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From: Janek Kozicki (janek_listy_at_[hidden])
Date: 2006-06-17 19:27:43

Leland Brown said: (by the date of Fri, 16 Jun 2006 14:14:31 -0700 (PDT))

> --- Janek Kozicki <janek_listy_at_[hidden]> wrote:
> > One last note: above design does not allow to resize vectors and
> > matrices in runtime. This limits the library usage. Should there be
> > added a slower complementary library that will allow to resize data in
> > runtime? Any ideas?
> I agree this is useful, and also that it has a
> run-time penalty. But I have an idea to reduce that
> penalty, as well as reducing the work for the library
> implementer to add this feature, without having to
> write a whole parallel library. I had hoped to add
> this to my own matrix library, but I haven't yet.
> (Actually, I need this capability, but I'm using a
> less efficient workaround for now.)

heh, perhaps you can help writing it here in boost, and then use it in
your program ;)

> A compromise that works in many situations is
> to set a maximum size for each object at compile-time,
> allocate memory for the maximum size, and then only
> use part of the structure depending on its current
> size set at run-time. This allows the data and its
> operations to have an adjustable size (up to a
> predetermined maximum) while avoiding heap allocation
> - the best of both worlds.
> As an example of how this can be implemented, suppose
> we augment our vector template with an extra template
> argument of type bool, which tells if the vector is
> resizable - e.g.:
> vector<3,double,false> // 3D vector of double

where the last argument defaults to false, so it doesn't need to be
specified always.

> vector<5,double,true> // vector *up to* 5D
> The resizable vector needs an extra member to store
> the actual current size, and that size should be used
> as the upper limit in loops, instead of the size in
> the template argument (see example below).
> Both kinds of vectors can be implemented with one
> definition, by inheriting from separately specialized
> base classes. Here's an example:

<snip, interesting stuff>

> FWIW, I'm not happy with the syntax above of adding an
> extra bool parameter. For my code, I planned to use a
> negative value of N to indicate resizable:
> vector<3,double> // fixed 3D vector
> vector<-5,double> // up to 5D vector
> matrix<4,-14,double> // 4x1 to 4x14 matrix
> matrix<-14,4,double> // 1x4 to 14x4 matrix
> matrix<-6,-6,double> // both values resizable
> but this is admittedly nonintuitive (though it does
> make both the library and user code more concise).

that is a very intersting idea. Especially because matrices would
require two bools. If stated out clearly in the documentation, perhaps
we could use it? What others think?

> BTW, this is where I'd really like to see templatized
> typedefs in C++, allowing us to define:
> template< int N, type T >
> typedef vector< N, T, false > fixed_vector;
> template< int N, type T >
> typedef vector< N, T, true > resizable_vector;
> Thus we'd have:
> fixed_vector<N,T>
> resizable_vector<N,T>
> while using the common vector template defined above.

uh, wait. It's not possible currently? I'm not 100% sure but I think
that I was using similar code some time in the past...

> In _The Design and Evolution of C++_, Bjarne
> Stroustrup says adding templatized typedefs would be a
> "technically trivial" extension, but questions the
> wisdom of the idea. Perhaps there's been some
> discussion about this topic in Boost?

At the end I want again to recall my "table of" .. err " concepts"?
Because resizable_vector really niecely fits here, so that we can see
more similarities in the design.

everything determined| something determined | everything determined
during compilation | during compile time | during runtime
stage | sth during runtime |
t1_quantity | t2_quantity | t3_quantity
fixed_quantity | scalable_quantity | free_quantity
                     | |
vector<3> | resizable_vector | vector.resize(3)
matrix<4,4> | resizable_matrix | matrix.resize(4,4)
fixed_vector ? | | free_vector ?

Janek Kozicki                                                         |

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