the mentioned libraries do employ expression templates, while the
> initialization library does not. 

IIRC, Blitz has the following syntax to initialize an array

  A = 1, 2, 3;

and I believe it's almost the same which this library does, and I *guess*
the above does not involve expression templates -- just overloaded
operator, which pushes data to the array.

> So I am unsatisfied. Anyone here can 
> soothe my concerns? Is overloading the comma operator considered harmless
> after all?

I think in this context it's more or less harmless. One can do:

   vector<int> v;
   v += 1, 2, 3;

but the += operator is only defined for specific containers, and it's not
defined for vector by default. So there's little risk that user will
inadvertently use it, meaning something else.

- Volodya