eigen/doc/D01_StlContainers.dox

namespace Eigen {

/** \page StlContainers Troubleshooting - Using STL Containers with Eigen

\b Table \b of \b contents
  - \ref summary
  - \ref allocator
  - \ref vector

\section summary Executive summary

Using STL containers on \ref FixedSizeVectorizable "fixed-size vectorizable Eigen types" requires taking the following two steps:

\li A 16-byte-aligned allocator must be used. Eigen does provide one ready for use: aligned_allocator.
\li If you want to use the std::vector container, you need to \#include <Eigen/StdVector>.

These issues arise only with \ref FixedSizeVectorizable "fixed-size vectorizable Eigen types" and \ref StructHavingEigenMembers "structures having such Eigen objects as member". For other Eigen types, such as Vector3f or MatrixXd, no special care is needed when using STL containers.

\section allocator Using an aligned allocator

STL containers take an optional template parameter, the allocator type. When using STL containers on \ref FixedSizeVectorizable "fixed-size vectorizable Eigen types", you need tell the container to use an allocator that will always allocate memory at 16-byte-aligned locations. Fortunately, Eigen does provide such an allocator: Eigen::aligned_allocator.

For example, instead of
\code
std::map<int, Eigen::Vector4f>
\endcode
you need to use
\code
std::map<int, Eigen::Vector4f, std::less<int>, Eigen::aligned_allocator<Eigen::Vector4f> >
\endcode
Note that here, the 3rd parameter "std::less<int>" is just the default value, we only had to specify it because we needed to specify the allocator type, that is the 4th parameter.

\section vector The case of std::vector

The situation with std::vector was even worse (explanation below) so we had to specialize it for the Eigen::aligned_allocator type. In practice you \b must use the Eigen::aligned_allocator (not another aligned allocator), \b and \#include <Eigen/StdVector>.

Here is an example:
\code
#include<Eigen/StdVector>
\/* ... *\/
std::vector<Eigen::Vector4f,Eigen::aligned_allocator<Eigen::Vector4f> >
\endcode

<span class="note">\b Explanation: The resize() method of std::vector takes a value_type argument (defaulting to value_type()). So with std::vector<Eigen::Vector4f>, some Eigen::Vector4f objects will be passed by value, which discards any alignment modifiers, so a Eigen::Vector4f can be created at an unaligned location. In order to avoid that, the only solution we saw was to specialize std::vector to make it work on a slight modification of, here, Eigen::Vector4f, that is able to deal properly with this situation.
</span>

*/

}