# Boost Basic Linear Algebra - Configuration Options

## NDEBUG

Make sure you define NDEBUG The only way uBLAS knows you want a release configuration is to check if you have defined NDEBUG. If you don't it assumes you want a debug configuration and adds a lot of very useful runtime check. However these are very slow!

## BOOST_UBLAS_MOVE_SEMANTICS

The patch and description was provided by Nasos Iliopoulos.

An immediate effect of this option is the elimination of the need for noalias in types `vector<T>` and `matrix<T>`, when assigned to the same type. This option doesn't have an effect on bounded and c types. Although it is rare, not all compilers support copy elision (that allows for move semantics), so a test must be performed to make sure that there is a benefit when it is enabled. A small demonstration and test can be found in `test_move_semantics.cpp`

In the test example two tests are defined, one for vectors and one for matrices. The aim of this example is to print the pointers of the storage of each of the containers, before and after the assignment to a temporary object. When move semantics are enabled, the `vector<T>` and `matrix<T>` storage is moved from the temporary and no copy is performed.

If move semantics are supported by your compiler you will get an output like the following:

    matrix<double> --------------------------------------------------------------------
    Temporary pointer r: 0x94790c0
    Pointer (must be equal to temp. pointer if move semantics are enabled) : 0x94790c0

Notes:

- It should be no surprise to see matrices and vectors been passed by VALUE, the compiler takes care and either moves (if the underlying code does not modify the object), or copies (if the underlying code modifies the object).
- There might be some space for some improvements (like clearing the data, before swaping)
- Move semantics don't eliminate temporaries. They rather move their storage around so no copies are performed.
- MSVC does no implement Named Return Value Optimization in debug mode. So if you build in debug with this compiler you might get different behaviour than a release build.
- Enabling move semantics is done via #define BOOST_UBLAS_MOVE_SEMANTICS.
- There is plenty of room for optimizations when c++0x standard is out, taking advantage of rvalue references. (I have a sweet vector implementation using that).
- If you enable move semantics and your compiler does not support them, the operation will just be as passing by const reference.

Interesting links

- Want Speed? Pass by Value.
- Rvalue References: C++0x Features in VC10, Part 2
- Move It With Rvalue References

## BOOST_UBLAS_CHECK_ENABLE

When BOOST_UBLAS_CHECK_ENABLE is defined then all index and parameter checks are enabled. This is enabled in debug mode and disabled in release mode.

## BOOST_UBLAS_TYPE_CHECK

When BOOST_UBLAS_TYPE_CHECK is enabled then all possibly expensive structure checks are enabled. If this is not desireable then use `#define BOOST_UBLAS_TYPE_CHECK 0` before including any uBLAS header. The define BOOST_UBLAS_TYPE_CHECK_EPSILON can be used to control the acceptable tolerance, see `detail/matrix_assign.hpp` for implementation details of this check.

## BOOST_UBLAS_USE_LONG_DOUBLE

Enable uBLAS expressions that involve containers of 'long double'

## BOOST_UBLAS_USE_INTERVAL

Enable uBLAS expressions that involve containers of 'boost::numeric::interval' types

## Configuring uBLAS with Macros

Many macro's appear in ublas/config.hpp and elsewhere. Hopefully in the future some of these will disappear! They fall into 4 groups:

- Automatically set by 'boost/numeric/ublas/config.hpp' based on NDEBUG. Makes the distinction between debug (safe) and release (fast) mode. Similar to STLport
  - Release mode (NDEBUG defined)
    - BOOST_UBLAS_INLINE Compiler dependant definition to control function inlining.
    - BOOST_UBLAS_USE_FAST_SAME
  - Debug mode
    - BOOST_UBLAS_CHECK_ENABLE Enable checking of indexs, iterators and parameters. Prevents out of bound access etc.
    - BOOST_UBLAS_TYPE_CHECK Enable additional checks for the results of expressions using non dense types. Picks up runtime error such as the assignment of a numerically non-symmetric matrix to symmertic_matrix. Use `#define BOOST_UBLAS_TYPE_CHECK 0` to disable expensive numeric type checks. (Note: "structure check" would be a much better name.)
    - BOOST_UBLAS_TYPE_CHECK_EPSILON default: sqrt(epsilon), controls how large the difference between the expected result and the computed result may become. Increase this value if you are going to use near singular or badly scaled matrices. Please, refer to `detail/matrix_assign.hpp` for implementation of these type checks.
- Automatically set by 'boost/numeric/ublas/config.hpp' based on compiler and boost/config.hpp macro's. Augments the compiler deficiency workarounds already supplied by boost/config.hpp
  - BOOST_UBLAS_NO_NESTED_CLASS_RELATION A particularly nasty problem with VC7.1 Requires that uBLAS and the user use begin(it) rather then it.begin()
  - BOOST_UBLAS_NO_SMART_PROXIES Disable the automatic propagation of 'constantness' to proxies. Smart proxies automatically determine if the underling container they reference is constant or not. They adjust there definition of iterators and container access to reflect this constantness.
- For use by uBLAS authors to test implementation methods. Preset in config.hpp
  - BOOST_UBLAS_USE_INVARIANT_HOISTING
  - BOOST_UBLAS_USE_INDEXING
  - BOOST_UBLAS_USE_INDEXED_ITERATOR
  - BOOST_UBLAS_NON_CONFORMANT_PROXIES Gappy containers may be non-conformant, that is contain elements at different indices. Assigning between proxies (vector ranges for example) of these containers is difficult as the LHS may need insert new elements. This is slow.
  - BOOST_UBLAS_USE_DUFF_DEVICE Near useless on all platforms (see GCC's -funroll-loops)
- User options. Can be predefined by user before including any uBLAS headers. They may also be automatically defined for some compilers to work around compile bugs.
  - BOOST_UBLAS_USE_LONG_DOUBLE Enable uBLAS expressions that involve containers of 'long double'
  - BOOST_UBLAS_USE_INTERVAL Enable uBLAS expressions that involve containers of 'boost::numeric::interval' types
  - BOOST_UBLAS_SIMPLE_ET_DEBUG In order to simplify debugging is is possible to simplify expression templateso they are restricted to a single operation
  - BOOST_UBLAS_ENABLE_PROXY_SHORTCUTS enable automatic conversion from proxy class to matrix expression
  - BOOST_UBLAS_NO_ELEMENT_PROXIES Disables the use of element proxies for gappy types.
  - The Gappy types (sparse, coordinate, compressed) store non-zero elements in their own containers. When new non-zero elements are assigned they must rearrange these containers. This invalidates references, iterators or pointers to these elements. This can happen at some surprising times such as the expression "a [1] = a [0] = 1;". Element proxies guarantee all such expressions will work as expected. However they bring their own restrictions and efficiency problems. For example as of Boost 1.30.0 they prevent the assignment of elements between different types.
  - BOOST_UBLAS_REFERENCE_CONST_MEMBER Enable to allow refernces to be returned to fixed (zero or one) elements of triangular or banded matrices
  - BOOST_UBLAS_NO_EXCEPTIONS Disable the use exceptions of uBLAS internal checks and error conditions. BOOST_NO_EXCEPTIONS has same effect.
  - BOOST_UBLAS_SINGULAR_CHECK Check the for singularity in triangular solve() functions

Copyright (©) 2000-2009 Joerg Walter, Mathias Koch, Gunter Winkler
Use, modification and distribution are subject to the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt ).

Last modified: Wed Sep 16 23:16:45 CEST 2009