Re: [boost] [GTL] - geometric template library - determininginterest

From: Simonson, Lucanus J (lucanus.j.simonson_at_[hidden])
Date: 2007-10-04 23:32:10

John wrote:
>What is not lightweight about the get<0>(pt)  syntax? Doesn't it
compile
>away? Is the issue a dependency on another library -- because the
>get<0>(pt) syntax seems really easy to provide without bothering with
>Fusion. 
>
>i.e. 
>template <int Index> float get(MyPoint & p) {return p[Index];}
>OR
>template <> float get<0>(MyPoint & p) {return p.x;}

Dependency is an issue, since my library depends only on the STL, which
ships with the compiler and compiles currently in gcc 3.2.2, 3.4.2, and
4.2.1; icc 9 and 10 and most painfully Visual Studio 2003/2005.  The
barrier to adoption into a legacy build system is extremely low.  Adding
a dependency, even to a boost library, would actually hurt my ability to
integrate it into legacy applications.

Moreover, the technique you are proposing works fine at compile time,
but won't work if the axis of the coordinate you are accessing is a
runtime parameter:
int my_axis = 0;
get<my_axis>(pt);
will throw an error.  We specify the axis we access at run time.  If we
specify a constant it should compile away and be light weight.  gcc
4.2.1 still needs us to play with the compiler flags to get decent
inlining.
pt.get(orient.getPerpendicular()); //orientation determined at run time.
My scanline algorithm actually parameterizes the orientation of the
scanline at runtime, so you can sweep left to right or bottom to top.

Lucanus Simonson