Re: [Boost-users] proto: analytical-only math functions

Subject: Re: [Boost-users] proto: analytical-only math functions
From: Eric Niebler (eric_at_[hidden])
Date: 2009-03-14 13:37:03

Hicham Mouline wrote:
> Eric Niebler wrote:
>> And if I'm right in assuming that the first parameter of 
>> derivative is which derivative to take, you'll need to make that 
>> information available at compile-time as well, so:
>>   let( f(x) = log(x) )[ g(x) = derivative<1>(f, x) ];
> 
> I am thinking about user friendliness.
> I think I will have the user define this:
> 
> typedef   proto::terminal<  function_tag< mpl::size_t<3> > >::type
> function3;
> function3 f( x1, x2, x3,  x1+ x2+ x3+ c6 );  // after all, store the
> expression tree at runtime
> 
> 1. Can I define a constructor for the terminal like this?

You could, but ...

> 2. Can I store the expression tree somewhere? Inside the function_tag
> template?

You could store the expression tree within such a function3 object, but 
only by erasing its type. (See 
http://www.artima.com/cppsource/type_erasure.html for more on type erasure.)

Look at the type "function3" you've defined above. It has absolutely no 
compile-time information whatsoever about what expression might be 
stored inside it. If you want to use this variable in more complicated 
expressions and do expression template magic to compute derivatives and 
stuff at compile time, you can't.

The options I offered above make all the compile-time information *about 
the full expression* available to Proto so you can compute interesting 
things at compile time.

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