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Subject: Re: [ublas] Help with raw pointers and matrix/vector interoperability with other libraries through casts
From: Vardan Akopian (vakopian_at_[hidden])
Date: 2008-09-15 13:32:27


I think there is a lot of confusion in your understand of ublas, but
also STL and c++ in general. I'll try some short answers to your

On Sat, Sep 13, 2008 at 12:32 PM, Jesse Perla <jesseperla_at_[hidden]> wrote:
> Hi,
> As I am using a variety of libraries for my projects, and while I will
> standardize on using ublas::matrix and ublas::vector, the other libraries
> use a variety of C-style array/matrix, stl::vector, and homegrown
> libraries. I am looking for a relatively straightforward way to pass in my
> ublas vectors/matricies to these functions without copying all of the
> values, etc. For now, I can focus on dense column:major only, with
> assumption of no reallocation/resizing.
> I have seen some discussion about getting the raw pointer to the
> matrix/vector libraries, but I am having trouble getting it to work. Also
> have heard about adaptors, but don't quite understand what people are
> talking about.

Because of the needs you describe above, it would definitely be a good
idea to get a better understanding of the adaptors. In brief, they
provide the underlying storage and handle all the memory management
details for all the ublas containers. More in the storage section of
he documentation

> It key cases are:
> 1) std::vector
> The faq suggests using a copy iterator to go between them, but I don't want
> to copy the data. std::vector does support initialization from a data
> pointer...

No, std::vector never shares the pointer. In fact it does not have a
constructor that takes a pointer. It has one which takes 2 iterators
though, and in which case it copies the data.

> I would love something like:
> ublas::vector<double> v(4);
> std::vector<double>& v_std = ublas_to_std_vector<double>(v);
> //Or better yet, if I could define a cast for this?

Casting is definitely not possible (nor desirable) for this types of
issues. Reusing an existing std::vector for a ublas vector is possible
_in principle_, but one would have to write a ublas adaptor that is
based on std::vector (which in itself is not that difficult). Then one
could implement a swap() type of method to take over std::vector's
storage and use it for the ublas vector adaptor. See below for a
different, unsafe way of reusing the storage of std::vector.

> 2) C-style array and matrix:
> a) Want to be able to pass in the matrix or vector to C style functions:
> //Couldn't get matrix type operators to work. I tried to various
> permutations on:
> ublas::matrix<int> A2(2,2);
> int** Ap2;
> Ap2 = &([0]);
> Ap2[0][0] = 1;
> //It appears that for matrices, the data() returns back an int* instead
> of an int** which is what most functions require. Perhaps there is an
> unsafe cast that would solve this problem? I can't figure out how to trick
> C style int** matrices to think they are of size NxN from a raw pointer
> without allocating data to determine pointer offsets.

Again this is not possible, since the storage for dense matrices (and
in fact for most of the sparse ones as well) is linear. However, this
is the standard way of designing a dense matrix class, and code that
expects a matrix as int ** should be reexamined and fixed. Note that
if you plan interacting with 3rd party libraries (e.g. lapack), you
don't need this types of casts, there are ublas bindings available for
most of the popular libraries.

> //The following seemed to work for ublas::vector though.
> ublas::vector<int> A(2);
> int* Ap;
> Ap = &([0]);
> Ap[0] = 1;
> b) If C style functions return data, would like to put the ublas wrapper
> around them. I assume this is done with calling a constructor on
> ublas::matrix or ublas::vector with the pointer.

The preferred and safe way of using ublas containers is to copy the
data, and thus allow ublas to handle all the tricky memory management
associated with copy, assignment, resize etc. However there has been
lots of discussions on this list on this subject, and ublas already
has ways of doing this. Search for BOOST_UBLAS_SHALLOW_ARRAY_ADAPTOR.
I'd strongly discourrage the use of this technique however, until
you've proven (through profiling and such), that copying the data is
where your bottleneck is.

> 3) Home Grown matrix libraries:
> A million exist, and nearly all seem to have constructors for passing in a
> pointer to data, so I think the problem
> Maybe a cast/function could be written to do this generically since the
> concept of a copy constructor with pointers may be all that is needed from
> the library? But I don't know enough about generic programming/casts to
> write this myself. Is the best way to deal with this stuff to define a
> reinterpret_cast? If so, any good examples of how to do so and how to add
> these to libraries without modifying the underlying boost::ublas, etc.?
> Any ideas or example code for this kind of stuff?

If a matrix constructor takes a pointer, it either does a copy and
manages its own memory, or shares the pointer and does not manage the
memory. Both capabilities are there in ublas. Use std::copy for
copying data (recommended), and shallow array adaptor for sharing the
data (not recommended).