Re: [Boost-users] [proto-docs] Customing Expression Trees - why three args in operator()?

Subject: Re: [Boost-users] [proto-docs] Customing Expression Trees - why three args in operator()?
From: Eric Niebler (eric_at_[hidden])
Date: 2008-09-25 14:09:50

Markus Werle wrote:
> Hi!
> 
> [@Eric: The docs look much greater now. Thank you for the hard work. 
> As I can afford time I will go through the docs step by step, 
> give comments again and ask silly questions like before.
> Is it OK for You that this happens on the public ML?
> IMHO this may help others, too. 
> Some remarks are rather nitpicking stuff, but still considered to be helpful
> for first time users of proto]

Here is fine. Thanks for taking the time to offer feedback.

> In this post I refer to 
> <http://boost-sandbox.sourceforge.net/libs/proto/doc/html/
> boost_proto/users_guide/getting_started/hello_calculator.html>
> 
> Q1: Probably a typo: Is it "Customing" or "Customizing"?

Thanks.

> Q2: It is not clear to me why operator() has 3 arguments:
> 
> double operator()(double a1 = 0, double a2 = 0, double a3 = 0) const
>     {
>         calculator_context ctx;
>         ctx.args.push_back(a1);
>         ctx.args.push_back(a2);
>         ctx.args.push_back(a3);
>         
>         return proto::eval(*this, ctx);
>     }
> 
> 
> IMHO the whole thing should work even without the 3rd argument.
> Can you explain?

I've only shown placeholders _1 and _2, so you're right, the third 
argument will never be used. I'll remove it.

> R1: For didactic reasons I would add examples that do not rely on the
> assumption that the user knows STL by heart, e.g. immediately before
> 
> <cite>
> // Use std::transform() and a calculator expression
> // to calculate percentages given two input sequences:
> std::transform(a1, a1+4, a2, a3, (_2 - _1) / _2 * 100);
> </cite>
> 
> please add the following code:
> 
> double const result = ((_2 - _1) / _2 * 100)(45.0, 50.0);
> BOOST_ASSERT(result == (50.0 - 45.0) / 50.0 * 100)); // is this portable?
> 
> This proposal is in analogy to the code presented in the boost::bind docs
> at http://www.boost.org/doc/libs/1_36_0/libs/bind/bind.html, which I 
> always found _very_ helpful, since they separate one thing from another.

Good suggestion.

> R2: In "Controlling Operator Overloads" it would help a lot if the
> difference between the previous approach and the new approach
> was written in *bold*, in order to hinder a scroll
> 
> proto::domain< proto::generator<calculator>, <BOLD>calculator_grammar</BOLD> >
> 
> or like this:
> 
> struct calculator_domain
> //  OLD version had default grammar  
> //: proto::domain< proto::generator<calculator> >
>   : proto::domain< proto::generator<calculator>, calculator_grammar >
> {};

OK, will do.

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com