Re: [boost] Numeric constants

Subject: Re: [boost] Numeric constants
From: John Maddock (john_at_[hidden])
Date: 2009-01-31 05:37:22

>How to use those constant ? It rather simple. Things like
>double k = pi_;
>int u = ten_;
>works as intended by auto-casting the valeu to the destination type.
>
>You can force a constant to be of a given type : abs( float(log2_) );
>Alternatively, the basic operator (+,-,*,/,unary -, ~, ! and comparison)
>are overlaod to autocast the constant into a proper type when used like :
>
>double k,v;
>k = v + pi_;
>bool test = x > log10_;
>
>A set of result_of enabled traits also allows to computes the correct
>type of a constant when used in a complex expression.
>Most constant type-cast is performed at compile-time, unless if you use
>a user-defined type-casting overload.
>
>I am overthinking it or could this be viable ?

Well.... this is one of those "complex" solutions that was suggested last 
time this was discussed that I've been trying to avoid ;-)

Off the top of my head, the main problems I see are:

* What is the result of 2 * pi?  How about 2 * pi * r where r is a real?
* If you mistakenly call a function passing a constant as argument, for 
example foo(2.0, pi), then if foo is a template you will likely get an 
inscutable failure very deep inside foo's body (or worse some other function 
that foo calls).
* In order to avoid ODR violations I presume the constants have to be 
defined in an anonymous namespace?  Does this cause code bloat simply by 
#including the constants header?

Now I suspect that all of these issues are probably solvable to some degree, 
it's a question of whether it's worth the hassle, and/or whether the 
resulting code is likely to turn into a problem to maintain.

Cheers, John.